THE COMMERCIAL EXPLOITATION OF THAMES MITTEN CRABS: A FEASIBILITY STUDY

Transcription

1 THE COMMERCIAL EXPLOITATION OF THAMES MITTEN CRABS: A FEASIBILITY STUDY A report for the Department for Environment, Food and Rural Affairs by the, Cromwell Road, London SW7 5BD, England (DEFRA reference FGE 274, dated 17 October 2008) Pilot Project on the feasibility of commercially exploiting Thames Chinese mitten crabs Research Team: Paul F. Clark*, Patrick Campbell, Brian Smith, Philip S. Rainbow, David Pearce and Roberto P. Miguez With contributions from:- Susanne Surman-Lee, Director; Health Protection Agency London Regional Food Water & Environmental Microbiology Services, 61 Colindale Ave, London NW9 5EQ, England Sariqa Wagley, CEFAS, Weymouth Laboratory, Barrack Road, The Nothe, Weymouth, Dorset DT4 8UB, England * Corresponding author: Crustacea Research Group, Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, England p.clark@nhm.ac.uk 1

2 Report to be cited as:- Clark PF, Campbell P, Smith B, Rainbow PS, Pearce D and Miguez RP The commercial exploitation of Thames mitten crabs: a feasibility study. A report for the Department for Environment, Food and Rural Affairs by the Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, England. DEFRA reference FGE 274. pp appendices

3 CONTENTS Page TITLE PAGE 1 CONTENTS 3 EXECUTIVE SUMMARY 4 ACKNOWLEDGEMENTS 7 1. INTRODUCTION 8 2. METHODS POTS VERSUS NETS FISH: THE BY-CATCH THAMES PHYSICAL DATA THAMES MITTEN CRAB FISHING THAMES MITTEN CRABS: FIT FOR 51 HUMAN CONSUMPTION? 8. BRITISH GOVERNMENT POLICIES: 67 A CONFLICT OF INTERESTS 9. CONCLUSIONS 71 REFERENCES 75 APPENDIX Page Appendix 1 Photographs Appendix 2 Classification, morphology and biology of Eriocheir sinensis H. Milne Edwards, 1853; Photographs 28-36, Figures Appendix 3 Invasive distribution and spread of Eriocheir sinensis outside of UK; Photographs 37-40; Figures Appendix 4 Thames mitten crab field data from 9 November 2005 to 5 January L = left pereiopod, R = right pereiopod 36 Appendix 5 By-catch Fish Report compiled by Patrick Campbell, Fish Section, Department of Zoology 53 Appendix 6 Eriocheir sinensis: increasing awareness 70 3

4 EXECUTIVE SUMMARY 1. A large population of Eriocheir sinensis is now well established in the River Thames, with an autumn migration pattern, ovigerous crabs, juveniles and burrowing behaviour. This population continues to expand westwards with a new record (13 October 2007) from Boveney Lock, just upstream of Windsor. 2. In South East Asia mitten crabs are eaten. The ovaries and testes are considered to be a delicacy; consequently mitten crabs are a seasonal product being only consumed during the autumn migration period when the gonads are ripening. 3. One method of controlling the Thames population may be the commercial exploitation of this species. This feasibility study, although limited, considers that the Thames mitten crab population is large enough to support an artisanal fishing industry. This could reduce mitten crab numbers from the catchment and provide additional financial benefits for local fishermen. 5. A major concern of the London Port Health Authority was the suitability of Thames mitten crabs for human consumption with respect to the lung fluke parasite Paragonimus westermani, the bacteria Vibrio parahaemolyticus and, concentrations of trace metals, PAHs and organochlorines. 6. With regard to Paragonimus westermani, Stentiford (2005) reported to the London Port Health Authority that the lung fluke parasite is not present in the Chinese mitten crabs from the Thames estuary and therefore, the infection risk for those consuming the raw product is negligible. 7. Levels of Vibrio parahaemolyticus in crabmeat peaked during the summer months, in August 2006 with total counts exceeded a median of 10 9 cfu/g. The high levels of total V. parahaemolyticus cells found in this small study indicate that subpopulations of clinically significant strains may be present purely by stochastic variation. Thus consumption of the product particularly raw or lightly cooked may pose a public health risk. 8. During the analysis of Thames mitten crabs for concentrations of trace metals, PAHs and organochlorines, concentrations of dioxins and PCBs in the brown meat (hepatopancreas/digestive gland and the confluent ovary/testis) were found to be high. Concentrations were actually above the European mandatory limit in tissues other than crab brown meat which is specifically excluded from the recommendation. This finding raises the question as to whether the brown meat of Thames mitten crabs is safe to eat. It should be borne in mind that the adverse effects of exposure to dioxins and PCBs are chronic, not acute; furthermore, mitten crabs are ripe for harvest only for a limited period of the year the autumn - and consumption will be restricted to a period of 3 to 4 months annually. Based on average data, an adult consuming a single 16g portion of mitten crab brown meat per week over a year would have an averaged daily intake of dioxins and dioxinlike PCBs at around the estimated tolerable daily intake (TDI). On the other hand, taking into account the limited season of availability of the crabs, a male or female past child-bearing age could consume several portions per week in this season. With a caveat over excessive partaking of mitten crab brown meat, particularly in 4

5 the case of children and women of child-bearing age, it does appear that the harvesting of mitten crabs from the Thames for culinary use need not be discouraged. 9. Although this report suggest that commercial exploitation of the mitten crab is possible with a view to reducing the population in the Thames, the final decision and control for such a fishery ultimately must taken by those Government organization with an invested interest including DEFRA, the Environment Agency and the London Port Health Authority. 10. If permission for commercial exploitation is forthcoming, mitten crab fishing on the Thames would be limited to three months of the year starting in September to the end of November, being timed to catch the annual downstream migration. Fishing could start in the River Lee area and possibly down as far as Barking Power Station water outlet point at ' N, ' E. Then during late September, when the dying off of the previous year s population is complete, mitten crab fishing could move further downstream to include the Dartford crossing area and further east to Tilbury Power Station and possibly beyond. 11. During this feasibility study baited pots and fyke nets were used to trap Thames mitten crabs over a fifteen-month period at four permanent sites along the river. A total of 539 crabs was caught during the present trial. 495 crabs were caught by fyke nets compared to 44 in baited pots; therefore baited pots are not considered to be a suitable trapping method for capturing mitten crabs in the River Thames. 12. During the fifteen-month fishing period a total of 2013 fish was trapped as bycatch including 1397 common eels (Anguilla anguilla) being caught in fyke nets. 13. During this extremely limited feasibility study, 308 Thames eels (Anguilla anguilla) were caught using fyke nets in the proposed fishing season and locality. But a large number of fishermen using fyke nets in the River Lee area to trap mitten crabs would be detrimental to the eel population because this species would be a considerable part of the by-catch. Consequently mitten crab fishing would have to be strictly controlled and probably licensed including possible monitoring of increased eel captures. 14. To avoid large captures of eels during the proposed mitten crab fishing season more research could be untaken into the trapping of Eriocheir including the possible development of pheromone bait in pots. Alternatively the use of fyke nets with openings, 40mm brass ring or a 40 mm mesh size, to release the eels and retain the crabs could be a cheaper but effective option. 15. This report does not suggest that a mitten crab fishery should be subject to future protective and conservation measures. The rational behind a Thames mitten crab fishery is to deplete the population. 16. A number of Government Policies and attitudes with regard to contentious invasive species require revisiting with a view to amendment. These measures include the banning of live imports of mitten crabs particularly into Scotland and perhaps generally into the UK. 5

6 17. A slowing in the flow rate of the River Thames by water abstraction and, changes in rainfall patterns and land management practices, may have been a contributory factor in allowing the mitten crab population to enter the estuary and migrate westwards. It would be interesting to record whether the mitten crab population would maintain itself if Thames water flows returned to something like normal. 6

7 ACKNOWLEDGEMENTS This feasibility study was funded by the Financial Instruments for Fisheries Guidance (FIFG), Projects in England, Innovative Measures, Application for Fisheries Grant, Department for Environment Food and Rural Affairs (DEFRA), grant number FGE 274; Esmée Fairbairn Foundation, grant reference EN/ ; Simon Birch Memorial Fund, Fishmongers Company, Fishmongers Hall, London; London Port Health Services, London Corporation, Sea Fish Industry Authority and the Food Standards Agency. We are indebted to Steve Bailey, FIFG Grant Development Officer, Sea Fish Industry Authority, for much support and valuable advice while applying for DEFRA funding. In fact without considerable input from Mr Bailey, this project may never have been funded. Jon Averns, London Port Health Services, City of London Corporation, has been extremely active with regard to Thames health regulations and the consumption of mitten crabs. We thank him for continued support. Tom Cousins, Fisheries Technical Specialist, South London & Tidal Thames, South East Area, Thames Region, Environment Agency, attended several meetings at the Museum and we thank him for his valuable time, permission to fish in the Thames and the loan of pots for the duration of this project. Eric Edwards, Fishmongers Company has always shown a keen interest in this project by providing support and helpful information as requested. Captain Christopher Mendoza, Port of London Authority, Harbour Master for the upper river, arranged our fishing sites at Lots Road and Richmond including Notes to Mariners. We also thank him for waiving lock charges at Richmond. We are grateful to the Lock Keepers at Richmond, Port of London Authority; they were always helpful and passed on useful local information. Alan Gratton, Cadogan Pier, granted us permission to use the land stage at Chelsea thereby allowing us to board Bumble B in safety. Bill Cohen, Scientific Analysis Laboratories, Manchester, guided us through the trace metal and organochlorine tests by providing information regarding the preparation of crabs. The project was grateful for his wise advice. David Mortimer, Food Standards Agency, has provided us with considerable scientific information regarding the permissible concentrations of dioxins in food products. His contributions have been gratefully received. Grant Stentiford, CEFAS Weymouth Laboratory, provided us with a copy of his report into the prevalence of lung fluke cysts in the Thames mitten crab population. We thank him for his various helpful comments on this human parasite. Alastair Wilson, Environment Agency, provided relevant information with regard to water abstraction from the Thames. Most of the photographs taken in this report are those of Phil Hurst with contributions from Phil Crabb and Harry Taylor; all from the Natural History Museum Photo Unit. Many thanks. We thank Jeni Stewart, for managing our various grants. 7

8 1.1 UK & IRISH RECORDS 1. INTRODUCTION England: The first British record of the Chinese mitten crab was in 1935 (Harold, 1935) from the River Thames. A second specimen was found in Southfields Reservoir near Castleford, Yorkshire in 1949 (Wall and Limbert, 1983). This reservoir tops up the canal system and it is likely that the crab was introduced through shipping on the Humber River. Ingle and Andrews (1976) reported further mitten crab records from the Thames and Clark (1984) for the Humber. Current records (Figure 1) have shown that the mitten crab is now common in the Rivers Thames, Medway, Ouse, Humber and Tyne (Herborg et al. 2003). Isolated captures have been reported from the River Teign, near Exmouth, Dungeness in Kent and ovigerous females from the Duddon Estuary near Morecambe Bay (see Figure 1) Wales: On the Welsh border there are recent records (November 2006) from the River Dee Estuary (Gabrielle Wyn, Countryside Council for Wales pers. comm.) Scotland: To date there are no records of mitten crabs from Scotland (Clark et al. 2002) Ireland: A single male record from collected from Waterford Harbour on the Suir Estuary at Bellview on 21 January 2006 (Minchin, 2006). 1.2 THAMES MITTEN CRABS River Thames Records: The first River Thames Chinese mitten crab record was a single specimen captured at Lots Road power station, Chelsea in 1935 (Harold, 1935). But it was over forty years before the next report of this exotic species from the Thames, when Ingle and Andrews (1976) recorded the capture of three crabs at West Thurrock power station and further sporadic sightings of the crab from the Thames were noted by Ingle (1986). Attrill and Thomas (1996a) suggested that the pattern of mitten crabs collected at West Thurrock Power Station had changed during with adults appearing more regularly until thirty individuals were captured on 13 November According to them this figure of thirty crabs was more than the mitten crabs caught in the previous seven years at the power station and suggested that downstream migration (first evidence in Thames) was occurring since all the females were ovigerous (see Figure 2). Clark and Rainbow (1997) and Clark et al. (1998) provided further evidence that the mitten crab population in the Thames appeared to have increased during the early 1990s and was continuing to rise. They used Environment Agency mitten crab records collected from the filter screens at West Thurrock ( when it closed) and Tilbury (opened in ) power stations. From a relatively constant background baseline the mitten crab captures at West Thurrock suddenly increased in 1992 and for the first three months of 1993 before the power station closed on 31 March of that year (see Figure 3). Large numbers of crabs were collected from Tilbury Power Station in 1994 and 1995 with at least as many in the first half of 1996 when collecting ceased (Figure 4). The West Thurrock data show an increase in 1992 that had been further enhanced and 8

9 Figure 1. Current distribution of UK Eriocheir sinensis. Courtesy of Marlin Figure 2. Number of mitten crabs recorded from surveys at West Thurrock Power Station from from Attrill and Thomas (1996a). Their data were aggregated to give number of crabs per quarter year (every three months). According to them the thirty crabs in the last quarter of 1992 suggested that downstream migration was occurring since all the females were ovigerous. 9

10 Figure 3. Numbers of Eriocheir sinensis collected at West Thurrock Power Station from 1976 to the 31 March 1993 (incomplete data hatched) when electricity generating ceased. Figure 4. Numbers of Eriocheir sinensis collected annually at Tilbury Power Station from October 1993 (incomplete data hatched) to June 1996 (incomplete data hatched). 10

11 maintained at Tilbury from 1993 to Figure 5 presents data in terms of the numbers of crabs collected in separate 4 h counting periods at Tilbury from 1994 to Data for more than one collection in a given month have been averaged to provide objectively comparable data. These data confirm the increase in crab numbers continued in The seasonal occurrence of crabs at Tilbury corroborates the view that crabs are migrating through the estuary. Peak numbers appear at Tilbury in May. The first juvenile was found in 1992 (Attrill and Thomas, 1996a) while Robbins et al. (2000) reported that high numbers of juveniles were found in the river suggesting that E. sinensis was established and successfully breeding. Recently, Gilbey et al. (2007, figure 1) sampled seven sites from Teddington Weir to Greenwich. They found juvenile crabs (Photograph 1) at all sites and their high numbers suggested a significant increase in the population size since previous studies (Atrill and Thomas, 1996a, b; Clark and Rainbow, 1997; Clark et al., 1998) River Thames distribution: Clark and Rainbow (1997) and Clark et al. (1998) also updated the distribution data in the Thames catchment. In 1996 they made an appeal to the general public in for mitten crab sightings and from 27 August to 15 November, 162 mitten crab reports were received. The updated distribution of the mitten crab in the Thames catchment is shown in Figure 6. At that time Eriocheir sinensis was known from as far west as the River Colne at Staines and was recorded from most of the Thames tributaries downstream of that point. In the east, the crab was been found in the Cray, Darent, Quaggy, Pool and Ravensbourne. In the northeast, sightings were common in the rivers Roding and Lee. The most northerly report of E. sinensis was in the River Lee at Enfield, some 15 km upstream of the River Thames. The survey also recorded E. sinensis in almost every tributary from Chelsea upstream to Chertsey. These include the Beverley Brook and the Rivers Wandle, Brent, Duke of Northumberland, Crane, Hogsmill, Longford, Ember, Mole and Ash. The furthest upstream record at Staines was ~ 65 km from Tilbury. However during the writing of this report the Museum received (13 October 2007) a record of a mitten crab caught by David Heller whilst fishing at Boveney Lock (just upstream of Windsor). This is now the furthest westward mitten crab record in the Thames River Thames burrows: There appears to be evidence of mitten crab burrows along some unprotected river banks of the Thames especially on Chiswick Eyot and along the banks of Syon Park, Middlesex. The Phragmites sp. bed on Chiswick Eyot has been particularly eroded away in recent years by the probable burrowing behaviour of crabs (see Photograph 2). According to Chris Dutton, Environment Agency (pers. comm.) the bank at Chiswick Eyot has receded ca. 6 metres. The burrows along the banks of Syon Park (see Photograph 3) were studied by Zucco (1999) Control of the Thames mitten crab population: Although the population in Thames had remained low from 1976 to the late 1980s, recent evidence suggests that mitten crab numbers have increased and continue to do so (Atrill and Thomas, 1996a, b; Clark and Rainbow, 1997; Clark et al., 1998; Gilbey et al. 2007). This sudden increase has potential environmental implications including the damage to unprotected river banks by the burrowing habitats of Eriocheir sinensis (Photographs 2, 3). To date there has been no attempt to control this invasive pest with a view to reducing its population size. One method of control may be the commercial 11

12 Figure 5. Monthly captures of Eriocheir sinensis from the intake screens at Tilbury Power Station (average number in 4 hour collections at 2 week intervals) from January 1994 to July Figure 6. Distribution of Eriocheir sinensis in the Thames catchment from information received between the 27 August 1996 and 15 November

13 exploitation of this species as it is considered to be a delicacy by South East Asia community while providing financial benefits to local fishermen. Also, as eels are now considered an endangered species, commercial exploitation of the mitten crab could provide Thames eel fishermen with an alternative fishery, while reducing their impact on eels Proposal: The population and density of Eriocheir sinensis in the River Thames should be investigated to determine whether commercial exploitation of the mitten crab is feasible, including the use of fishing gear (fyke nets versus baited pots) and to assess the crab s suitability for human consumption. The by-catch of fish caught in the traps should be identified with a view to assessing possible depletion of fish stocks while catching mitten crabs. 13

14 2. METHODS 2.1 Fishing gear: Trawling and dredging in the Thames on a commercial scale are not allowed, as such practices will damage the ecosystem and may be hazardous to shipping navigating the relatively narrow deep-water channel. Further they would also damage the product (the crab). Traditional fishing methods in the UK to catch crabs and lobsters have involved baited pots. However, Thames eel fishermen had already reported that they collect large numbers of mitten crabs in their fyke nets. Therefore this project will trial the trapping efficiency of baited pots versus fyke nets. 2.2 Thames sites: After discussions with Captain Christopher Mendoza, Harbour Master (Thames Upper), Port of London Authority (PLA) in early February 2005 four sites along the Thames were selected as permanent sampling stations. Site 1. West Thurrock Power Station water intake jetty: N, E. West Thurrock Power Station has now been demolished, but the water intake jetty still remains. As this was the site where a number of mitten crabs were captured during the late 1970s (see Ingle and Andrews, 1976; Ingle 1983) to 1990s (see Atrill and Thomas, 1996a, b; Clark and Rainbow, 1997; Clark et al., 1998), this area was selected as a regular sampling station (Photographs 4, 5). Nets and pots were set in one long line. Site 2. River Lee, N, E. In 1996 when the Natural History Museum (Clark and Rainbow, 1997; Clark et al. 1998) made an appeal to the general public for mitten crab sightings, many reports came from anglers fishing the River Lee (Figure 30), which suggested that this tributary was heavy populated. Consequently the entrance to Bow Creek where the River Lee joins the Thames was selected as a permanent trapping station (Photographs 6, 7). Nets and pots were set in one continuous line. Site 3. Lots Road Power Station, north bank, N, W; south bank, N, W. The Natural History Museum had captured over 350 mitten crabs from the intake screens of Lots Road Power Station, Chelsea (Robbins et al., 2000). Although the power station closed down on 21 October 2002, this part of the Thames was considered as a permanent collecting site for this project. Notes to Mariners were issued by PLA for a two year fishing period for two sites in the vicinity of Lots Road Power Station. The fyke nets were always set on the north bank and the pots on the south (Photographs 8, 9). Site 4. Richmond Lock. Downstream Twickenham Road Bridge, west buttress; N, W. The Harbour Master (Thames Upper) suggested two sites for trapping just upstream of Richmond Half Lock, downstream Richmond Railway Bridge, west buttress and downstream Twickenham Bridge, west buttress (Photographs 10, 11). The appropriate Notes to Mariners were issued for two years. However, the Environment Agency would not allow the project to set fyke nets in this area because of concerns about trapping and killing many freshwater fishes including large carp. Only baited pots were set in this area. 2.3 Environment Agency: Tom Cousins, Fisheries Technical Specialist, South London & Tidal Thames, approved the mitten crab programme and the suggested 14

15 four permanent sites on behalf of the Environment Agency (Figure 7). There were two main conditions, one no nets were to be seta at site 4 (Richmond Railway Bridge west buttress) because this may catch and kill too many fish over a period of 48 h, and otter guards were to be used in the fyke nets at site 3 (Lots Road Power Station) so that no large fish could be trapped. However the lifting of nets within 24 h was in error and should have read 48 h. 2.4 Fyke nets: The nets (Photograph 12) were the property of David Pearce (DP) and for 2005 he held eel licence No. th0003 fn 05. All licences are now controlled by Fisheries Admin Support Team, Environment Agency, PO box 32, Richard Fairclough House, Knutsford Road, Warrington, WA4 1HH. Catherine Mitchell was Admin Officer. The fyke nets of DP were tagged with nos th to th which covered 60 nets. One fleet of fyke nets comprised 4 double nets ca. 30 m long with ca. 9 m of rope at each end to attack anchors. The nets stood approximately 55 cms above the river-bed and were positioned in areas least likely to impact on navigation. Although the nets were set as close to the bank as possible, they always remained covered at low water. In order not to draw attention to the nets were not buoyed but marked using GPS. The nets were retrieved by use of a grapple. The Environment Agency requested that the fyke nets were set with otter guards at site 3, Lots Road, so as not to trap any large freshwater fish. 2.5 Pots: These were supplied by the Environment Agency (Photograph 13) and comprised four to a fleet. At sites 1 and 2 these were always attached to one end of the fyke nets. Initially flounder was used as bait, but this was later changed to herring because it was considered that its oil may attract the mitten crabs. The setting and recovery of the pots were the same as for the fyke nets. 2.6 Data collection: At each site crabs had their carapace width measured (Photograph 14), they were sexed (Photographs 29, 30) weighed, missing limbs counted and ovigerous crabs noted. The by-catch of fish was identified, and (with exception of the eels) fish body length were measured (Photograph 15). Physical measurements include water temperature, salinity, ph, dissolved O 2 and air temperature. These data were gathered using a YSI Multiparameter Environmental Instrument, model 556 with the probe sampling only the river surface. The scales used to weigh the crabs was an Ohaus FD series including and animal chip to dampen the scale for crab movement. A Draper expert digital caliper was used to take carapace width measurements. 2.7 Contaminant analysis: The project collaborated with London Port Health Authority under the guidance of Mr Jon Averns to analyse the crabs for trace metals and organic chemical contaminants including lead, cadmium, mercury, dioxin and polycyclic aromatic hydrocarbons. All crabs captured in the traps at sites 1-4 were placed in plastic bags, labelled, dated and put into a freezer ( 20 C) in the Zoology Department, NHM for dissection at a later date. 2.8 Sampling method: The boat used was an orange and white fast dory type name Bumble B, moored at Erith. Sites were sampled once a month during neap tides (if possible) starting on 7 November 2005 and finishing on 5 January 2007 to include 2 months overlap of data collection. 15

16 Figure 7. The Environment Agency letter of consent, which included seven conditions. 16

17 Figure 7. Continued. 17

18 2.9 Equipment: Scales, fish measuring boards, recording sheets, callipers, life jackets and spare cylinder, pens and pencils, labels, note book, digital camera, crab record sheets, Hydrodata Instrument (calibrate), plastic bags, cool box for CEFAS, food and drink, accident report forms, warm clothes including hat, suntan lotion Notify: Contact Environment Agency at least 24 hrs before fishing and Port of London Authority, Harbour Master for the upper river Sampling timetable Monday: Site 3 near to Lots Road Power Station, north bank, N, W; south bank, N, W. Set pots and nets WITH OTTER GUARDS. 1. Physical measurements Site 4 Richmond Lock, downstream Twickenham Bridge, west buttress; N, W; downstream Richmond Railway Bridge, west buttress; N, W. 1. Set pots only at this site. 2. Physical measurements Wednesday: Site 3 near to Lots Road Power Station, north bank, N, W; south bank, N, W. 1.Retrieve nets and pots and record fish and crab data a. At each site captured crabs are to have carapace width measured (see fig. 1), sexed, weighed, ovigerous crabs noted and missing limbs identified. b. Fish to be measured and identified. 2. Physical measurements include water temperature, salinity, ph, dissolved O 2 ; air temperature will be taken using a YSI Multiparameter Instrument. 3. Bag all captured crabs for contaminant analysis and take to the NHM for deep freezing. Site 4 Richmond Lock, downstream Twickenham Bridge, west buttress; N, W; downstream Richmond Railway Bridge, west buttress; N, W. 1 3 as above. Site 2, River Lee area, N, E 1. Set pots and nets 2. Physical measurements. Site 1, West Thurrock Power Station water intake jetty, N, E. 1. Set pots and nets 18

19 2. Physical measurements Friday: Site 1, West Thurrock Power Station water intake jetty, N, E. 1 3 as above. Site 2, River Lee area, N, E. 1 3 as above. Afterwards By-catch fish data to be sent to Tom Cousins, Environment Agency by following Wednesday Health & Safety: Life jackets were worn at all times by NHM staff and other attendees while working on Bumble B. A first aid kit was made available during fieldwork and mobile telephones were accessible for shore communications Microbiology of Mitten Crabs: Mitten crab samples were submitted for microbiological examination to the Health Protection Agency London Regional Food, Water & Environmental Microbiology Services Laboratory between February 2006 and January 2007 to determine the microbiological quality and assess the potential as a harvestable foodstuff. The crabs submitted were small which limited the range of tests and it was necessary to pool them so each sample consisted of five frozen crabs. Samples were examined for Enterobacteriaceae; Escherichia coli, Campylobacter, Salmonella and Vibrio spp. using microbiological testing methods based on ISO or other published methods. Identification of presumptive Vibrio species was carried out by the Health Protection Agency Reference Laboratory at the Centre for Infections Vibrio parahaemolyticus: During , 14 samples of Chinese mitten crabs and 5 Thames water samples were collected from various stations along the River Thames on an ad hoc basis for Sariqa Wagley (PhD student, CEFAS, Weymouth). She was developing methods for the Detection and Characterisation of Pathogenic Vibrio species in Seafood. The crab samples were captured in fyke nets set mainly on the south bank of the River, approximately opposite site 1. These nets were laid on Wednesday and hauled on Friday. About 10 large crabs were selected, placed live in a cool box containing two freezer packs and were couriered overnight to arrive at CEFAS, Weymouth Saturday morning for testing. The crabs were examined for total and potentially pathogenic V. parahaemolyticus using conventional and molecular based methods. Total V. parahaemolyticus in crab homogenates was enumerated using ISO8914 in a 3 3 most probable number (MPN) format and by direct plating onto thiosulphate citrate bile sucrose (TCBS) agar. PCR and nucleic acid hybridisation techniques were used to determine presence of the species markers, toxr and tlh (Kim et al., 1999, Taniguchi et al., 1985 and McCarthy et al., 1999) and the pathogenicity markers (tdh and trh) described previously by (Tada et al., 1992, McCarthy et al., 2000 and Nordstrom et al., 2006). Pulsed field gel electrophoresis (PFGE) was used to evaluate the genetic diversity among V. parahaemolyticus strains obtained and assess the relatedness of strains with known significant pathogenic isolates. For more information see Wagley et al. (2008). 19

20 2.15 NHM Field Personnel: Paul Clark (Crustacea Research Group), Patrick Campbell (Fish Section), Roberto Portela Miguez (Curator) and Brian Smith (Trace Metal Group). 20

21 3. POTS VERSUS NETS 3.1 Mitten crabs caught: From November 2005 to January 2007 a total of 539 crabs (Table 1) was caught, weighed, measured and sexed including noting of females in an ovigerous condition (Appendix 4). This figure may be considered low but fishing with nets at site 4, Twickenham Road Bridge, was not permitted. 226 females, of which 57 were ovigerous, were trapped compared 333 males. 3.2 Nets versus Pots: Fyke nets overwhelmingly proved to be the most efficient (Figure 8) method of catching mitten crabs with the pots trapping just 44 crabs (Photograph 16) in total. Bait was changed from flounder to herring because it was considered that the oil from the latter might attract more crabs and the size of the entry to the pots was increased in an attempt to improve pot efficiency without success. Furthermore the pots were considered to be fishing because at site 1 in November 2005 thirty eight Carcinus maenas (Linnaeus, 1758) were collected (Photograph 17). Another factor that may have reduced the efficiency of the pots was the loss of bait at some sites (Table 2). Eels were considered to be responsible for bait removal. 3.3 Nets at site 1: West Thurrock Power Station, caught the largest number of crabs, 288, included all 57 ovigerous crabs with the majority of crabs being caught at this site between May and August 2006, 170 (Figure 9). 3.4 Nets at site 2: River Lee, the nets caught 122 crabs, with no ovigerous crabs. Most crabs were caught during May 2006, 28 and 57 between August and October 2006 (Figure 10). 3.5 Nets at site 3: Lots Road Power Station, 85 crabs were in the nets, with no ovigerous specimens. Most crabs were caught between May and July 2006, 33 and September and October 2006, 28 (Figure 11). 3.6 Experimental traps: Trials of bait to use in pots is certainly considered to be an option including the development of pheromones to attract crabs. However a simpler and much cheaper option could be the development of crab only fyke nets constructed from 40 mm mesh or with a 40 mm brass ring sown into the trap thereby allowing eels to escape capture. 21

22 Table 1. A summary of mitten crab numbers trapped by pots and nets during a fishing period form November 2005 and January Female crabs Site 1 Site 2 Site 3 Site 4 Pots Nets Pots Nets Pots Nets Pots Total Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan

23 Table 1. Continued. Ovigerous crabs Site 1 Site 2 Site 3 Site 4 Pots Nets Pots Nets Pots Nets Pots Total Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan

24 Table 1. Continued. Male Crabs Site 1 Site 2 Site 3 Site 4 Pots Nets Pots Nets Pots Nets Pots Total Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Total

25 Figure 8. Pots versus nets, total number of mitten crab caught. Table 2. The fishing efficiency of pots was probably reduced due to bait removal at some sites. Month Site 1 Site 2 Site 3 Site 4 Nov Dec Jan Feb Mar Apr May pots Jun pot + Jul pots Aug Sep pots Oct pots - 2 pots Nov Dec Jan = bait present on retrieval of pots - = bait absent on retrieval of pots 25

26 Figure 9. Numbers of mitten crabs caught by fyke nets at site 1, West Thurrock Power Station. Figure 10. Numbers of mitten crabs caught by fyke nets at site 2, River Lee. 26

27 Figure 11. Numbers of mitten crabs caught by fyke nets at site 3, Lots Road Power Station. 27

28 4. FISH: THE BY-CATCH 4.1 List of Fish: Twenty species of fish were identified from the by-catch of mitten crab fishing (see Table 3). Table 3. A list of the twenty species of fish caught as the by-catch during mitten crab fishing from November 2005 to January Flounder, Platichthys flesus Linnaeus, 1758 Common eel, Anguilla anguilla (Linnaeus, 1758) Sole, Solea solea (Linnaeus, 1758) Whiting, Merlangius merlangus (Linnaeus, 1758) Bass, Dicentrarchus labax (Linnaeus, 1758) Sprat, Sprattus sprattus (Linnaeus, 1758) Five-bearded rockling, Ciliata mustela (Linnaeus, 1758) Smelt, Osmerus eperlanus (Linnaeus, 1758) Herring, Clupea harengus Linnaeus, 1758 Perch, Perca fluviatilis Linnaeus, 1758 Red gurnard, Aspitrigla cuculus (Linnaeus, 1758) Sand-smelt, Atherina presbyter Cuvier, 1829 Bib (pouting), Trisopterus luscus (Linnaeus, 1758) Transparent goby, Aphia minuta (Risso, 1810) Cod, Gadus morhua (Linnaeus, 1758) Pogge (Hooknose), Agonus cataphractus (Linnaeus, 1758) Tadpole-fish, Raniceps raninus (Linnaeus, 1758) Roach, Rutilus rutilus (Linnaeus, 1758) Common or freckled goby, Pomatoschistus minutus (Pallas, 1770) Dab, Limanda limanda (Linnaeus, 1758) 4.2 Mortalities: After a possible maximum of 48 hrs in the traps (nets or pots) the vast majority of fish caught were returned back to the river alive, but there were two exceptions. Of the 14 smelts, Osmerus eperlanus, captured only one survived to be returned to the river alive. This species is rarely found alive in fyke nets (experience and Dave Pearce pers. comm.). Furthermore, on Wednesday 7 December 2005, site 3. Near Lots Road Power Station, north bank, N, W, 8 fish species were caught in the nets at this site; including 6 flounder, Platichthys flesus, 1 common eel, Anguilla Anguilla and 1 bass Dicentrarchus labax. Most of these fish were dead, due possibly to lack of oxygen caused by the sheer weight of leaves trapped by the nets from the river from the autumn fall (Photographs 18, 19). 4.3 Fish data: A total of 2013 fish (Table 4) was trapped as by-catch while fishing for mitten crabs in the Thames from 9 November 2005 to 5 January 2007 (for raw data see Appendix 4). Of the total number, only 5 fish were caught in pots including 2 flounder, Platichthys flesus, 2 perch, Perca fluviatilis and 1 five-bearded rockling, Ciliata mustela. In comparison the fyke nets efficiently captured over two thousand fish during the same period, with the five most common caught fish being Anguilla anguilla, Platichthys flesus, Dicentrarchus labax, Solea solea and Merlangius merlangus (Table 5). 28

29 Table 4. A total of 2013 fish was trapped as by-catch while fishing for mitten crabs in the Thames from 9 November 2005 to 5 January FISH SPECIES NUMBER Flounder, Platichthys flesus 400 Common eel, Anguilla anguilla 1397 Soles, Solea solea 49 Whiting, Merlangius merlangus 20 Bass, Dicentrarchus labax 64 Sprat, Sprattus sprattus 5 Five-bearded rockling, Ciliata mustela 5 Smelt, Osmerus eperlanus 14 Herring, Clupea harengus 3 Perch, Perca fluviatilis 6 Red gurnard, Aspitrigla cuculus 2 Sand-smelt, Atherina presbyter 1 Bib (pouting), Trisopterus luscus 2 Transparent goby, Aphia minuta 1 Cod, Gadus morhua 28 Pogge (Hooknose), Agonus cataphractus 1 Tadpole-fish, Raniceps raninus 2 Roach, Rutilus rutilus 10 Common or freckled goby Pomatoschistus minutus 2 Dab, Limanda limanda 1 Table 5. The five most common fish species caught in fyke nets from November 2005 to January MONTH eel flounder bass sole whiting Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan TOTAL

30 4.4 Anguilla anguilla: During the fifteen month fishing period 1397 common eels were netted. Catches were low from December to March because winter temperatures, below 6-7 C, induce torpor and reduce eel activity to a minimum. Eel fishing in the Thames is not permitted upstream of Tower Bridge but starts downstream as temperatures rise in mid-later spring and finishes towards the end of autumn (see Figure 12). 4.5 Eel status: According to Aprahamian and Firth (2004) the trend of glass eel numbers entering European rivers since 1980 is downward, with lowest level of recruitment recorded in They state that a number of factors have been suggested for this decline including change in ocean climate, habitat loss, predation, turbine mortality, over-exploitation, parasites and pollution. The pollution in the Thames and its subsequent recovery have been well documented (see Harrison and Grant 1972 for example). Dave Pearce (pers. comm.) comes from a family of commercial fishermen. His great grandfather fished eels out the Thames from and had four to five boats working out of Erith. In 1920 his family stopped fishing eels in the Thames due to the pollution in the river. Eel fishing started again around 1980 and Dave started fishing full time in Today there are about 3-4 fishermen commercially exploiting eels in the Thames estuary. There is a new threat to the eels in the Thames. It comes in the form of a swimbladder parasite, Anguillicola crassus, and this is cause for concern (Aprahamian, 2004). This parasite causes mortalities in farmed and wild population in the presence of other stress factors (Kirk, 2003). It is a nematode originating from Asia. Norton et al. (2005) examined eels from the River Thames between 2000 and 2003 for this infection. Their data suggests that prevalence of infection in the Thames estuary was higher during than for the period (mean 48% vs. 10% respectively). In June and July 2006, Dave Pearce (pers. comm.) started to get some eel mortalities in his Thames storage facility as temperature rose and 0 2 levels fell. He put this down to a combination of stress and possible swimbladder infections by nematodes. He set up a freshwater storage tank at home using aerated tap water and mortalities ceased immediately. He has used the storage facility during the 2007 eel season and considers this to be a better way to hold his eels. He has not had any eel mortality using this system to date. 30

31 Figure 12. Net captures at sites 1-3 of the common eel, Anguilla anguilla plotted against Thames water temperature. 31

32 5. THAMES PHYSICAL DATA A YSI Multiparameter, model 556, Environmental Instrument was used to collect physical data from the Thames. All measurements were taken with the probe just below the surface of the river. The data gathered at sites 1-4 for temperature, salinity and dissolved O 2 are shown in Figures The highest temperatures at the four sites in the Thames occurred between July and September 2006 with the maximum being 22 C in August 2006 at site 4. At sites 3 and 4 these high temperatures coincided with a drop in the dissolved O 2 levels with the lowest being 30% and 43% at each site respectively. The site with the most fluctuating dissolved O 2 levels was site 2, the River Lee, at Bow Creek; the lowest being 28 recorded during July This poor quality water was almost certainly due to the Abbey Mills power station releasing effluent into the River Lee when the current sewage treatment works such as Beckton cannot cope with storm drainage. Every net haul contained a certain amount of rubbish including condoms and the components of sanitary towels (see Photograph 20). 32

33 Temperature Salinity Dissolved O 2 Figure 13. Thames physical data, site 1, West Thurrock Power Station water intake jetty: N, E. 33

34 Temperature Salinity Dissolved O 2 Figure 14. Thames physical data, site 2, River Lee, N, E. 34

35 Temperature Salinity Dissolved O 2 Figure 15. Thames physical data, site 3, Lots Road Power Station, north bank, N, W. 35

36 Temperature Salinity Dissolved O 2 Figure 16. Thames physical data, site 4, Richmond Lock. Downstream Twickenham Road Bridge, west buttress; N, W. 36

37 6. THAMES MITTEN CRAB FISHING 6.1 Seasonality: The ovaries and testis are considered to be the delicacy with regard to eating mitten crabs and it is these together with the hepatopancreas (the brown meat) that are consumed. The white meat in the walking legs, sternum and claws is of secondary importance, but consumed. The ripening ovary is particularly sought after and large females (Photograph 21) in the perfect condition command a high price in restaurants. Consequently, mitten crabs are only eaten during the migration period in the autumn when the gonads are ripening. Females appear to be popular at the beginning of the season and males (Photograph 22) later towards the end of the season. Once the females have spawned their eggs they are no longer of commercial value. 6.2 Ovigerous crabs: Ovigerous (egg bearing or berried) mitten crabs do not have any commercial value in the food market (Photograph 23). Andrews et al. (1981, Table 1) appear to have recorded the first berried mitten crab for the Thames taken from West Thurrock Power Station in January 1979, which incidentally was returned live to the river, and later in December of the same year. Ingle (1986) too recorded an ovigerous mitten crab from the Thames stating only, During 1979 a female mitten crab bearing eggs was caught at Southend-on-Sea. Attrill and Thomas (1996, Table 2) listed all records of Eriocheir sinensis from 1986 to 1993 in the Thames including notes on ovigerous crabs. Table 6 of the present study summaries their collection of berried crabs from the Thames Estuary between The earliest capture of an ovigerous specimen reported by Attrill and Thomas was August ( ) and their latest December ( ). From this present study, egg-bearing females (Photograph 23) were only collected at Site 1, West Thurrock Power Station. The present data show (Table 7) that crabs are carrying eggs between December one year and the following June, with a total of fifty seven ovigerous crabs being trapped during this period. Incidentally, most females (11 out 12) caught at Site 1 during December were ovigerous. However, field experience has suggested that some females become ovigerous in early October although Attrill and Thomas (1996) record a specimen from August (Table 6). All female mitten crabs found in the Thames Estuary during December 2005 were ovigerous (Dave Pearce pers. comm.). Of interest is that some females captured at Site 1 during June carried a significantly reduced numbers of eggs under the abdomen to the point that these were score as ½ in the ovigerous column of the field data sheets. This suggested that these were still in the process of hatching or that Eriocheir sinensis may hatch off zoeas in batches and not all at once. Ovigerous females can be found in the Thames Estuary towards the end of August through to June the following year. Hatching takes place sometime in early spring through to June the same year. 6.3 Life cycle: For a schematic mitten crab life cycle illustration see Figure 42. During the reproduction phase mitten crabs migrate down stream becoming sexually mature. After mating and spawning, the crabs over-wintered and returned to the estuary during the spring and summer when the eggs hatch off zoeas, after which, 37

38 Table 6 Ovigerous crabs recorded in the Thames by Attrill and Thomas1996. Date Site Notes West Thurrock Power station 1 ovigerous crab West Thurrock Power station 1 ovigerous crab West Thurrock Power station 1 ovigerous crab Crossness 1 ovigerous crab West Thurrock Power station 17 ovigerous crabs Table 7. Ovigerous mitten crabs were only collected at site 1, West Thurrock Power Station. Ovigerous crabs MONTHS POTS NETS Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan TOTAL

39 adults of both sexes die. Eriocheir sinensis is semelparous, in that it reproduces only once during its lifetime, after which it dies. This downstream/upstream crab migration can be illustrated by reinterpretating the old Tilbury data (Figure 17). The numbers of specimens captured and illustrated in red are the previous year s migrating crabs returning to estuary and are migrating upstream past Tilbury Power Station. The females will hatch off zoeas and both sexes eventually die. These mass deaths of the previous years migration are completed by July and August at the latest. The new downstream migration of sexually mature specimens is represented in blue and crabs start to pass Tilbury Power Station to eventually over winter around early September. This scenario appears to be supported by the net capture data from Site 1, West Thurrock Power Station upstream of Tilbury Power Station (Figure 18). Again the numbers of specimens captured and illustrated in red are the previous year s migrating crabs returning to estuary and the new down stream migration in blue. Ovigerous crabs were caught at this site as late as June 2007 together with fifty one males (Table 7). These crabs appeared to be lethargic compared to the crabs captured and Site 2 (River Lee) for the same month. Crabs at the River Lee site appeared much more aggressive and are considered to be starting their downstream migration (see Figure 19). The crabs at Site 1 are now believed to be in the process of dying off. This dying of mitten crabs is completed at Site 1 by the end of July or early August. Further evidence of this overwintering and movement of mitten crabs back into the Thames estuary is presented by the capture of male and female (Photograph 24) mitten crabs with barnacle settlement on their carapaces. These crabs were collected on 27 July 2006 at 'N 'E, from the south bank of the Thames approximately opposite Site 1. Two species of barnacle have settled: Balanus improvisus, a native species only found in estuaries and Elminus modestus, which can be found outside the estuary and is an invasive species from Australasia. In the Thames even the invasive species have settlements of invasive species! All the spat is fouling from 2006; in fact the carapace had developing cypris larvae settling on its dorsal surface. 6.4 Time to fish: Mitten crab fishing on the Thames could be undertaken for three months of the year from the start of September through to end of November to catch the annual downstream migration. 6.5 A place to fish: Port of London Authority bylaws do not allow commercial fishing upstream of Tower Bridge, therefore trapping of migrating mitten crabs can only be undertaken downstream of this landmark. From net capture data, mitten crab numbers started to increase from July at Bow Creek. As the River Lee is known to be particularly infested with mitten crabs commercial fish could start downstream of Trinity Buoy Wharf at the beginning of September each year. However, although fishing could start during August (Figure 20) at Site 2 (River Lee), there is a perceived problem around the Dartford crossing and downstream thereof. Early fishing at Site 1 (August) in this area could still capture the remnants of the previous year s migrating population that is in the process of dying off (Figure 21). This population is not a commercially viable product as capture of such crabs would not be suitable for sale as the gonads are spent and these specimens have a limited survival time. Sale of such crabs would not be productive for this fledgling industry. However, crab fishing season could start in August between the River Lee outlet and say 39

40 Figure 17. Monthly captures of Eriocheir sinensis from the intake screens at Tilbury Power Station (average number in 4 hour collections at 2 week intervals) from January 1994 to July Red columns represent mitten crabs including ovigerous females returning into the estuary and migrating upstream past Tilbury Power Station from East to West. Blue columns are mitten crabs migrating downstream past Tilbury Power Station from West to East. Figure 18. Mitten crabs captured in fyke nets at site 1, West Thurrock Power Station. Red line represents mitten crabs including ovigerous crabs returning into the estuary and migrating upstream beyond Tilbury Power Station from East to West. Blue line represents mitten crabs migrating downstream towards Tilbury Power Station from upstream, West to East. 40

41 Figure 19. Mitten crabs captured in fyke nets at site 2, River Lee. Red line represents resident Bow Creek population of mitten crabs (no ovigerous crabs present). Blue line represents mitten crabs migrating downstream towards site 1, West Thurrock Power Station, from upstream, West to East. Figure 20. Proposed mitten crab fishing period at site 2, River Lee area. Red columns represent resident Bow Creek population of mitten crabs (no ovigerous crabs present). Blue columns represent mitten crabs migrating downstream towards site 1, West Thurrock Power Station, from upstream, West to East. 41

42 Figure 21. Proposed mitten crab fishing period at site 1, West Thurrock Power Station area. Red columns represent mitten crabs including ovigerous crabs returning into the estuary and migrating upstream beyond Tilbury Power Station from East to West. Blue columns represent mitten crabs migrating downstream towards Tilbury Power Station from upstream West to East. Barking Power Station water outlet point at ' N, ' E. Then during September, when the dying off of the previous year s population is finished, mitten crab fishing could move downstream to include the Dartford crossing area and further east to Tilbury Power Station and possibly beyond. 6.6 Size of crab: While on NHM fieldwork in Singapore, Paul Clark met with Mr Low of Allswell Marketing, Singapore an importer of live seafood including the razor shell, Ensis siliqua (Linnaeus, 1758) from Scotland. Mr Low imports 300 kg of live Eriocheir sinensis from Yang Cheng Lake, US$ per kg including cost and freight. Crabs can sell in restaurants for US$40 per crab. The developing gonads are eaten. Gonads become ripe during the migratory period in late autumn. Sold by weight, a high price is fetched for females between g and males gms. Females are steamed for between 4-5 mins until gonads are soft and males between 10-15mins. There could be an alternative market where the gonads are used as an ingredient in other food products. A photograph taken recently (21 November 2007) in a Singapore Restaurant (Photograph 25) indicated the following size ranges and price of male mitten crabs for human consumption. Male hairy crab 250 g plus 65SGD reduced to SGD (22 GBP to 11GBP each), and g 45 SGD reduced to SGD. From November 2005 to January 2007 a total of 539 crabs was caught during this limited feasibility study and weighed. Weight of crabs will vary from initial wet weight to drying as water drains from the gill chambers and evaporates from exoskeleton for example. Using the Singapore Restaurant (Photograph 25) as a rough guide, the total number of Thames mitten crabs captured were divided into weight classes, <50g, >50g, >100g, >150g and >200g. Figure 22 shows the distribution of 42

43 Figure 22. Distribution of weight classes for all mitten crabs collected from November 2005 to January weight classes for all mitten crabs collected from November 2005 to January A total of 23 crabs was caught with weight greater than 100g including 2 crabs greater than 150g and 1 at 200g. Of the total number, 339 male crabs were caught (Figure 23): 13 had a weight greater than 100g including 1 crab greater than 150g and 200g. Ovigerous crabs have no commercial value, therefore only the non-ovigerous crabs (149 in total) were place into weight class (Figure 24). Five non-ovigerous crabs were collected with a weight greater that 100g, none were greater than 150g. This study suggests that a mitten crab fishing season could be permitted from the beginning of August to the end of November. A total of 144 mitten crabs was caught during the propose fishing period (Figure 25). Five males had a weight greater than 100g with only 1 greater than 200g (Figure 26) and 3 non-ovigerous females with a weight greater than 100g with none greater than 150g (Figure 27). Size of crab fished is basically down to market forces although from a number of excursions to See Woo Chinese supermarket at Charlton that imports mitten crabs from Holland under the name of big gate crabs the minimum size appears to be around 60 mm in carapace width. But grading of crabs is probably with larger crabs especially females commanding a higher price tag. Mitten crabs at See Woo were selling for 5 50p per lb or 14 per kilo (17 October 2006). Only crabs with a full set of claws and walking are acceptable for the restaurant market. 43

44 Figure 23. Distribution of weight classes for all male mitten crabs collected from November 2005 to January Figure 24. Distribution of weight classes for all non-ovigerous mitten crabs collected from November 2005 to January Ovigerous mitten crabs have no commercial value. 44

45 Figure 25. Distribution of weight classes for all mitten crabs collected during the proposed fishing period from August to end of November the same year. Figure 26. Distribution of weight classes for all male mitten crabs collected during the proposed fishing period from August to end of November the same year. 45

46 Figure 27. Distribution of weight classes for all non-ovigerous male mitten crabs collected during the proposed fishing period from August to end of November the same year. Ovigerous mitten crabs have no commercial value. During this study 539 crabs were caught and measured across the carapace width. Using the See Woo Chinese supermarket at Charlton as a rough guide, the total number of Thames mitten crabs captured were divided into width classes, <60mm, >60mm and >70mm. Figure 28 shows the distribution of width classes for all mitten crabs collected from November 2005 to January A total of 18 crabs was caught with width greater than 60mm including 2 crabs greater than 70mm. Of the total number, 339 male crabs were caught with 10 having a carapace width greater than 60mm (Figure 29). Only 149 non-ovigerous crabs were captured (Figure 30). Five non-ovigerous crabs were collected with a carapace width size of greater than 60mm. This study suggests that a mitten crab fishing season could be permitted from the beginning of August to the end of November. A total of 144 mitten crabs was caught during the propose fishing period (Figure 31). Four males had a width greater than 60mm with only 1 greater than 70mm (Figure 32) and 3 non-ovigerous females with a carapace width greater than 60mm (Figure 33). 6.7 After crab fishing: Traditionally mitten crabs are tied as per Photograph 25, 46, Figure 45, so that they do not loose legs fighting and weight by moving around. This may be commercially cheap to do in SE Asia but may be an expensive labour cost in UK. Furthermore after capture and prior to onward sale, crabs should be placed in clean water for around two days to clear mud from the gills and exoskeleton. 6.8 Capture of eels: There is no point in setting up a mitten crab fishing industry if this activity is going to deplete the Thames eel (Anguilla anguilla) population downstream from below the River Lee. Consequently consideration must be given to the numbers of eels trapped during the proposed mitten crab fishing period at sites 1 and 2 (see Figure 34 and 35 respectively). At site 1 from September to November 46

47 2005, 35 eels were caught and at site 2 from August to November 2005 the number was 273. Therefore the total number of eels trapped by fyke nets set only 7 times for approximately a 48 h period was 308. To allow the eels to escape, crab fyke nets could be made from a 40 mm mesh size or have a 40 mm brass eye sown into the trap. Figure 28. Distribution of width classes for all mitten crabs collected from November 2005 to January Figure 29. Distribution of width classes for all male mitten crabs collected from November 2005 to January

48 Figure 30. Distribution of width classes for all non-ovigerous mitten crabs collected from November 2005 to January Ovigerous mitten crabs have no commercial value. Figure 31. Distribution of width classes for all mitten crabs collected during the proposed fishing period from August to end of November the same year. 48

49 Figure 32. Distribution of width classes for all male mitten crabs collected during the proposed fishing period from August to end of November the same year. Figure 33. Distribution of width classes for all non-ovigerous male mitten crabs collected during the proposed fishing period from August to end of November the same year. Ovigerous mitten crabs have no commercial value. 49

50 Figure 34. Number of common eels captured at site 1, West Thurrock Power Station. Figure 35. Number of common eels captured at site 2, River Lee. 50

51 7. THAMES MITTEN CRABS: FIT FOR HUMAN CONSUMPTION? 7.1 European mitten crab fishing: Mitten crab fishing is already in progress on the continental mainland of Europe being undertaken in Germany and Holland. For crab fishing in Germany see:- (1) Go to the middle of the page - you see a fisherman in his boat. The paragraph below says that he catches more crabs than eels in the Laacher Lake (NW Germany). (2) E7E3DB4B6EB371A20CF41BC5A9~ATpl~Ecommon~Scontent.html Go to second last paragraph - it says that several tonnes of crabs are marketed. (3) ets.html Fisherman in Elbe near Hamburg catches crabs. On 27 May 2005, Fishing News (Figure 36) carried a story: Dutch fisheries snap up Chinese crabs. This article finished by stating that 22 tonnes were supplied to the market at an average price of 3.80 kg. As far as this study is aware, there has been no attempt by the EU or the health authorities in Germany and Holland to investigate the suitability of this product for human consumption. 7.2 London Port Health Authority: The City of London is responsible as the London Port Health Authority (LPHA) for enforcing public health related legislation within a district which extends for 150 kilometres from Teddington Lock to the outer limits of the Thames Estuary (Figure 37). It is the health authority for the tidal Thames. The Authority, the largest in the UK, has been in existence since 1872 and is part of the City of London's Department of Environmental Services. The LPHA monitors the harvesting of the cockles, oysters, mussels and other bivalve molluscs, to ensure that they meet the appropriate standards for processing in order to prevent illness to consumers. Consequently, before the mitten crab population can be commercially exploited in the Thames, the LPHA must be satisfied that the product is suitable for human consumption and meets similar health standards as those set down for shellfish. 7.3 Lung fluke, Paragonimus westermani: LPHA has taken a pro-active interest in the possibility of commercially exploiting the Thames mitten crab population. For certain, a mitten crab population infested with the lung fluke parasite would present a potential health hazard to humans. Therefore LPHA commissioned a project to perform a seasonal parasitological and histopathological survey to address the potential for Eriocheir sinensis from the Thames Estuary to be infected by 51

52 Figure 36. The Fishing News, 27 May 2005, Dutch fisheries snap up Chinese crabs. Figure 37. London Port Health Authority district, which extends for 150 kilometres from Teddington Lock to the outer limits of the Thames Estuary. 52