Interim Report. J. R. Ellis, G. Burt and L. Cox. Cefas, Lowestoft

Transcription

1 Fisheries Science Partnership: 2007/08 Interim Report Programme 19: Thames Ray Tagging and Survival Prepared by: J. R. Ellis, G. Burt and L. Cox Cefas, Lowestoft February 2008

2 Summary This Fisheries Science Partnership project was developed to estimate the longerterm survivorship and movements of thornback rays in the southern North Sea, using traditional tagging methods. Thornback ray is the dominant skate species taken in targeted and mixed fisheries in this area, and is taken in various trawl, line and gillnet fisheries. Five inshore vessels were used in the study, with seven trips planned in order to examine the catches and survivorship following capture by small otter trawlers, gillnetters and linefishing vessels. A variety of skate and ray species are marketed as skate, and commercial landings data are combined for all species. The lack of species-specific landings data has hampered assessment of the status of individual stocks and species. Some of the larger-bodied skate species are now of conservation concern. The main objectives of this study were to (1) examine the species, sex and size composition of skates (Rajidae) taken in inshore fisheries in the southern North Sea, (2) assess the survivorship of commercially caught fish and (3) undertake a tag-and-release programme for thornback ray Raja clavata. Due to the size of the vessels involved in the fishery and the fishing locations, the utility of examining short-term survival using holding tanks was limited, although such studies have been undertaken during complementary studies in the Bristol Channel. A visual assessment of the health state of fishes caught under commercial conditions was made on board, with the majority of fish subsequently tagged with a Petersen disc and returned. Given that the trawlers and liners have short tow durations/soak times, most fish were in a good state of health. Studies with gillnet used 24 h soak times (as opposed to the commercial 48 h soak time), as it was felt that mortality would be very high with longer soak times. This report summarises data collected during studies on the inshore trawler Harvester. 1

3 Introduction The Government announced in January 2003 a package of funding for the industry, which included 1 million in 2003/2004 for fishers and scientists to work in partnership. Following the announcement, the NFFO and Cefas developed a programme to improve knowledge of our fish stocks. A further 1m was provided for projects in the following financial year, then another 1m per year guaranteed for each of the three financial years 2005/ /2008. The objectives of the Fisheries Science Partnership (FSP) programme are to build relationships between UK fishers and scientists and to involve fishers in the co-commissioning of science. This study examined the distribution, species and size composition of skates (Rajidae) taken in inshore fisheries operating in the Greater Thames Estuary, an area of regional importance to thornback ray Raja clavata, which is one of the most commercially important skate species in UK waters. Within the UK, the term skate is generally used for those species with an elongated snout, and the term ray used for species with shorter snouts. However, most species are marketed as skate. Skates of are of great commercial importance in Europe. In recent years, the total weight of the 7 8 most commonly landed skates in UK ports have been t per year (Defra, Landings statistics), worth an estimated 3 million. In all, 13 skate species have been recorded from UK shelf seas (Wheeler et al., 2004), with another dozen species occurring in deep-water west of the British Isles. The most abundant skate species in inshore waters of England are thornback ray Raja clavata, blonde ray Raja brachyura and spotted ray Raja montagui, and these species all occur in the southern North Sea and eastern English Channel. Smalleyed ray Raja microocellata and undulate ray Raja undulata are most frequently found in the Bristol Channel and English Channel respectively, although they have been reported from the southern North Sea. Farther offshore, in the central and northern North Sea and Celtic Sea, other skates such as cuckoo ray Leucoraja naevus, shagreen ray L. fullonica and common skate Dipturus batis become more frequent. Starry ray Amblyraja radiata is also abundant in the northern and central North Sea, but is of little commercial importance because of its small size. Objectives 1) To examine the species, sex and size composition of skates (Rajidae) taken in inshore fisheries in the southern North Sea. 2) To assess the health state/survivorship of commercially caught fish. 3) To undertake a tag-and-release programme for thornback ray Raja clavata. 2

4 Methods Over the course of the whole project, five vessels (and seven trips) were to be used to gauge differences between the various gears used by the inshore fleet and to cover wider parts of the southern North Sea. During these trips, the gears and fishing methods were described. With the exception of gillnets (which were usually fished for 24 h and not 48 h), gear deployments (tow length, soak time) were of commercial duration. For each gear deployment, the position (latitude, longitude, depth and times) were recorded. Once the catch was made, the total length (L, to the cm below), sex (Figure 1) and health state of all skates (1: lively, 2: sluggish; or dead) were recorded. The maturity of male skates (Figure 2) was also recorded using visual inspection of clasper length (see Table 1). Although the maturity of female skates could not be noted (it requires examination of the gonads), those that were exuding egg cases (stage D) could be noted. If a skate had a large part of the end of the tail missing, then disc width (D) was recorded, and the total length estimated using the length-width relationship (D = L 0.248) (See Annex 1 for further information on converting length to width of thornback rays) Skates were tagged using numbered Petersen discs (Figure 3). The main species of interest was thornback ray, although during the wider project, small numbers of spotted ray and smoothhound were also tagged. The tags were placed in the centre of the wing, because if attached near the margin of the wing they can be snagged on fishing nets and pulled out. Smaller skates (<30 35 cm) were generally not tagged, because allowing sufficient space for the growth of the fish would likely result in the tag snagging. Due to the size of the vessels involved in the fishery and the fishing locations, the utility of examining short-term survival using holding tanks was limited, although such studies have been undertaken during complementary studies in the Bristol Channel (Catchpole et al., 2007). The numbers of selected commercial fish were also recorded where time permitted, but these data are not included in this mini-report. Vessel and gear This report summarises work undertaken on the Harvester, an inshore trawler operating out of West Mersea. The report from this trip is given in Annex 2. Work was undertaken in the period June 2007, and used a twin rig trawl. In all, 33 stations were fished successfully. 3

5 Figure 1: Ventral surfaces of male (with claspers, left) and female (without claspers, right) thornback rays Figure 2: Ventral surfaces of immature (stage A, left) and mature (stage C, right) male thornback rays 4

6 Results (a) Species composition of skates The only skate species recorded during this survey was thornback ray Raja clavata (n = 937), although occasional specimens of blonde ray Raja brachyura and spotted ray Raja montagui were recorded from other surveys in the area. Other elasmobranchs observed were lesser-spotted dogfish Scyliorhinus canicula and smoothhounds Mustelus spp. (b) Sex ratio and size distribution The length distributions of thornback rays caught were similar for both sexes, ranging from cm and cm for females and males, respectively (Figure 4). A wide length range was recorded, with peaks in the length-frequency at cm and cm, although there are no associated age data. Slightly more female thornback rays were caught than males (n = 481 and 456 respectively). There were no differences in the sex ratio for the smaller length groups, although the largest size class was comprised of significantly more females (Table 2). (c) Spatial distribution and relative abundance Thornback ray was common throughout the area surveyed with smaller fish more abundant in the shallower north-western part of the survey area, and larger fish more common at the more offshore sites of the survey area (Figure 5). (d) Health state Although no data on short-term survival could be collected during this survey, the health state of each fish was recorded on a qualitative scale (lively, sluggish, dead). Most trawl stations were of commercial tow duration for this vessel. Of the 937 thornback rays caught, 591 (63.1%) were considered lively at the point of tagging, 323 fish (34.5%) were considered sluggish and 22 individuals (2.3%) were considered dead. Health state was unrecorded for one fish. Visual assessment of health state suggested that the two larger size categories (50 74 cm and 75 cm) were in better condition (74.1% lively; 25.5% sluggish; 0.2% dead) than fish less than 50 cm in length (51.8% lively; 43.6% sluggish; 4.5% dead). 5

7 Figure 3: Thornback ray tagged with Petersen disc Figure 4: Size distributions of male and female thornback ray captured in the trawl survey Harvester (Trawl) Numbers Total length Female Male 6

8 Figure 5: Numbers of thornback ray caught in the survey by length group, <50 cm (top), cm (middle) and 75 cm (bottom). Maximum bubble sizes are 77, 72 and 15 fish respectively. 7

9 Figure 6: Release (+) and recapture (red triangle) positions of thornback ray. 8

10 (e) Tag releases and recaptures During the course of this survey, 541 elasmobranchs were tagged and released, including 517 thornback rays and 24 smoothhounds. The numbers tagged and released by sex and length class are summarized in Table 3. As of 16 January 2008, 41 (7.9%) of the thornback rays tagged and released during this survey have been recaptured and their details returned (Figure 6). Summary 1. The inshore trawl fishery in the Greater Thames Estuary can capture large numbers of thornback ray, which is by far the predominant skate species in the area. A wide length range of thornback ray was observed, ranging from recently hatched fish (14 cm length) to mature fish of up to 89 cm in length. Most of the larger thornback rays taken were female, although it is unclear whether this skewed sex ratio occurs at other times of the year. 2. Visual inspection indicated that most of the thornback rays caught were healthy (63% lively, 34.5% sluggish), with about 2% dead. As the smallest juveniles were not tagged, it will not be possible to examine the survivorship of this life-history stage. 3. To date, 7.9% of the thornback rays tagged and released by this survey have been recaptured. Acknowledgements We thank the skipper and crew of the Harvester for their assistance in undertaking this survey, as well as all the fishermen who reported tag returns. References Catchpole, T.L., Enever, R. and Doran, S. (2007). Bristol Channel ray survival. Cefas, Lowestoft, Fisheries Science Partnership Report 21, 15 pp. (available at Wheeler, A.C. Merrett, N.R. and Quigley, D.T.G. (2004). Additional records and notes for Wheeler's (1992) List of the Common and Scientific Names of Fishes of the British Isles. Journal of Fish Biology 65 (Supplement B),

11 Table 1: Maturity stages for male skates Stage State Description A Immature Claspers undeveloped, shorter than extreme tips of posterior margin of pelvic fin. B Maturing Claspers just longer than posterior margin of pelvic fin, their tips more structured, but the claspers are soft and flexible and the cartilaginous elements are not hardened. C Fully mature Claspers longer than posterior margin of pelvic fin, cartilaginous elements hardened and claspers stiff (can be soft at some times of the year). D Active Clasper reddish and swollen, sperm present in clasper groove, or flows if pressure exerted on cloaca. Table 2: Sex ratio of thornback rays by length class Length category M F Ratio Significance < 50 cm : cm : cm :5.7 P < Table 3: Number of elasmobranchs tagged and released by sex and (a) species and (b) size class of thornback ray. (a) Species Sex M F U Total Thornback ray Smoothhounds (b) Size class Sex M F Total < 50 cm cm cm Total

12 Annex 1: Length-width relationship for thornback ray Fish biologists tend to examine most fish species in terms of their total length, although many fishermen (and some fisheries regulations) consider the width of skates. The relationships between the length (L) and width (D) of thornback ray (see Figure below) are given by the equations: D = L and L = D (Data as collected during Cefas beam trawl surveys in the Irish Sea) Disc width (cm) Total length (cm) 11

13 Annex 2: Trip Report Vessel: Harvester (CK 137) Cefas staff: G Burt, L Cox Dates: June 2007 Gear: Twin rig otter trawl Cefas staff travelled to West Mersea on 19 June to meet the skippers of the FV Harvester and FV Jessica M in order to discuss the content of the survey, prior to its commencement the following day. It was agreed that both vessels would steam to the same fishing grounds each day where they would normally fish for thornback ray. These grounds were within a 23 mile radius of West Mersea, which could be reached within about a three hour steam. This would ensure that there would be good spatial coverage of the fishing grounds, but as a consequence the number of achievable hauls per day would be compromised and the more productive grounds would not be fished repeatedly. The FV Harvester was rigged with a twin rig otter trawl, comprising of two 6 fathom 80 mm mesh nets, fitted with a 90 mm square mesh panel plus an additional 160 mm square mesh, 15 meshes deep behind the headline, and a 3 inch rubber disc ground rope. FV Harvester normally undertakes 48 hr fishing trips using the above gear to target thornback ray during the day and night and sole during the night or when the water holds colour. The soak times were minutess, the time the skipper would tow for under normal commercial operations. FV Harvester carried an 8 ft draught which meant that this would restrict fishing in very shallow waters, which some of the other vessels were able to exploit. All tows were conducted during daylight hours. FV Harvester left port at 04:30 (BST) on 20 June at high water. The vessel returned to port some 12 hours later at high tide, a pattern that continued for the next five days until 24 June when the fishing was called off due to northerly force 8 winds forecast for the following day. Fishing resumed on the 27 June and the FV Harvester left port at 01:00. It was decided that the vessels would remain relatively inshore due to moderate to rough conditions further offshore. The vessel steamed to fish in the Wallet, a known fishing ground for thornback ray, but not a ground as reliable for good catches of thornback rays as the grounds further offshore. On the final day both Cefas staff and the skippers agreed that as good weather could not be relied upon, both vessels would remain inshore and fish where juveniles would be caught. FV Harvester returned to port at 10:00 on 28 June, after which staff returned to Lowestoft. Whilst at sea the sea state varied between calm to rough. Thornback rays and other elasmobranchs to be tagged were processed as soon as possible once the net was onboard. When larger numbers of fish were caught, fish for tagging were held in a tank of seawater, which was replenished with fresh seawater between hauls. Fish were released into the sea as soon as they were tagged either whilst towing or steaming and the release position documented as the previous haul position. None of released tagged fish were re-caught whilst towing. An average of five hauls per day were completed and over the duration of the survey a total of 33 stations were fished. On two of the stations, the starboard net came fast and suffered damage, which was repaired and did not compromise the completion of the survey. A total of 937 thornback rays were caught, of which 541 were tagged and released. Thornback ray was one of the predominant species caught and the skipper commented that the catch rates were generally to be as expected, bearing in mind the design of the survey, although it was mentioned that catches had been greater two weeks prior to the survey. The vessel was in excellent order and Cefas staff thank the skipper and crew for their constructive support and help throughout the survey. 12

14 Annex 3: Comments from skipper After reading through the report, I am happy to agree the findings, as with all surveys, it s true to say results are all about "on the day". That said, it is worth noting that a week prior to the survey, my boat had 150 stone of good mixed year classes of thornback ray, in two 1 hour tows on the edge of the Black Deep, and again some two weeks after the survey, I had 180 stone of the same for 3 one hour tows in the same area. My own conclusion is that there is a healthy local stock. What might be a good idea is to consider some form of technical measures for those fisheries targeting the inshore breeding grounds in the spring, as these could have impacts on the skate stocks, with large numbers of females being taken. 13