Lydia Kapapa, Sihaba Ramadhani, Bigeyo Kuboja, Patroba Matiku, Juma Kangwe and Joseph Sululu.

Transcription

1 Assessment of demersal fish stock in the SWIOFP countries: The technical report for shallow water demersal fish trawl survey in Tanzania 13th 30th August Compile by: Baraka Kuguru and Robert Kayanda Contributors: Mathias M. Igulu, Silas Mathew, Muhaji Chande, Said Mgeleka, Lydia Kapapa, Sihaba Ramadhani, Bigeyo Kuboja, Patroba Matiku, Juma Kangwe and Joseph Sululu. 1. Introduction Demersal fishes constitute the largest category of reported fish landings by SWIOFP countries. The WIO demersal fish catches comprise almost 600 fish species, about which very little is known of their fishery. There is a need of collaborative management intervention to investigate how many stocks or populations exist. An extensive gap analysis prioritization process was undertaken by SWIOFP participants in Component 3, and a final list of four priority demersal fish species (Otolithes ruber, Trichurus repturus, and Pomadasys maculatum) was selected to be investigated. The widespread and occurrence of these species strongly suggest that they are shared stocks among the SWIO countries. None of the relevant fisheries are managed on a collective regional level. Most countries that exploit marine fish stocks conduct regular stock monitoring programmes which provide data on the current status of the fish stocks. These monitoring programmes aim at understanding and altimetry balancing the levels of fishing efforts to the harvestable surplus stock. Such programmes are limited or inconsistence in most of WIO countries due to luck of funds and technical knows how. The SWIOFP demersal fish priority species have characteristic of high productivity (i.e. intrinsic biological attributes) however vulnerable for trawl fishery. The objective of this survey was to determine the status of demersal fishes in coastal waters of Tanzania. Specifically the survey assessed species composition, distribution and population structure of the demersal fish stock in 3 fishing zones along the coast of Tanzania; collect genetic samples of prioritized demersal fish species; identify and quantify non-fish by-catch (for Component 2 and 5); and Measure and record data of physico-chemical parameters of sea water at each station

2 2. Material and methods Study area The study covered three geographical areas along the coast of Tanzania (Fig1). The demersal fish trawl SWIOFP survey No TZN2011C311 took place between 13 th of August 2011 and 29 th August The area covered were Pemba and Zanzibar channel (Fig.2a &b) north Mafia Channel Fig 2c&d and south Mafia channel Kilwa- Jaja fishing grounds. The estimated area covered for the survey was 11,280 km 2 for Zone 1 (Bagamoyo, Pangani, Tanga, Unguja and Pemba), 4,349 km 2 for Zone 2 (Kisiju & Mafia) and 9,372 km 2 for Zone 3 (Jaja & Kilwa), making a total area covered to be 25,001 km 2. Figure 1 below shows the position of the sampling stations.

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4 Figure 1 map showing fishing area (zone 1, 2, and 3)

5 Data collection The study took place in the coastal waters of Tanzania. In order to achieve the above objectives, we assessed fish abundance indices, population structure indices and environmental parameters. (i)abundance indices were determined by using swept area and acoustic methods; (ii) Biometric indices were assessed using measuring boards and weighing scales; (iii) Physiochemical properties of water were assessed by using CTD, and water sample collection for nutrients analysis (Niskin bottle). These parameters were chosen because they explain patterns of demersal fish stock abundance and distribution. Sampling protocol The RV Mafunzo surveyed along the coast of Tanzania and covered the same three geographical areas covered during the survey by R/V Dr Fridtjof Nansen along the Tanzania coast ( ) and survey by Myklevoll (1984) along Zanzibar channel (; Zone 1-; Zone 2- and ; Zone 3 Figure 1). Sampling was done using Standard Operating Procedure by randomly carrying out sampling in defined tracks within the fishing blocks in pre-determined stations. The Survey was stratified by depth (principally) covering the bathymetric range between 10m 150m. Biological data and samples were collected to assess the basic biology of priority species. Tissue samples were collected for genetic studies to determine stocks abundance and structure in relation to nation jurisdictions (i.e. does a stock belong to a specific country or is it shared between 2 or more countries in the region?). Specimens of selected species were collected on request and preserved (frozen/formalin) for reference collection or for processing at shore based laboratories. Retained and discarded by-catches were identified and quantified, biological data and samples were collected as per sampling protocols provided by Component 3 (demersal fish) TZA2011C311. Biodiversity impacts of the fisheries were quantified as per protocols provided by Component 5. Environmental data (water temperature, salinity and density and dissolved oxygen) will be collected through the use of a CTD to explain patterns in distribution / abundance. Sampling design strategy was done using Standard Operating Procedure by randomly carrying out sampling in defined tracks within the fishing blocks in pre-determined stations. Fishing zones 1-3 were covered as major sampling blocks in which random sampling was carried out within 10 x 10 nautical miles squares. The sampling points chosen were among those covered by Dr. Fridjoft of Nansen and managed to be reached by M.V. Mafunzo. The guide to the selection of the strata was; the size of the sampled area, the availability of resources, and the previous information obtained from the area. Sampling period lasted for fifteen days. M.V. Mafunzo from Mbegani Fisheries Development Centre was used for data collection. The vessel is a stern trawler with 380 HP, overall length 22 m and gross registered tonnage of 115 tons. The trawling speed was between knots while a cruising speed was 8 knots. It has a high keel that enables it to fish and ply in deep waters in addition to its capacity to fish inshore. The vessel possessed other navigational aids equipments such as autopilot, GPS, echo sounder, radar and thermometer. A trawl net with 40 mm mesh size codend and head rope length of 33.5 m was used during fishing operations.

6 At each station, before starting trawling, environmental data were collected. Surface water sample was collected using a Niskin bottle for quantification of the chemicals/ nutrients and biological information such as phytoplankton, Chl a and salinity. CTD was used to collect environmental information such as Sea surface temperature, Oxygen, conductivity, and depth profiles. Other environmental information collected at the start of each trawl were 1) wind strength and direction, 2) sea condition, 3) current speed estimates, 4) transparency using a secchi disc, 5) depth, and bottom-type. Trawling was operated only during the day time (6h00 to 18h00), at speed of knots for a duration of 1 hr. After one hour trawling, the net was hauled from water then all unwanted debris and plant materials were first removed. All large sized fish were sorted and placed in boxes/buckets and later included in the sample. The remaining catch was then divided into equal proportions or portions (e.g. 2, 4, 8 etc) depending on the catch size and one proportion or portion was taken as sub sample. The number of proportions or portions was used as a raising factor on the calculation of total catch. Sometimes the total catch was very small therefore the whole catch was taken as a sample. The total weight of the catch was obtained by adding weight of large fish (from 1 Kg and above) and weight of small fish (after multiplying by the raising factor). Weight of the sample was obtained by adding weight of big fish and the weight of the sub sample. The samples were sorted to the lowest possible taxonomic level using FAO species identification keys, and then weight and number of individuals of each species were recorded. The priority species were selected and examined for biological parameters such as sex and maturity stages. Other measurements such as total length (TL) ( 0.1 cm), and weight of individuals ( 0.1g) and length-frequency for each category of species in the sub sample were also recorded. The different types of fish species encountered were collected and preserved as sample specimen Population structure indices Genetics sample collection Fish and prawn specimens were collected from three fishing zones, Indian Ocean, Tanzania, using bottom trawl net. Fin clips and muscle tissues (tail) from fresh-caught fish and prawns respectively were preserved in 95% ethanol in 1.5 ml vials for molecular analysis. After the research cruise, the old ethanol was replaced by new ones and stored in freezer at -20 C. Photographs were taken for each individual of fish and prawns retained for molecular analysis. Mainly prawn specimens were fixed in 40% formalin as a voucher and for morphological analysis. Physiochemical parameters A study on interaction of both physical and chemical properties of water plays a significant role in the composition, distribution and population structure of dermasal fish stock. Apart from this, it also gives an insight into the relationship between an organism and their environment. The physico-chemical study helps in understanding the structure and function of a particular water body in relation to its inhabitants. Nutrients have direct effects on biological productivity (primary productivity) and indirectly to fisheries through secondary productivity. The proper balance of physical, chemical, and biological parameters in water bodies is an essential ingredient for successful production of aquatic living resources.

7 This study was aimed at determining various environmental and oceanographic factors that influence the composition, distribution and population structure of demersal fishery in the coastal waters of Tanzania. The factors evaluated included the water temperature, conductivity dissolved oxygen, Total phosphorus, Total nitrogen, Dissolved phosphorus, and Nitrate-Nitrogen Data processing Species composition Determination of species composition was done in order to identifying the potential of the demersal fish species specifically the priority species and their population distribution. The percentage species composition by number of fish, prawn, and other by-catch resources caught were calculated and recorded in all there surveyed zones. All captured data were geo-referenced and were stored in an excel database. Spatial distribution of fish species Spatial distribution of fish species according to depth was considered to describe the percent contribution of the species at different depth range. Spatial distribution was also used to describe the bathymetric distribution of different fish catches observed at different depth range. Length weight relationships The length weight relationship is useful in fishery assessments for predicting weights from the more easily measured lengths.the length weight relationship of the dominant fish species was established using the following formula; W = al b Where: W = individual wet weight, L = total length of individual, a and b are constant values to be determined from log transformed values of length and weight, i.e. log W = log a + b log L, via least square linear regression (Zar, 1984). The total length (cm) of each fish was taken from the tip of the snout (mouth closed) to the extended tip of the caudal fin using a measuring board. Body weight was measured to the nearest gram using a weighing balance. Size at 50% Maturity Size at 50% maturity for the priority species was determined using maturity ogives. To obtain the length at 50% maturity, the length of each individual in the sample was measured and the maturity of each individual assessed. The percentage of mature individuals in each size class is described with a logistic curve as a function of the length of the fish. For establishing the length at 50% maturity, a coding of maturity stages with only 2 stages (immature, mature) sufficed. Determination of catches, fishing effort and catch per unit effort Catch rate is an indices of abundance which is used as a parameter to evaluate the amount (Kg) of resource exploited per unit effort (time, fishers, gear). Catch per unit effort/ catch rate was obtained by dividing total catch (Kg) obtained in each fishing station by the fishing time (60 minutes, i.e., one hour). x CPUE n Where x = total catch in each station (haul) and n= number of station per zone Biomass estimation

8 Biomass was estimated by using swept area method as described by Sparre and Venema (1998). The data collected from current survey was used to make estimates Cw/ a of the total biomass of the area using the following formula: B * A Where: q Cw = catch weight, A= total area under investigation, a=area swept by trawl, q = catchability coefficient, The value of q was considered 0.5 (Pauly, 1980) The swept area a was given by: a D*hr * 2 and D V * t. Where: V= velocity of the trawl over the ground when trawling (km/hr), t = time spent trawling (hr), hr = length of the head rope which is also equal to the width of the path swept by trawl (km) From the recorded catch in weight of a haul and the calculated swept area, the catch weight per area was calculated using formula Cw 2 CPUA kg/ km a Having mean catch per unit area ( Cw/ a) of all hauls then the average biomass per unit area b was estimated using the following formula: Physiochemical parameters Water samples for nutrients were drawn using a 4 L capacity niskin bottle, filtered through a 0.45 µm GF/C filter paper. Filtered water and filter papers were separately and safely stored in freezers pending to laboratory analysis. Salinity, temperature, nitrate saturations, dissolved oxygen and Fluorescence were measured using a CTD (Seabird Electronics, SeaCat SBE 19 Plus). The CTD was lowered into the sea using a winch from the shady side of the vessel. The standard procedures as described by the manufacturer were observed all the time when the CTD was lowered into the sea. In the laboratory, soluble reactive phosphorus (orthophosphate) was determined using the ammonium molybdate method (Wetzel and Likens, 2001). Determination of Nitrate-Nitrogen was done using the Cadmium reduction method (APHA, 2000). Physico-chemical data were uploaded from the CTD into MS-Excel and averaged over 1 m intervals to provide profiles for each station. These were then combined by fishing grounds Pangani, Dar es Salaam, and Mafia for further analysis. 3. Results Species composition and distribution Varieties of species mainly small fishes were dominating in each haul during the survey. Percentage composition of dominant fish species with relative abundance (beyond 1%) by number was calculated. The results indicate that a total number of 21 dominant fish species were obtained (Table 1). The tables below show percentage species composition of the most encountered demersal fish species during the survey

9 Table 1 show species composition of demersal fish %Species Species No composition Leiognathus leuciscus Upeneus sulphureus Upeneus bensasi Nematopalaemontenuipes Upeneus taeniopterus Saurida tumbil Johnioeps sina Leiognathus equulus Penaeus indicus Metapenaeusmonoceros Gerres filamentosus Secutor insidiator Pellona ditchela Lethrinus lentjan Nemipterus bleekeri Pomadasys stridens Upeneus moluccensis Polynemous sextarius Scolopsis bimaculatum Terapon theraps Otolithes ruber Others Distribution of fish species by depth (m) at the three fishing zones The results revealed that most fish were concentrated in shallow waters among the surveyed fishing ground (Appendix A). Catch rates and Biomass The analysis of species abundance by zones was done to indicate the different catch rates observed among three fishing zones (Fig 2). The result indicated that fishing zone 3 (Kilwa) had significantly lower catch rates compared to fishing zone 1 (Bagamoyo) and 2 (Rufiji).

10 Catch rate (Kg/hr) Zone 1 Zone 2 Zone 3 Fig 2 Mean fish catch rate (kg/ hr) from the three sampling zone. Error bar = ± SE Catch rates at different depth The analysis of species abundance by depth was done to indicate the different catch rates observed at different sampling depth among fishing zones (Fig 3). The result indicated that catch rates were higher in shallow waters between 10m and 40m. Distribution of catch rates at different sampling depth among fishing zones varied. Fish catch rates at different sampling depth decreased with depth (10-40m) in fishing zone 1 and 2. Contrarily, fish catch rates at different sampling depth increased with depth (10-40m) in fishing zone 3 (Fig 3) and 2.. In general fishing zone 1 had wider range of fish catch rates at different sampling depth among the three fishing zone Fig 3. Catch rate (Kg/hr) Zone 1 Zone 2 Zone to to to to to to to to 90 Depth (m) Figure 3. Demersal fish catch rate at different depth among the three fishing zones

11 Biomass estimation In terms of biomass estimations, fishing zone 1 (Bagamoyo ) had more demersal fish compared to zone 2 (Rufiji) and zone 3 (Kilwa) Fig Biomas (Tones) x zone 1 zone 2 zone 3 Fishing zone Figure: 4 Fish biomass from the three sampling zone Population structure indices The length weight parameters and maturity stage of the priority species is shown in the table below Table 2 Population structure parameters of the priority species collected in the three fishing zones SPECIES N Means SE Min. Max. L 50 a b r 2 Otolithes ruber Trichurus repturus Pomadasys kaakan Pomadasys maculatum Genetics The number of fish and prawn specimens collected in each zone is shown in Table 1. The specimens will be used to extract DNA for molecular analysis. The extraction of DNA will be performed according to procedure described in the DNeasy Tissue Handbook (QIAGEN, German). The DNA extracted for each individual will be used to:- Investigate on the patterns of genetic differentiation in the three species of prawns according to the demographic events Investigate whether the three zones composed of genetically distinct species Investigate the Isolation by Distance (IBD) within and among population The investigations will be based on mtdna control region or d-loop and microsatellites of the nucdna. The primers for DNA amplifications and sequencing have already been designed. The fish molecular analysis will be done at University of

12 Pretoria, while prawns molecular analysis will be done by PhD student funded by SWIOFP, Ms. Catherine Mwakosya. Table 3: Specimens of fish and prawn collected for molecular analysis in three fishing zones Number Zone Fish Prawns P. kaakan P. indicus P. monodon M. monocerous I II III The results will be used for management purpose, as current and historical population sizes will be estimated. Though the species occurs sympatrically, knowing their genetic distinct status is of very importance for whether species-specific management should be applied. Therefore, the findings will be useful for drawing management measures for conservation and development of fisheries resources. Primary productivity Levels of surface chlorophyll a concentrations were measured in two zones (Fig5 ). There were equal values of chl a concentration in both sites Chla mg/l zone 1 zone zone 1 zone2 Figure:5 Chlorophyll a concentration levels from zone 1 and zone 2 Physico-chemical parameters Nutrients The figure below, show variation in nutrients as recorded at different fishing zones. Levels of phosphorus and nitrates were not different among the three fishing zones (Fig 6). However zone 3 and zone 2 had values of both total nitrogen and silca concentrations which were relatively higher compared to zone 1.

13 Figure:6 Nutrients levels (nitrate, phosphate, total nitrogen and silca) from the three sampling zone Depth profiles of CTD cast A total of 34 stations were sampled for oceanographic information. In general, temperature ranges from to C with a mean of 25.93±0.22 C. Zone III was the warmest, recording an average water column of C. Temperature ( o C) Bagamoyo Kilwa Rufiji Depth (m) Figure: 7 Temperature profiles from the three sampling zone Dissolved oxygen was 6.1±0.01 mg/l in the sampled area with no significant difference (p > 0.05) between the zones. The highest values were found between 15 and 50 m, thereafter it decreases sharply with depth.

14 DO (mg/l) Depth (m) 40 Bagamoyo Kilwa Rufiji Figure: 8 Dissolved oxygen profiles from the three sampling zone Mean salinity was ± 0.01, with Rufiji being less saline up to almost 20 m. Salinity (PSU) Depth (m) Bagamoyo Kilwa Rufiji 80 Figure: 9 Salinity profiles from the three sampling zone

15 Mean fluorescence was 0.10 ± 0.05, with Rufiji having more chl a fluorescence up to almost 15 m Figure: 10 Fluorescence profiles from the three sampling zone Mean nitrogen saturation was 10.8 ± 0.04, with Rufiji having higher levels of nitrogen saturation up to almost 15 m Figure:11 Nitrogen saturation profiles from the three sampling zone

16 4. Discussion 1. Abundance Indices This is the first detailed shallow water demersal fish survey and other marine by-catch in Tanzania showing types of species, their composition and their abundance among three trawlable fishing sites (Fig 1).Primary productivity and physiochemical parameters such as nutrients, Water column of dissolved oxygen, chlorophyll a fluorescence, Sea surface temperature, salinity and levels of nitrogen saturations were also measures. The water physiocahemical parameters were determined because they are necessary for the life sustainability of marine organisms. Most of productivity activities were observed between 15m and 20m. Between these depths values of fluorescence which is measurement of chlorophyll a increased. This is supported by higher values of oxygen, as result of photosynthetic activities from phytoplankton. Values of nitrogen saturation also increased within the same depth range due to nitrogen fixation activities from phytoplankton. With exception to salinity values salinity which was low in surface water in zone 2, the temperature values followed the same patterns as fluorescence, oxygen, and nitrogen. It is hypothesized that the observed salinity and temperature values within that depth ranges were favorable for growth of phytoplankton. Species composition and distribution Most species by number/weight were concentrated in shallow waters among the surveyed fishing zones. Our results are in agreement with previous studies conducted by Mbegani Fisheries Development centre and also Dr Fridtjof Nansen 1982 which found coastal waters of Tanzania are dominated with small sized demersal fish which are concentrated in shallow water. The stock assessment abundance indices revealed that zone 2 (rufiji) had rich stock biomass in terms of catch rates and biomass than zone 1 (Bagamoyo) and zone 3 (Kilwa) fishing areas. This may be due to higher water productivity observed in this fishing area as revealed by higher values of oxygen and chlorophyll a and linked to food web chain. The presence of such large quantities of demersal fish is an indication of high abundance of food, particularly fish larvae and other marine fauna. Relatively higher abundance of the priority species have been reported in shallow water crustacean survey 2011 and from previous study conducted by TAFIRI It has been observed there are caught mostly by prawn trawler than demersal fish trawler. All species were found in zone two and to lesser extent in zone 1 (Apendix A). Our result is in agreement with Sean et al2004 who reported the same results in south Africa. Comparison between the status of demersal fish in Zanzibar channel 1980s and 2000s (all data collected by MV Mafunzo) using spatial maps revealed that catch rates has decreased 3 times fold. In 1980s there was higher catch rates in Zanzibar channel toward the Zanzibar Island however the situation has been changed after 20 years, currently more fish are skewed towards the Tanzania main land. This finding suggests that there have been more fishing effort in from Zanzibar fisher may be due to hotel industry development. Secondly may be due to population increase and limited land in Zanzibar.

17 Fig 12 comparison of the status of demersal fish in Zanzibar channel between 1980s and 2000s. All survey data were collected by using Mafunzo. General conclusion There is a need of immediate management intervention on the depletion of the shallow water demersal fish stock in Tanzania. This survey reveals there are a higher fishing pressure on shallow demersal fish from artisan fishers because there is no demersal fish trawler which is operating in Tanzanian coastal waters. Acknowledgment Many individuals were involved in the preparation and execution of this survey. This survey was supported by South West Indian Ocean Fisheries Project (SWIOFP). We would like to thank SWIOFP Regional Management Unit for logistics and facilitation. We would like to express our special thanks to the Captain and all crews of MV Mafunzo, for their tireless assistance during fishing cruise. Special thanks to the director general, Dr. Benjamin Ngatunga, the director of research Dr. Semvua Mzighani and the director Dar es Salam of centre Dr. Juma Kangwe for their guidance and administrative support during the survey time. Lastly, to TAFIRI staffs and everybody who in one way or the other has participated in this survey.

18 Appendix A Percentage distribution of fish species by numbers at different depth strata at zone 1 Species m 31-50m 51 60m 61-80m 81-90m Leiognathus leuciscus Upeneus sulphureus Upeneus bensasi Nematopalaemon tenuipes Upeneus taeniopterus 0.7 Saurida tumbil Johnioeps sina Leiognathus equulus 0.36 Penaeus indicus 0.03 Metapenaeus monoceros 0.1 Gerres filamentosus 1.15 Secutor insidiator Pellona ditchela 0.04 Lethrinus lentjan Nemipterus bleekeri Pomadasys stridens Upeneus moluccensis Polynemous sextarius Scolopsis bimaculatum 0.94 Terapon theraps Otolithes rubber Percentage distribution of fish species by numbers at different depth strata at zone 2 Species m 31-50m 51 60m 61-80m 81-90m Leiognathus leuciscus Upeneus sulphureus Upeneus bensasi Nematopalaemon tenuipes Upeneus taeniopterus Saurida tumbil 0.46

19 Johnioeps sina Leiognathus equulus Penaeus indicus Metapenaeus monoceros Gerres filamentosus Secutor insidiator Pellona ditchela Lethrinus lentjan Nemipterus bleekeri Pomadasys stridens Upeneus moluccensis Polynemous sextarius Scolopsis bimaculatum Terapon theraps Otolithes rubber Percentage distribution of fish species by numbers at different depth strata at zone 3 Species m 31-50m 51-60m 61-80m 81-90m Leiognathus leuciscus Upeneus sulphureus Upeneus bensasi Nematopalaemon tenuipes Upeneus taeniopterus Saurida tumbil Johnioeps sina Leiognathus equulus Penaeus indicus Metapenaeus monoceros Gerres filamentosus

20 Secutor insidiator Pellona ditchela Lethrinus lentjan Nemipterus bleekeri Pomadasys stridens Upeneus moluccensis Polynemous sextarius Scolopsis bimaculatum Terapon theraps Otolithes rubber