THEME: BIODIVERSITY ACCORDING TO BEAM TRAWL FISHING

Transcription

1 FIELD PRACTICAL THEME: BIODIVERSITY ACCORDING TO BEAM TRAWL FISHING MARINE SYSTEMS (AEW-22806) Supervisor: Ingrid van de Leemput Prepared by: A.Capriati ( ) D. Coskun ( ) L.L. Hakim ( ) T. Rosalie ( )

2 1 INTRODUCTION The Wadden Sea is located in the southeastern part of the North Sea. The Wadden Sea is in intertidal zone which has marine resources in abundance. This abundance attracts researchers to study the changing conditions and the diversity. Moreover, due to the anthropogenic impact such as over-fishing and sea grass losses, the fish diversity has been declining (Polte et al., 2006). The absence of management, properly affected to ecosystem. Nevertheless, fish communities have a correlation with environmental factors such as weather conditions (van der Veer et al., 2015). The course of Wageningen University, provides field work programs for monitoring fish biodiversity in the Wadden Sea. This term is necessary to determine the impact of human activity such as shipping, fishing and tourism in the Wadden Sea. OBJECTIVES The study purpose is to observe and record community diversity, particularly in fish diversity in the Wadden Sea, in order to relate to nature and human intervention. DATE AND LOCATION This study was conducted on 9 September 2015, at 09.30am 04.00pm. The field trip was taken place on board of the RV Navicula from the NIOZ. The location of the sampling site was divided into four sites (from Afsluitdijk until the Marsdiep) (Fig.1) Figure 1. Map of different sampling sites, the red dots indicate the sites were there has been fished with a beam trawl on the 9 th of September The results are solely from these four sites.

3 2 METHODS Procedure The data collected, consist of primarily fish communities. Sampling method was conducted by operating the beam-trawl. The mesh size net of the beam trawler was 1cm 2. The mouth or opening of the net is kept open by a beam and the beamtrawl is towed on outriggers along 410m, 460m, 340m and 370m respectively from the first until the fourth site (Fig.2a). Afterwards, the beam-trawl was landed by a machine on the deck (Fig. 2b). The fishes that landed were separated from other non-fish organisms for the next measurement. Since there were high number of shrimps caught, the first until the fourth shrimp sampling is taken in samples of 1/128, 1/128, 1/256 and 1/32 respectively due to the total number of shrimp. Fish catches were selected according to the different types. For instances, crab, plaice, sand goby, shrimp, sea star, hermit crab, fivebeard rockling and others (Fig. 3). Furthermore, the species length was measured and was written down in the data sheet (Fig. 4). a b Figure 2. a) Beam-trawl is towed and b) fish catch is landed on the deck Figure 3. Crab sorting (male and female) Figure 4. Colleting and measuring process

4 3 Composition of fish diversity RESULT AND DISCUSSION Fish observation in the Wadden Sea is divided into four different sites along Texel. During the day, there were 28 fish species found. Sites 3 were a ste which had the highest fish production. In general, the most abundance species was shrimp which was more than 95% in site 1, 2, and 3. However, shrimp was not found in site 4. Some of species, such as plaice, sand goby, sea star, crab, fivebeard rockling and brittle serpent star fish were also found in each site (Table 1). Plaices, sea star, crabs, fivebeard rockling and brittle serpent star fish were found in each site, because these species are able to live on a wide variety in depth. Other species were found in small number such as Agonus cataphractus, Cyclopterus lumpus, Trachurus trachurus and so forth (Table 2). These species have not been found in each sampling site. Table 1. The percentage of biodiversity of fish caught by beam trawl Species Percentage of fish each station (%) Plaice Sand goby Sea star Shrimp Male crab Female crab Hermit crab Fivebeard rockling Sea snail Bullrout Brittle serpent star fish Others Table 2. The Composition of fish abundance in the Wadden Sea No Biodiversity in a beam trawl at Wadden Number of fish each Station Sea Plaice Sand goby Sea star Shrimp Male crab Female crab Hermit crab Fivebeard rockling Sea snail Bullrout Brittle serpent star fish (ophiuridea) Others

5 4 Note that almost all fish in the Wadden Sea have a small size due to the fact that the Wadden Sea is nursery ground area. The highest size distribution of the fishes was found around 6-9cm (Table 2). Besides that, the water current needs to be taken into account. When there is a strong current like in the Wadden Sea, smaller fish have an advantage as they can rest behind rocks. Based on the graph of the first sites, shrimp and crabs were the dominant species in this place. Also, the male crabs have a bigger size and are more abundant than the female crabs. Looking to at the second site, sand goby shows here the highest abundance in comparison to the other sites. Despite that, the size of crabs was relatively equal to the sizes found in the first sampling site. The third sites showed that the sea star abundance has size apportionment. In contrary from the first site, the existence of crabs showed that female crabs have here a larger size than the males. Although in the first, second and third site shrimp were found, they have not been found in the fourth site. The different depth in each site while trawling for fish has to be taken into account since it has a strong influence on the existence of more or less different species (Sajdlová, et al., 2015).

6 No measurement for shrimp size, but there were 50 individuals for sub sampel 1/256 5 Sample size (cm) Table 2. Size Ditribution of Fish Abundance in The Wadden Sea Species Plaice Sand goby Sea star Shrimp Male carb Female rab Fivebeard Sea snail Bullrout rockling Sites

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8 7 CONCLUSION The dominant groups are belonging to shrimp, plaice, sand goby, crabs and fivebeard rockling due to the fact that the other species have been found in fewer numbers. The small sizes of the dominant fish species are correlated with the net mesh size and fishing ground. REFERENCES Polte, P., & Asmus, H. (2006). Intertidal seagrass beds (Zostera noltii) as spawning grounds for transient fishes in the Wadden Sea. Marine Ecology Progress Series, 312, Sajdlová, Z., Draštík, V., Jůza, T., Říha, M., Frouzová, J., Čech, M.,... & Kubečka, J. (2015). Fish behaviour in response to a midwater trawl footrope in temperate reservoirs. Fisheries Research, 172, Van der Veer, H. W., Dapper, R., Henderson, P. A., Jung, A. S., Philippart, C. J., Witte, J. I., & Zuur, A. F. (2015). Changes over 50 years in fish fauna of a temperate coastal sea: Degradation of trophic structure and nursery function.estuarine, Coastal and Shelf Science, 155,