Indirect Effects of Bottom Fishing on the Productivity of Marine Fish

One quarter of marine fish production is caught with bottom trawls and dredges on continental shelves around the world. Towed bottom-fishing gears typically kill 20-50 per cent of the benthic invertebrates in their path, depending on gear type, substrate and vulnerability of particular taxa. Particularly vulnerable are epifaunal species, which stabilize the sediment and provide habitat for benthic invertebrates. To identify the habitats, fisheries or target species most likely to be affected, we review evidence of the indirect effects of bottom fishing on fish production. Recent studies have found differences in the diets of certain species in relation to bottom fishing intensity, thereby linking demersal fish to their benthic habitats at spatial scales of ~10 km. Bottom fishing affects diet composition and prey quality rather than the amount of prey consumed; scavenging of discarded by-catch makes only a small contribution to yearly food intake. Flatfish may benefit from light trawling levels on sandy seabeds, while higher-intensity trawling on more vulnerable habitats has a negative effect. Models suggest that reduction in the carrying capacity of habitats by bottom fishing could lead to lower equilibrium yield and a lower level of fishing mortality to obtain maximum yield. Trawling effort is patchily distributed - small fractions of fishing grounds are heavily fished, while large fractions are lightly fished or unfished. This patchiness, coupled with the foraging behaviour of demersal fish, may mitigate the indirect effects of bottom fishing on fish productivity. Current research attempts to scale up these localized effects to the population level

Experimental validation of geosmin uptake in rainbow trout, Oncorhynchus mykiss (Waldbaum) suggests biotransformation

The bioconcentration of waterborne geosmin in rainbow trout, Oncorhynchus mykiss (Waldbaum) was assessed. Fifty rainbow trout with a mean (SD) weight of 226.6 (29.0) g and lipid content of 6.2 (0.6) % (w/w) were exposed to geosmin in static water for 0, 2, 4, 6, 8, 12, 24, 36, 48 and 120 hr, with one tank containing five fish for eac h exposure period. Geosmin concentrations were measured in fish tissue and water samples collected over time. With time the geosmin concentration in the fish increased and decreased in the water. However, the total absolute amount of geosmin in the system declined over time which could be explained by induction of biotransformation. This is in accordance with the decreasing lipid normalized geosmin levels in the liver compared with the liver-free carcass. Geosmin distribution within rainbow trout clearly is not exclusively governed by the lipid content of tissues. In vivo geosmin bioconcentration in rainbow trout is slower and the body burden reached is lower than the generally accepted theoretical model predicts

Evaluating the effect of fishery closures: Lessons learnt from the Plaice Box

To reduce discarding of plaice Pleuronectes platessa in the North Sea flatfish fisheries, the major nursery areas were closed to large trawlers in 1995. The area closed was named the ‘Plaice Box’ (PB) and beam trawl effort fell by over 90% , while the exemption fleets of small flatfish beam trawlers, gill netters targeting sole (Solea solea) and shrimp (Crangon crangon) trawlers increased their effort. Contrary to the expectation, plaice landings and biomass declined. The initial support for the PB from the fisheries was lost, whereas other stakeholder groups claimed that any failure was due to the fact that fishing had never been completely prohibited in the area. To evaluate whether the PB has been an effective management measure, the changes in the ecosystem (plaice, demersal fish, benthos) and fisheries are analysed to test whether the observed changes are due to the PB or to changes in the environment unrelated to the PB. Juvenile growth rate of plaice decreased and juveniles moved to deeper waters outside the PB. Demersal fish biomass decreased, whereas the abundance of epibenthic predators (Asterias rubens and Cancer pagurus) increased in the PB. Endobenthos, in particular the main food items of plaice (polychaetes and small bivalves) remained stable or decreased both inside and outside the PB. Currently catches of both plaice and sole from within the PB are lower than in the late 1980s and the exemption fleet often prefers to fish outside the Plaice Box alongside much larger competitors. It is concluded that the observed changes are most likely related to changes in the North Sea ecosystem, which may be related to changes in eutrophication and temperature. It is less likely that they are related to the change in fishing. This case study highlights the importance setting testable objectives and an appropriate evaluation framework including both ecological and socio-economic indicators when implementing closed areas. Key words: Marine Protected Area, MPA, spatial management, fisheries management, discards, climate change, trawling impact, North Sea, benthos, ecosystem change, stakeholder perceptio

Shrimp fishery and natural disturbance affect longevity of the benthic invertebrate community in the Noordzee-kustzone Natura2000 area

The Noordzeekustzone is an important fishing ground for fishing vessels targeting brown shrimp (Crangon crangon). Shrimp trawling is by far the dominant fishing activity in this area. However, the effect of shrimp fishery on the benthic invertebrate community has never been clearly established. It is important to establish this effect (or lack of effect) because the Noordzeekustzone is a designated Natura 2000 area, with a policy target to improve the quality of the seafloor habitat (so-called H1110b,permanently submerged sand banks).In this paper, the longevity composition of the benthic community is studied in relation to environmental variables. First the longevity composition is estimated for seafloor habitats and the effect of depth,grainsize, tidal shear stress and trawling intensity on the longevity composition is estimated and used to derive quantitative relationships that can be used to determine the changes in the benthic community and the effect of natural and human pressures. The analysis is carried out using dredge sampling data,which effectively samples only the larger individuals (>0.5cm) in the upper 7cm of the sediment. The methodology developed in the FP7-project BENTHIS (Rijnsdorp et al, 2015) was used to assess the changes in the benthic community in the Noordzeekustzone and the importance of all those candidate factors with special attention to fishing effort. In this study we find clear evidence that intensive shrimp trawling is associated with a reduction in the longevity of the benthic invertebrate community. However, the direction and intensity of that impact is determined by the wind regime in the area, which we take as a proxy for the degree and/or frequency of natural disturbance of the seafloor. The entire Noordzeekustzone area is subject to strong natural disturbance, and it has often been suggested that trawling has no effects in such areas. Our analysis shows otherwise. Even within this highly dynamic area there is a clear gradient along the magnitude of natural disturbance, in the effect of shrimp fishing. At the lower end, we find that shrimp trawling truncates community longevity, while at the higher end shrimp trawling actually enhances longevity. The mechanism for this reversal remains to be studied. To our knowledge, this is the first study wherea clear effect of shrimp trawling on the benthic ecosystem has been found in empirical data.This study shows a statistically significant effect of an admitted economic activity on the seafloor in a Natura2000 area where protection of seafloor habitat is one of the key reasons for the protection. The admission of shrimp trawling in this area has been granted based on an appropriate assessment which concluded that there are no known significant effects of the activity on the seafloor habitat. The statistically significant results of this study indicate that there are effects and hence warrant furtherstudy to determine significance in the sense of the natura2000 framework