107Auswirkungen der Garnelen-fischerei auf Habitate und Lebensgemeinschaften im Küstenmeer der Norddeut-schen Bundesländer Schleswig- Holstein, Hamburg und Niedersachsen (CRANIMPACT)

Study design The CRANIMPACT project investigated the effects of beam trawl shrimp fisheries on 2 predominant fisheryrelevant habitat types in the sublittoral of the Wadden Sea National Parks of the northern German states. Two complementary approaches were used to investigate the short-term effects following an experimental fishing event and the chronic changes caused by sustained fishing pressure of varying intensity. In the experimental approach, the short-term, small-scale effects on endo- and epifauna after experimental fishing and their effect duration were investigated on a total of 4 study sites (A, B, B2 in the tidal flat system of the Sylt backshore tidal flat; C in the tidal flat system near Norderney). The experiments were conducted as before-after-control-impact studies (BACI) exclusively in habitat type fine and medium sands with ripple structure (a sufficiently large lanice field could not be sampled). The large-scale and chronic effects of fishing were determined along gradients of fishing intensity in the Wadden Sea of Lower Saxony, Schleswig-Holstein and Denmark. For this purpose, among other things, methods were developed to represent small-scale differences in fishing effort using satellite data in tidal flat systems. Gradient analysis (GA) was carried out on fine and medium sands with ripple structure as well as on fields with colonization of the tree tube worm Lanice conchilega for the endofauna alone. All surveys were conducted in the sublittoral. A total of 427 endofauna samples and 52 epifauna samples were examined from 2019, 2020, and 2021. Results Results from 28 stations were included in the gradient analysis (GA). As a reference area, 5 sites were located on tidal flats in the Danish Wadden Sea, where fishing has been prohibited since 1977. The community analysis of all stations in the GA revealed 2 main associations: A Bathyporeia spp. association on fine and medium sand and a Lanice conchilega association on substrates with increased silt content. In both associations, sub-associations could be identified that could be differentiated based on fishing intensity. Besides fishing intensity, only silt content in the sediment exerted a significant effect on faunal composition. The explicitly included natural disturbance was not significant. However, these two factors explained only just under 20 % of the total variability of the endofaunal community. Of this, 10.3% explanatory contribution came from the level of silt content and 8.9% from differences in fishing intensity. The high similarity of some stations from the no-fishing area with stations with low fishing intensity can be interpreted as the endofauna being resilient to low fishing intensity, which can be explained by adaptation to the existing, comparatively high natural disturbance in the Wadden Sea. In contrast, at stations with high fishing intensity, the total biomass of endofauna increased with increasing fishing intensity in both associations. The significant highest value of 39.5 g ash-free dry weight per m² was measured in the intensively fished Lanice conchilega sub-association at about 100 hours of fishing per year and km². The 'biological-traits' analysis showed that at these stations with high fishing intensity, community characteristics also changed and the abundance of small species < 1 cm body size decreased, while the abundance of larger predatory species increased. Disturbance in the Before-After-Control-Impact experiments (BACI) was established by 4-fold fishing with a commercial shrimper to ensure that the area of the experiment was affected by short-term but significant fishing disturbance. Significant spatiotemporal variability was detected for the endofauna through ANOSIM analysis. Significant differences were also found for individual variables or taxa between the experimentally fished and control plots. In the fewest cases, the differences were consistent across all experiments. In the sum of all considered taxa as well as total number and biomass of all analyzed animals in the respective experiments, no statistical clustering of significant results was found for the differences between disturbance and control after experimental fishing (p=0.23). Where significant negative abundance effects occurred, caused by changes in Bathyporeia spp. or spionids, among others, the modeled effect duration was in the range of 13-20 days and can thus be characterized as short-term. The epifauna was characterized by a few species with high continuity, i.e. species that appeared in almost every sample. In the BACI experiments, in contrast to the endofauna, no significant small-scale variability could be detected in the undisturbed samples. Reasons for this are, on the one hand, the high constancy of the species inventory, but on the other hand also a small number of samples resulting from the comparatively large space required for the hauls with the beam trawl. There was also no statistical clustering of significant test results for the difference between control and disturbance. Individual effects at the species level were detected for Crangon crangon and Asterias rubens. In experiment B2 off List, the so-called "scavenger effect", i.e. the immigration of epibenthic, scavenging species after the fishing disturbance, was additionally investigated. Such an overcompensation effect by food opportunists could not be proven. Conclusions The close-meshed sampling from the experiments showed that the habitat types studied are characterized by considerable small-scale variability in the predominant communities, which made the detection of fishing impacts difficult. Nevertheless, for individual parameters or species of both endo- and epifauna, significant influences by experimental fishing could be detected. In the overall picture, however, this was not reflected in a statistically significant accumulation of effects due to fishing disturbance. Where effects were measured at the species level, the modeled effect duration ranged from 13 to 20 days. In the summary meta-analysis of all experiments, fishing effects were confirmed for total abundance and biomass of endofauna at 4-fold impact level while being insignificant if scaled to a single haul, and for Crangon crangon among epifauna. Consistent with these experimental results, communities in gradient analysis (GA) were found to be resilient to low fishing intensities and changed at high fishing intensities. The former can be interpreted as an adaptation to the comparatively high natural disturbance in the Wadden Sea and by small-scale local exchange processes of drift fauna between disturbed and undisturbed areas, which overlaid possible effects of shrimp fishing. At higher fishing intensities, shifts in communities to so-called "high fishing effort subassociations" occurred in the gradient analysis. The transition to these communities was observed at fishing intensities of 19.7 to 23.2 hours per year per km2. From the gradient analysis, the overall fish-free effect on endofaunal composition can be reported as 8.9%. Against the background of the heterogeneous distribution of fishing intensity in tideway systems and the local exchange processes between disturbed and undisturbed areas, which are essential for recolonization times, it can be concluded that the benthic communities on fine and medium sand with ripple structure in unfished and lightly fished tideway areas are in a resilient state. To what extent the present communities are also a result of the long-term fishing exploitation of the North Sea cannot be answered with the research approaches from CRANIMPACT, as well as the question of the fishing impact on other, rare or especially sensitive habitats not considered in this study. The impact of fishing is always a result of the specific combination of fishing gear, habitat and species community and is not directly transferable to other combinations.

Fock, Heino & Dammann, Robin & Mielck, Finn & Lauerburg, Rebecca A. M. & López Ganzález, Alfonso & Nielsen, Pernille & Nowicki, Margarethe & Pauli, Matthias & Temming, Axel, 2023. "107Auswirkungen der Garnelen-fischerei auf Habitate und Lebensgemeinschaften im Küstenmeer der Norddeut-schen Bundesländer Schleswig- Holstein, Hamburg und Niedersachsen (CRANIMPACT)," Thünen Report 334336, Johann Heinrich von Thünen-Institut (vTI), Federal Research Institute for Rural Areas, Forestry and Fisheries.

Handle: RePEc:ags:jhimwo:334336
DOI: 10.22004/ag.econ.334336

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