IMIS

Mussel farming in the Netherlands is traditionally based on bottom culture. Mussel seed is collected in Autumn and Spring from natural stocks and translocated to on-bottom culture plots. Fishery starts in Autumn on beds which are most vulnerable for starfish predation or for being lost during winter storms. In Spring a second fishery takes place. We investigated the impact of seed fishery on sediment characteristics and the macrobenthic communities. A BACI designed experiment was run for 4 years in the subtidal western part of Wadden Sea. Sediment and benthos samples were taken directly before and after the seed fishery. Sampling was repeated for a longer period after fisheries. In this study the data before, directly after and 1–1.5 years after the fishery were analysed. The number of locations studied was less than initially planned. As a result the effect sizes that could be detected were larger than chosen at the start of the study. Yet, our study has shown a potential direct effect on sediment characteristics and the benthic communities. Although the effect was not significant, sediment became coarser and the percentage of clay lower in the fished areas compared to the control areas, and the effects were most pronounced after Spring fisheries. The effect on the number of species was significant, and although the interaction term with season was not significant, we did find a larger decrease in the Impact parts after Autumn fisheries, while after Spring fisheries there was a smaller increase. The short term effects on total density were most pronounced after Spring fisheries: a significant decline in total density in the Impact parts while density increased in the Control parts. Species composition was less similar between Impact and Control parts and was lower after fisheries, for both fishery seasons. After 1–1.5 years, the dissimilarity between the Impact and Control part gradually disappeared. Differences between Impact and Control parts in changes in median grain size and the number of species were no longer visible. For the Spring plots, however, also after 1.5 year, the percentage clay and the total densities were lower in the Impact parts then in the Control parts.

The observed differences are mainly caused by the changes in species associated to mussel beds, and as a consequence of the changes in mussel densities themselves. In conclusion, changes appears to follow the development of the mussel densities, including the effect of fishery on the mussel densities.