Broad-scale abundance changes are more prevalent than acute fishing impacts in an experimental study of scallop dredging intensity

There is limited understanding of the relationship between mobile gear fishing intensity and the magnitude of the resulting perturbation in marine benthic communities. Theory predicts that instantaneous fishing-induced mortality should generally scale directly with fishing intensity. This study used a before–after-impact experimental design with 16 fishing intensity levels to assess the impact of scallop dredging on benthic communities at two experimental sites. Few instances of statistically significant effects of fishing intensity were detected for individual taxa, although there was some evidence of impacts on community structure at one site. In contrast, short-term natural abundance fluctuations were much more prevalent and were of a magnitude similar to that estimated to be produced by fairly intense fishing, as would occur in only very limited geographic areas in the commercial fishery. Post-hoc simulations used to estimate the statistical power of the study suggest that true effects of elevated fishing mortalities could reasonably be detected, but that power was low for small instantaneous fishing mortalities. This situation is comparable to other well-designed mobile gear impact studies that reported power. The simulations also revealed comparable levels of statistical power for correctly detecting abundance changes due to fishing and due to natural fluctuations. The results of this study taken in the context of the commercial fishery suggest that impacts of scallop dredging on the local benthic communities are small with respect to natural variation in the ecosystem. This study highlights the need to account for natural spatio-temporal variation in the designs of studies of mobile fishing gear impacts. Failure to do so increases the risk of drawing incorrect conclusions.