Response of potential fish community indicators to fishing

Fish community metrics and diversity indices have often been proposed as indicators to support an Ecosystem Approach to Fisheries Management. The response of eight fish community indicators to spatio-temporal changes in fishing effort was studied by comparing the rate of change in (i) areas subject to different levels (high, medium, and low) of fishing effort and (ii) areas inside and outside a closed area (the plaice box) where management measures led to changes in fishing effort over time. Several indicators showed broadly consistent responses to fishing effort (slope of the biomass-size spectra, mean weight, and mean maximum length) while a range of biodiversity indices and biomass did not. The response of the indicators to spatio-temporal changes in fishing effort showed that, with the exception of the slope of the biomass-size spectra, none of the indicators reliably detected the effects of spatial management measures at the scales of time and space that were used for these analyses. While some of the size-based indicators we tested did provide evidence for fishing impacts on communities, the response of the indicators to fishing was not straightforward and may have depended on environmental conditions and historic fishing regimes. Therefore, the indicators would provide limited support for assessing the effects of short-term and small-scale management actions and must be applied with caution until we have an improved theoretical understanding of their response to fishing and the environment