Performance of indicators derived from abundance estimates for detecting the impact of fishing on a fish community

Population and community indicators for the impact of fishing are often estimated using abundance estimates instead of raw sampling observations. Methods are presented for testing null hypotheses of non-significant impacts and where possible, for calculating the statistical power. The indicators considered concern populations (intrinsic growth rate, total mortality, exploitation rate and a new indicator, the change in fishing mortality required to reverse population growth) and communities (kand partial dominance curves, a biodiversity index, size spectrum and proportions of various population groups). The performance of these indicators is compared for the Celtic sea groundfish community based on achieved precision, statistical power and availability and estimation method of reference points. Among population indicators, mean length in the catch was most precisely estimated and the corresponding hypotheses tests had consistently large powers. Total mortality performed reasonably well. In contrast, both the intrinsic population growth rate and the exploitation rate gave unreliable results. All tested community indicators performed similarly well. Indicators for which the direction of change caused by fishing is predictable, such as the proportion of non-commercial species or piscivores in the community, are promising indicators at the community level.