Evaluating fishery impacts using metrics of community structure

Evaluation of the ecosystem effects of fishing presupposes that it is possi ble to measure the state of an ecosystem, detect differences among measures of its state, and determine if the differences are due to chance or impact s of fishing. Ecologists have developed or adopted four main classes of com munity/ecosystem metrics For such tasks: diversity and similarity indices, ordination methods, metrics of aggregate community properties. and metrics derived from ecosystem models. Each class is explained briefly, and guidanc e is given to the Literature. Size spectra, k-dominance curves, and multidi mensional scaling emerge as promising methods for these uses. Some modellin g frameworks also may be useful, but more work is needed on sensitivity tes ting and development of independent theory for these models. The current th eoretical basis for most ecosystem models assumes that competition structur es the system bring modelled. Problems with applying that framework to expl oited marine systems are reviewed. Resampling methods are essential when te sting for differences among community metrics. Practices for determining ca usality when ecosystem metrics change are reviewed, and additional knowledg e and data necessary to link change to cause are highlighted. (C) 2000 Inte rnational Council for the Exploration of the Sea.