Notes on nine biological indicators estimable from trawl surveys with an illustrative assessment for North Sea cod

This paper reviews a selection of nine indicators for assessing the biological state of a fish population monitored only by using trawl surveys in which catchability is assumed constant from time to time and place to place. The intention is to inform or remind of the range of possibilities and of associated practical and statistical issues. The indicators, grouped according to relationships with abundance, length, age, weight, and reproduction, are documented in a standard format covering estimation, interpretability, sensitivity, considerations for reference points where possible, and an example of their application. Lastly, selected indicators together with year-class curves fitted to abundance indices-at-age are applied to the North Sea cod to demonstrate the feasibility of an assessment based on only one trawl survey. Results support findings of the International Council for the Exploration of the Sea (ICES) working group on the assessment of demersal stocks in the North Sea and Skagerrak.