Using size-based indicators to evaluate the ecosystem effects of fishing

TLDR

The study reviews the usefulness of size‑based indicators for ecosystem fisheries management and calls for further modelling and worldwide comparative studies, noting that longer‑term strategic management may facilitate their use. The authors compile and tabulate SBIs—such as mean length metrics and size‑spectrum parameters—detailing their definitions, data needs, biases, time‑series availability, and expected responses to fishing pressure, and analyze fishing versus environmental effects to distinguish causes. The review finds that most SBIs can be derived from standard survey data, but no single indicator effectively signals heavy fishing pressure; instead, suites of SBIs and reference directions are recommended, though their slow response limits applicability to short‑term management.

Abstract

Abstract The usefulness and relevance of size-based indicators (SBIs) to an ecosystem approach to fisheries (EAF) are assessed through a review of empirical and modelling studies. SBIs are tabulated along with their definitions, data requirements, potential biases, availability of time-series, and expected directions of change in response to fishing pressure. They include mean length in a population, mean length in a community, mean maximum length in a community, and the slope and intercept of size spectra. Most SBIs can be derived from fairly standard survey data on length frequencies, without the need for elaborate models. Possible fishing- and environment-induced effects are analysed to distinguish between the two causes, and hypothetical cases of reference directions of change are tabulated. We conclude that no single SBI can serve as an effective overall indicator of heavy fishing pressure. Rather, suites of SBI should be selected, and reference directions may be more useful than reference points. Further modelling and worldwide comparative studies are needed to provide better understanding of SBIs and the factors affecting them. The slow response to fishing pressure reflects the complexity of community interactions and ecosystem responses, and prohibits their application in the context of short-term (annual) tactical fisheries management. However, movement towards longer-term (5–10 years) strategic management in EAF should facilitate their use.