Size-based indicators for assessments of ecological status of coastal fish communities

Abstract

Human impact does not only affect the abundances of fish, but also the age- and size-distributions. Indicators of fish age and size-str uct ures can hence be useful tools for fisheries- and environmental management. Size-based indicators have been tested and proposed for large, homogenous marine ecosystems with high fishing mort alit y, but rarely for fine-scaled heterogeneous ecosystems in coastal zones. Here we analyse a suite of size indicators for coastal fish communities in the Baltic Sea, including mean and median length, 10th and 90th-percentile of the length distribution (L10, L90), mean length of the 10% largest fish ( L max ), large fish indices, size-spectra, and siz e-div ersity. R esults sho w good precision and accuracy of most indicators at realistic sample siz es, e x cept f or siz e-spectra and siz e-div ersity, making them less suitable. Different indicators sho w ed correlations among sites, indicating similar responses to environmental variation. Most size indicators responded positively to lower fishing pressure, especially indicators emphasizing the largest individuals in the population (e.g. L90 and L max ), whereas eutrophication and ph y sical disturbances had less impact. We conclude that size-based indicators aiming at describing the occurrence of larger fish, like L90 and L max , are useful for establishing management targets and assessing the status of coastal fish.