Indicators of change in the size structure of fish communities: A case study from the south coast of South Africa

Abstract

Heavy fishing is believed to change the size structure of fish communities through the differential removal of larger sized individuals and species. The size structure of the demersal fish communities off the south coast of South Africa is assessed using a variety of size-based indicators. The communities on this coast are subjected to different fisheries with wide ranging impacts. Changes in the size structure were assessed over the period from 1986 to 2003 for the whole south coast and two depth strata. There was a reduction in the mean length, mean maximum length ( L max), and proportion of large fish in these fish communities, and a corresponding increase in the proportion of small fish. Species with a large L max declined in mean catch rate and mean length in contrast to species with a small L max. The trend in the size-based indicators confirms that there has been a change in the size structure of the communities. This change is characterized primarily by a reduction in the abundance of larger sized individuals and a reduction in species with a large maximum length.