Publication Abstract

Size structural change in lightly exploited coral reef fish communities: evidence for weak indirect effects

N.K. Dulvy, N.V.C. Polunin, A.C. Mill and N.A.J. Graham

Exploitation can potentially influence community structure via the preferential removal of larger-bodied fishes, which may exert top-down control upon other community members. We test for both direct and indirect effects of subsistence exploitation upon coral reef fish communities. Three size metrics are used to describe large-scale community structure along a 13-island spatial gradient of exploitation. Size islands were surveyed three times over a one-year period. The size metrics varied significantly with sample date and fishing intensity, however fishing intensity consistently explained more of the variation in the size metrics. The steepness of size spectra slopes increased with increasing fishing intensity and the mean weight of individuals and the mean maximum length of the community declined by 45% and 25% respectively in response to exploitation. We examined the numerical and biomass response to exploitation of each 5 cm size category. The larger size classes (> 26 cm) declined in numbers and biomass by 63 and 61% respectively in response to exploitation. The average number and biomass of fishes in the three smallest size classes (<25 cm) increased by 31% and 9% respectively in response to exploitation. We suggest this indicates some form of weak compensatory response at the community-level to the direct removal of larger fishes by exploitation.

Reference:

N. K. Dulvy, N. V. C. Polunin, A. C. Mill , N. A. J. Graham (2004) Size structural change in lightly exploited coral reef fish communities: evidence for weak indirect effects. Canadian Journal of Fisheries and Aquatic Sciences, 61: 466-475