Relative influence of ecological factors and fishing on the structure of reef fish stocks in six fishing grounds of the kingdom of Tonga (south Pacific)

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Two fishing grounds were selected in each of the three archipelagos of Tonga, according to fishing pressure (high and low). In each ground, socioeconomic surveys provided an evalua-tion of fishing pressure. Reef fish stocks were assessed through underwater visual censuses along 241 transects and their habitat was described with a novative method (Medium Scale A p p r o a c h) , better adapted to their life territories. The analysis of ecological and fishing data showed a variation between global fishing pressure at the archipelago level, which was responsible, in combination with ecological factors, for more differences than between the pair of sites within each archipelago. The fishing factor explains globally less variance of fish populations (1.6 to 5.7%) than factors acting at micro-scale (depth, hard substrate and live coral coverages, hetero-geneity and topographic complexity) and at meso-scale (oceanic influence), that explain 23.3 to 34.3% of variance. The study con-firmed fishing effects already known (such as reduction of fish populations average size, compensatory increase of density in the small size classes) and showed, at least for scarids, a "shifting dom-inance" phenomenon, based on the decrease of large-size targ e t species, which benefits to small size species, less vulnerable to fishing. Clustering of species according to diet or life history traits revealed gradual changes of density and biomass in specific groups according to fishing pressure gradient. This result provides poten-tial for setting up indicators of stock status for better management of reef fish resources.

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