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

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Abstract:

Exploitation influences community structure directly through preferential removal of larger-bodied fishes and indirectly because larger-bodied fishes may exert top-down control upon other community members. We consider the direct and indirect effects of subsistence exploitation upon the size structure of coral reef fish communities along an island-scale spatial gradient of fishing intensity. We explored the effect of fishing intensity and sample date (three dates over a year) at six islands and the overall effect of fishing intensity averaged over sample dates at 13 islands. Fishing intensity consistently explained more of the variation in the size metrics than sample date. In response to exploitation, the mean weight of individuals declined by 52%, the weighted average maximum size (Lmax) declined by 25%, and slopes of community size spectra steepened. The larger size classes (>26 cm) declined in relative numbers by 63% and relative biomass by 61% in response to exploitation. However, the numbers and biomass of the three smallest size classes (<25 cm) increased by 31% and 9%, respectively, in response to exploitation. This increased abundance is consistent with a weak compensatory response presumably from a reduction in predation upon smaller size classes as an indirect effect of exploiting larger size classes.

L'exploitation affecte la structure de la communauté directement par le retrait préférentiel des poissons de grande taille et indirectement parce que les poissons de grande taille peuvent exercer un contrôle descendant sur les autres membres de la communauté. Nous avons examiné les effets directs et indirects de l'exploitation de subsistance sur la structure en taille de communautés de poissons de récifs coralliens le long d'un gradient d'intensité de pêche à l'échelle spatiale des îles. Nous avons étudié les effets de l'intensité de la pêche et de la date d'échantillonnage (trois dates au cours d'une année) à six îles, ainsi que l'effet général de l'intensité moyenne (toutes dates confondues) de la pêche à 13 îles. Dans tous les cas, l'intensité de la pêche explique une plus grande proportion de la variation des métriques reliées à la taille que la date d'échantillonnage. En réaction à l'exploitation, la masse moyenne individuelle diminue de 52 %, la taille maximale moyenne pondérée (Lmax) baisse de 25 % et la pente des spectres de taille de la communauté augmente. En réaction à l'exploitation, les densités relatives des classes de grande taille (>26 cm) diminuent de 63 % et leur biomasse de 61 % . Cependant, les densités et les biomasses des trois classes les plus petites (<25 cm) augmentent respectivement de 31 % et de 9 %. Cette densité accrue s'accorde bien avec l'existence d'une faible réaction compensatoire probablement causée par une réduction de la prédation chez les classes de plus petite taille comme effet indirect de l'exploitation des classes de plus grande taille.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: March 1, 2004

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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