C and N stable isotope ratios in fishes from marine protected areas and areas open to fishing

Here, we assess whether fishery exploitation affects the trophic structure of carnivorous fishes. We censused fishes and analysed the stable isotope ratios of C and N of species targeted by fishermen in areas open to fishing and marine protected areas in the Mediterranean Sea and the north-eastern Atlantic Ocean. Results demonstrated a major impact of fishing on the biomass and the size structure of nektobenthic carnivorous fishes. However, those changes did not modify the diversity of the trophic resources used by the assemblage, the pattern of resource partitioning between species or the degree of trophic redundancy. These results add to recent evidence suggesting that marine protected areas implemented in fished seascapes may fail to restore the original structure of the food webs that once existed in pre-fished ecosystems, because regional decimation and extinction of highly mobile predators prevents recovering the original diversity of predators at local scales, even at no-take areas. If so, more strict local fishing regulations are unlikely to restore the original diversity of high trophic level carnivores and restoration goals should be reframed in terms of an objective that is less unrealistic than restoring the pre-fished condition while still recovering aspects of the historical trophic structure.

本研究旨在评估渔业开发是否会影响肉食性鱼类的营养结构。我们对地中海及东北大西洋海域的捕捞开放区与海洋保护区内的渔民捕捞目标鱼种开展了普查，并分析了这些鱼种的碳、氮稳定同位素比值。研究结果显示，捕捞活动对游泳底栖肉食性鱼类的生物量与体型结构存在显著影响。然而，这些变化并未改变鱼类群落所利用的营养资源多样性、物种种间资源分隔模式，亦未改变营养冗余程度。本研究结果进一步佐证了近期的相关研究结论：在已开发捕捞的海域景观中设置的海洋保护区，可能无法恢复捕捞前生态系统原本存在的食物网结构——这是因为区域范围内高度移动捕食者的大量消亡与灭绝，使得即便在禁捕区内，也难以在局域尺度上恢复捕食者的原有物种多样性。若果真如此，更严格的地方性捕捞法规仍不太可能恢复高营养级肉食性鱼类的原有多样性，因此修复目标应当重新界定：在仍需恢复历史营养结构部分特征的同时，设定比恢复捕捞前状态更为现实的可行目标。