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对肉食性鱼类的营养结构产生影响。我们对地中海及东北大西洋海域内允许捕捞区域与海洋保护区（marine protected areas）中的渔民目标捕捞鱼类进行了普查，并分析了其碳、氮稳定同位素比值。结果表明，捕捞活动对游移底栖（nektobenthic）肉食性鱼类的生物量与体型结构具有显著影响。不过，此类变化并未改变该鱼类集合群所利用的营养资源多样性、物种种间的资源分隔模式，亦未影响营养冗余程度。

本研究结果进一步补充了近期相关研究证据：在已受捕捞影响的海域景观中设立的海洋保护区，或无法恢复未受捕捞的原始生态系统中原有的食物网结构——究其原因，高度移动捕食者的区域性大量捕杀与灭绝，即便在禁捕区内，也会阻碍当地尺度下捕食者原始多样性的恢复。

若该结论成立，则更为严苛的地方性捕捞法规，仍难以恢复高营养级肉食性动物的原始多样性；因此，渔业修复目标应进行调整，设定为相较于恢复未受捕捞的原始状态更为切合实际的方案，同时仍需恢复历史营养结构的相关特征。