Detecting conservation benefits of marine reserves on remote reefs of the northern GBR

The Great Barrier Reef Marine Park (GBRMP) is the largest network of marine reserves in the world, yet little is known of the efficacy of no-fishing zones in the relatively lightly-exploited remote parts of the system (i.e., northern regions). Here, we find that the detection of reserve effects is challenging and that heterogeneity in benthic habitat composition, specifically branching coral cover, is one of the strongest driving forces of fish assemblages. As expected, the biomass of targeted fish species was generally greater (up to 5-fold) in no-take zones than in fished zones, but we found no differences between the two forms of no-take zone: ‘no-take’ versus ‘no-entry’. Strong effects of zoning were detected in the remote Far-North inshore reefs and more central outer reefs, but surprisingly fishing effects were absent in the less remote southern locations. Moreover, the biomass of highly targeted species was nearly 2-fold greater in fished areas of the Far-North than in any reserve (no-take or no-entry) further south. Despite high spatial variability in fish biomass, our results suggest that fishing pressure is greater in southern areas and that poaching within reserves may be common. Our results also suggest that fishers ‘fish the line’ as stock sizes in exploited areas decreased near larger no-take zones. Interestingly, an analysis of zoning effects on small, non-targeted fishes appeared to suggest a top-down effect from mesopredators, but was instead explained by variability in benthic composition. Thus, we demonstrate the importance of including appropriate covariates when testing for evidence of trophic cascades and reserve successes or failures.

大堡礁海洋公园（Great Barrier Reef Marine Park，GBRMP）是全球规模最大的海洋保护区（marine reserve）网络，但针对该生态系统中开发程度相对较低的偏远区域（即北部海域）内禁渔区（no-fishing zone）的实施效果，目前学界认知仍较为有限。本研究发现，保护区效应的检测颇具挑战性，而底栖生境（benthic habitat）组成的异质性——尤其是分枝珊瑚盖度（branching coral cover）——是驱动鱼类群落（fish assemblage）结构的最关键因素之一。正如预期，目标捕捞鱼类（targeted fish species）的生物量在禁捕区（no-take zone）普遍高于捕捞区（fished zone），最高可达5倍；但本研究未发现两种禁捕区类型——"no-take"与"no-entry"——之间存在显著差异。在偏远的远北近岸礁与中部外礁区域，分区管理的效应十分显著；但令人意外的是，在距离人类活动更近的南部海域，未检测到捕捞效应。此外，远北区域捕捞区内高度目标捕捞物种的生物量，几乎是南部所有保护区（禁捕或禁入）的2倍。尽管鱼类生物量存在较高的空间变异性，但研究结果表明，南部海域的捕捞压力（fishing pressure）更高，且保护区内的偷捕（poaching）行为可能较为普遍。研究结果同时显示，捕捞者存在“沿禁捕区边界捕捞”的行为模式：在大型禁捕区附近的开发海域，鱼类种群（stock size）规模会出现下降。有趣的是，针对小型非目标鱼类的分区管理效应分析，最初似乎指向中型捕食者（mesopredator）介导的下行控制效应，但该结果实则可通过底栖生境组成的异质性得到解释。因此，本研究证实，在检测营养级联效应（trophic cascade）以及评估保护区的成败时，纳入恰当的协变量至关重要。