Movements and growth rates of tagged bronze whaler sharks (Carcharhinus brachyurus) in southern Africa

Context: Shark species that are targeted by recreational anglers and commercial fisheries are vulnerable to over exploitation when fishing effort is not informed or regulated by data on the relative distribution and growth of different age/size classes.
Aims: We investigate the spatiotemporal distribution, movement patterns and growth rates of Carcharhinus brachyurus in southern Africa. 
Methods: We analysed a 36-year cooperative shore-angling tag-recapture dataset.
Key results: Distribution was centred in Namibia and the South Coast of South Africa during the austral summer. Large-scale regional movement of individual sharks (n = 21) supports the lack of population structure in southern African. The South Coast of South Africa represented a potentially important nursery region where 96% of all juveniles were tagged. Sub-adults and adults were more widely distributed and recorded significantly greater time at liberty than juveniles, but the distance moved for adults was significantly lower, indicating higher site fidelity. Growth model predictions showed annual growth rates of C. brachyurus were among the slowest when compared to other carcharhinids.
Conclusions and implications: These slow life-history traits, affinity to coastal regions and cross-border movements leave the southern African C. brachyurus population vulnerable to overexploitation in the absence of regionally aligned research and management.

背景：针对休闲钓鱼者和商业渔业的鲨鱼种类，在缺乏关于不同年龄/体型级别相对分布和生长数据的指导或监管下，容易遭受过度开发。目标：本研究旨在探讨南非地区Carcharhinus brachyurus的时空分布、迁徙模式和生长速率。方法：我们分析了长达36年的合作海岸钓鱼标签-重捕数据集。关键结果：在澳大利亚夏季，分布中心位于纳米比亚和南非南岸。21条个体鲨鱼的区域性大规模迁徙支持了南非地区缺乏种群结构的事实。南非南岸可能是一个重要的育幼区，其中96%的幼鲨被标记。亚成体和成体分布更广，且记录的自由时间显著长于幼鲨，但成体的移动距离显著较低，表明更高的地域忠诚度。生长模型预测显示，与其它鲨鱼科相比，C. brachyurus的年生长速率最为缓慢。结论和影响：这些缓慢的生命史特征、对沿海地区的偏好以及跨境迁徙使得南非地区的C. brachyurus种群在没有区域协调研究和管理的背景下，容易遭受过度开发。