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.

研究背景：作为休闲垂钓者与商业渔业捕捞目标的鲨鱼物种，若捕捞作业未基于不同年龄/体型组的相对分布与生长数据进行指导与监管，则极易遭受过度捕捞风险。
研究目标：本研究旨在探究南非南部海域短鳍真鲨（Carcharhinus brachyurus）的时空分布、移动模式与生长速率。
研究方法：我们分析了一份历时36年的合作岸钓标记重捕（tag-recapture）数据集。
主要研究结果：南半球夏季时，该物种的分布集中于纳米比亚与南非南海岸。21头个体的大规模区域移动证实，南非南部海域的该种群不存在种群结构。南非南海岸是一处潜在的重要育幼区，此处标记的幼鲨占所有幼鲨标记总量的96%。亚成体与成体的分布范围更广，放流后自由活动时长显著长于幼鲨，但成体的移动距离显著更低，表明成体具有更高的栖息位点忠实性。生长模型预测结果显示，与其他真鲨科（Carcharhinidae）物种相比，短鳍真鲨的年生长速率处于较慢水平。
结论与启示：上述缓慢的生活史特征、对近岸海域的栖息偏好以及跨境移动特性，使得南非南部海域的短鳍真鲨种群在缺乏区域协同研究与管理的背景下，极易遭受过度捕捞威胁。