Ecosystem modelling

The seagrass ecosystem of Shark Bay is characterised by top-down control through tiger sharks. Adult tiger sharks migrate to shallow habitats in Shark Bay during the austral summer. During such times of high tiger shark abundance, the megafauna of the ecosystem leave the seagrass meadows. Despite the fact that only some species of the megafauna are major components of the adult tiger sharks diet, the majority of these species and individuals are impacted by the presence of tiger sharks and leave the feeding grounds in the seagrass beds in summer. This control system has the potential to become imbalanced if, in common with other populations of this species worldwide, there is a decrease in the abundance of adult tiger sharks in the Shark Bay population. Targeting of tiger sharks by fishers in the waters of Northern Australia and Indonesia has increased steadily during the last years, which might impact the tiger shark stock in Shark Bay. Using qualitative modelling, model analyses indicate a strong top-down control of tiger sharks. By implementing a direct and behaviour-induced impact, tiger sharks affect the use of the seagrass habitat by the megafauna and thus impose alternating predation pressure on different prey groups within the seagrass habitat in Shark Bay. Tiger sharks are clearly affected by the Northern Shark Fishery, but the activities of recreational fishermen within Shark Bay reduce the prey availability for juvenile tiger sharks, which might negatively affect the whole tiger shark population and thus may impact the dynamics of this seagrass ecosystem. Recognizing the ecological importance of this species and the potential impact that further decrease of the population is likely to have on the ecosystem, it is important that appropriate management action is taken to maintain the abundance of the tiger shark population in Shark Bay.

鲨鱼湾海草生态系统的核心调控机制为虎鲨介导的下行控制（top-down control）。成年虎鲨会在南半球夏季洄游至鲨鱼湾的浅海生境。在此类虎鲨种群密度较高的时段，该生态系统中的大型动物群会撤离海草草场。尽管该大型动物群中仅有部分物种是成年虎鲨的主要猎物，但绝大多数此类物种与个体仍会因虎鲨的存在而受到惊扰，并在夏季逃离海草床内的觅食区域。若如同全球范围内该物种的其他种群一样，鲨鱼湾的成年虎鲨种群数量出现下降，这套下行调控系统便有可能失衡。过去数年中，澳大利亚北部与印尼海域的渔民针对虎鲨的捕捞量持续攀升，这可能会对鲨鱼湾的虎鲨种群资源造成影响。通过定性建模开展的模型分析显示，虎鲨对该生态系统存在显著的下行控制作用。虎鲨通过直接作用与行为介导的影响，改变了大型动物群对海草生境的利用模式，进而对鲨鱼湾海草生境中的不同猎物类群施加交替变化的捕食压力。虎鲨显然会受到北部鲨鱼渔业的影响；而鲨鱼湾内部的休闲垂钓活动，则会降低幼年虎鲨的猎物可获得性，这可能会对整个虎鲨种群产生负面影响，进而干扰该海草生态系统的动态平衡。鉴于虎鲨的生态重要性，以及种群数量进一步下降可能对该生态系统造成的潜在影响，采取恰当的管理措施以维持鲨鱼湾虎鲨种群的规模，显得尤为关键。