Risky business: consistent breeding migration in a benthic shark despite environmental variability

The timing of migration is essential for the survival and reproductive success of migratory species. Environmental and biological factors can influence the breeding success of these migrations. However, there is a lack of long-term studies on benthic migratory species that analyse their response to climatic variation. Climate change may threaten the breeding migration of species that rely on environmental cues to initiate migration. Port Jackson sharks, Heterodontus portusjacksoni, display high philopatry to their breeding grounds. Using passive acoustic telemetry, this study investigated the annual migration patterns of Port Jackson sharks at a breeding aggregation site in Jervis Bay, Australia. The influence of biological (sex and size) and environmental factors (temperature, East Australian current strength, rainfall, moon phase and period of the day) on these patterns was assessed over a 7-year period (2013–2019). The results of the linear mixed-effects models revealed annual and individual variability in migration timing, with males arriving and departing earlier than females. There was no influence of environmental variables on the migration timing of Port Jackson sharks. We found that more sharks arrived and departed at night. Overall, there was significant repeatability in migration timing for both arrival and departure. This interannual individual consistency has important implications for breeding success at the population level. Migration patterns that are unresponsive to environmental cues may lead to a mismatch between migration timing and suitable conditions for juvenile survival.迁徙时机对于洄游物种的生存与繁殖成功至关重要。环境与生物因子可影响此类洄游的繁殖成效。然而，目前尚缺乏针对底栖洄游物种响应气候变化的长期研究，气候变化或会威胁依赖环境线索启动迁徙的物种的繁殖洄游过程。杰克逊港鲨（Port Jackson sharks，学名*Heterodontus portusjacksoni*）对其繁殖场表现出高度的归巢性（philopatry）。本研究借助被动声学遥测（passive acoustic telemetry）技术，针对澳大利亚杰维斯湾一处繁殖聚集场的杰克逊港鲨展开年度洄游模式调查，并在2013至2019年的7年周期内，评估了生物因子（性别与体型）及环境因子（水温、东澳洋流强度、降雨量、月相以及每日时段）对该洄游模式的影响。线性混合效应模型（linear mixed-effects models）结果显示，迁徙时机存在年度与个体间差异，雄性的抵达与离开时间均早于雌性；未发现环境变量对杰克逊港鲨的迁徙时机存在显著影响。研究发现，更多的杰克逊港鲨于夜间完成抵达与离开行为。总体而言，杰克逊港鲨的抵达与离开时机均存在显著的可重复性。这种年际个体一致性对种群层面的繁殖成功具有重要意义。对环境线索无响应的洄游模式，可能会导致迁徙时机与幼体生存所需的适宜环境条件之间出现错配。