Data from: Juvenile survival, competing risks, and spatial variation in mortality risk of a marine apex predator

Reliable estimates of mortality have been a major gap in our understanding of population ecology for marine animals. This is especially true for juveniles, which are often the most vulnerable age class and whose survival can strongly influence population growth. Thousands of pop‐up archival satellite tags (PAT) have been deployed on a variety of marine species, but analysis of these data has mainly been restricted to movement ecology and post‐handling survival following fisheries bycatch. We used PAT data to provide empirical estimates of annual survival and cause‐specific mortality for juveniles of a marine top predator.
We tagged and tracked juvenile white sharks in the north eastern Pacific Ocean to (1) estimate survival rates and competing risks and (2) investigate intrinsic and environmental influences on mortality risk. We also evaluated the use of PAT data with respect to meeting assumptions of known‐fate survival models.
Annual juvenile survival rate was 0.632 (SE = 0.15) and annual natural mortality rate (0.08, SE = 0.06) was lower than the rate of gillnet interactions (0.48, SE = 0.15). Mortality risk decreased with greater body length and was significantly greater (hazard ratio = 9.05, SE = 0.70) for juvenile sharks in Mexican waters, relative to California waters.
The PAT data allowed for unambiguous determination of fate in most cases, aided by collaborative relationships with fishers and secondary tags deployed on a subset of sharks. Although caution must be exercised to establish whether assumptions are met, our work demonstrates that PAT data represent a widely available, untapped data source that could dramatically increase our understanding of marine population ecology.
Synthesis and applications. Our research shows fisheries bycatch to be the main source of mortality for juvenile white sharks in the northeastern Pacific Ocean, highlighting the need for best practices, such as releasing sharks quickly following incidental capture. Furthermore, mortality risk for juveniles was greater in Mexican waters, such that survival may be lower in colder years when juveniles are likely to move south seeking warmer water. This could increase stochasticity in juvenile survival and negatively influence population growth for this apex predator.

可靠的死亡率估算，始终是海洋动物种群生态学认知体系中的核心空白之一。对于幼体而言更是如此——它们往往是种群中最脆弱的年龄组，其存活状况会对种群增长产生极强的调控作用。目前全球已有数千个弹出式档案卫星标记（pop-up archival satellite tag, PAT）被部署于各类海洋物种身上，但相关数据的分析大多仅局限于运动生态学以及渔业兼捕后的存活情况研究。本研究借助PAT数据，针对一种海洋顶级捕食者的幼体，开展了年存活率与致死原因特异性死亡率的实证估算。
我们在东北太平洋海域对幼年大白鲨进行标记与追踪，旨在达成两个研究目标：其一，估算其存活率与竞争风险（competing risks）；其二，探究内在与环境因素对死亡风险的影响。此外，本研究还针对已知命运存活模型（known-fate survival model）的假设前提，评估了PAT数据的适用性。
幼年个体的年存活率为0.632（标准误SE=0.15），年自然死亡率为0.08（SE=0.06），显著低于与刺网接触的死亡率（0.48，SE=0.15）。死亡风险随体长增加而降低；相较于加利福尼亚海域的幼鲨，墨西哥海域幼鲨的死亡风险显著更高（风险比hazard ratio=9.05，SE=0.70）。
借助与渔民的合作关系以及对部分个体部署的次级标记，PAT数据可在绝大多数情况下明确判定个体的命运结局。尽管在验证假设前提时仍需谨慎行事，但本研究证实，PAT数据是一种广泛可得却尚未充分开发的数据源，可极大推动我们对海洋种群生态学的认知。
综合与应用。本研究表明，渔业兼捕是东北太平洋幼年大白鲨的主要致死因素，这凸显了推广最佳操作规范的必要性，例如在偶然捕获后尽快放归鲨鱼。此外，墨西哥海域的幼鲨死亡风险更高，因此在气候较冷的年份，幼鲨可能会向南迁徙以寻找温暖水域，其存活率或有所下降。这会加剧幼体存活的随机性，对这一顶级捕食者的种群增长产生负面影响。