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.

准确可靠的死亡率估算，长期以来都是海洋动物种群生态学研究中的关键空白。这一点在幼年个体上尤为突出：幼年个体通常是最脆弱的年龄组，其存活状况对种群增长具有极强的调控作用。目前已有数千个弹出式档案卫星标签（pop-up archival satellite tag, PAT）被应用于各类海洋物种，但此类数据的分析大多仅局限于运动生态学研究，以及渔业兼捕后的捕获后存活情况分析。本研究利用PAT数据，为一种海洋顶级捕食者的幼年个体提供了年度存活率与死因特异性死亡率的经验估算值。

我们在东北太平洋海域对幼年大白鲨进行了标记与追踪，旨在达成两项研究目标：其一，估算其存活率与竞争风险；其二，探究内在与环境因素对死亡风险的影响。此外，本研究还针对已知结局生存模型（known-fate survival models）的假设前提，评估了PAT数据的适用性。

幼年个体年度存活率为0.632（标准误SE=0.15），年度自然死亡率为0.08（SE=0.06），显著低于刺网误捕死亡率（0.48，SE=0.15）。死亡风险随个体体长增加而降低；相较于加利福尼亚海域的幼年鲨鱼，墨西哥海域的幼年鲨鱼死亡风险显著更高（风险比（hazard ratio）=9.05，SE=0.70）。

借助与渔民的合作关系，以及对部分鲨鱼部署的二级标记，本研究在绝大多数案例中均可明确判定个体结局。尽管在验证假设前提时仍需谨慎操作，但本研究结果表明，PAT数据是一种广泛可得却尚未充分开发的数据源，可大幅提升我们对海洋种群生态学的认知水平。

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死亡来源，这凸显了推行最佳操作规范的必要性，例如在意外捕获鲨鱼后尽快将其放归。此外，墨西哥海域的幼年鲨鱼死亡风险更高，因此在寒冷年份，幼年个体可能会向南迁徙以寻找温暖水域，其存活率或随之降低。这一现象可能会加剧幼年个体存活的随机性，并对该顶级捕食者的种群增长产生负面影响。