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

1. 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. 2. We tagged and tracked juvenile white sharks in the northeastern 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. 3. 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. 4. 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. 5. 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.