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

Abstract

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.

Non‐trophic impacts from white sharks complicate population recovery for sea otters

“…After 2004, shark‐bitten live strandings are virtually absent or infrequent during this focused foraging period (Figure ). The underlying incentives during the migration‐foraging transition may be influential here (Benson et al, ; Lyons et al, ). Following a 3–6 week migration from distant oceanic habitat, adult sharks are likely resource depleted (Brown et al, ; Carlisle et al, ; Jorgensen et al, ) upon arriving at the coast.…”

“…Therefore, the recent (2015–2017; Figure ) cluster of strandings during these spring months may be attributed to immature white sharks that do not yet venture offshore (Hoyos‐Padilla et al, ). The seasonal movements of this age class are not well known (Benson et al, ; Lyons et al, ), but such demographics generally show strong environmental forcing (e.g., Ascani, Houtan, Lorenzo, Polovina, & Jones, ) and this appears to hold for white sharks as they develop endothermy (Weng et al, ; White, ). Due to episodic ocean warming, the thermal habitat envelope for juvenile white sharks has extended seasonally north of Point Conception into Monterey Bay (White, ), to within the core of the sea otter distribution.…”

Pop-up satellite archival tags reveal evidence of intense predation on large immature Chinook salmon (Oncorhynchus tshawytscha) in the North Pacific Ocean

“…For this analysis, we used a timesince-release time scale, in which Chinook salmon entered the model on the day of its deployment, which was considered day 0. Survivorship was then estimated across the monitoring period, and individual fish exited the model upon mortality (predation or unknown) or were right-censored on the pop-up date or the date of prematurely releasing from a Chinook salmon (Benson et al 2018;Fieberg and DelGiudice 2009). Premature release was assigned when a tag released from a fish and reported to satellites before the programmed release date for unknown reasons, but the tagged Chinook salmon appeared to be alive according to depth data immediately before the tag reported to satellites.…”

“…On a preprogrammed date, the tag releases from the fish, floats to the surface of the ocean and transmits data to satellites, which are then retrieved by project investigators. Because PSATs do not rely on recapture of tagged individuals for data retrieval, they are a valuable tool to assess survivorship, because data can be retrieved from individuals that experience mortality (Benson et al 2018;Lacroix 2014;Nielsen et al 2018). Therefore, the objectives of this study were to examine diagnostic evidence of predation on large Chinook salmon from depth, temperature, and light records collected during recent satellite tagging research.…”

Implications of life history uncertainty when evaluating status in the Northwest Atlantic population of white shark (Carcharodon carcharias)

“…There are several ways that fishing mortality or survival rates may change over ontogeny, where differences in survival or susceptibility to fisheries among ages are expected (Benson et al, 2018; Dewar et al, 2013). The annual survival rates for the two longevity scenarios were 89% for the short lifespan and 94% for the long lifespan.…”