Wanted dead or alive: characterizing likelihood of juvenile Steller sea lion predation from diving and space use patterns

Abstract

Understanding linkages between behaviors and mortality risk is critical for managing populations. Juveniles constitute a particularly vulnerable life stage, with growing evidence that within stages, individual strategies may be associated with greater predation risk and mortality. These forms of predator−prey dynamics are rarely explored in marine environments due to difficulties in confirming vital status of individuals, and the lack of data sets that link mortality to behavior. We analyzed 2 concurrently collected data sets for juvenile Steller sea lions Eumetopias jubatus in the Gulf of Alaska to examine associations between mortality and specific behavioral patterns. Forty-five juvenile Steller sea lions were instrumented with external satellite tags and internally implanted vital rate transmitters (LHX tags). From 2005 through 2018, 25 juveniles remained alive and 20 died (18 confirmed predation). Using a binomial generalized linear mixedeffects model, we tested whether the probability of individual mortality was associated with seasonally specific dive patterns (time wet, dive depth) or horizontal movement patterns (home range size, average trip distance). Additionally, in 3 cases, external tags were transmitting until death. To examine links between fine-scale ante-mortem behavioral patterns and predation susceptibility, we compared dive patterns recorded during the last 3 d of data transmissions (both for animals which survived and for those which died) to dive patterns recorded during other periods at sea. Results suggest individuals that spent more time dry or dove shallower had a greater mortality probability, which could reflect foraging tactics of predators. This study highlights the effectiveness of combining multiple telemetry systems for exploring the vulnerability of individuals to the consumptive effects of predators.

An Integrative Method for Characterizing Marine Habitat Features Associated With Predation: A Case Study on Juvenile Steller Sea Lions (Eumetopias jubatus)

“…Distance from rookeries and haulouts had a slight positive association with predation risk increasing up to 20 km, but at greater distances the risk diminished again. This finding may appear to contradict the observation by Bishop et al (2019) of slightly elevated predation risk for individuals with increased haulout times during summer, which must occur near rookeries and haulouts, even while overall risk was lower in summer than winter. This disconnect may be due to the differences in temporal scales across studies.…”

“…Overall, between the present study and Bishop et al (2019), the observed diving, haulout, spatial and habitat use associated with resource-risk trade-offs are consistent with a mix of at least two predator types-transient killer whales and Pacific sleeper sharks, as proposed and theoretically modeled by Frid et al (2009). Interestingly, the observations by Bishop et al (2019) that may indirectly suggest closer-to-rookery predation in the summer were derived primarily from telemetered sea lion dive behavior data and were informed by predation event timing, but not location. The positive association of predation risk with deeper bathymetry, and distance from rookeries we report here were derived from predation event telemetry timing and location, and was only spatially constrained by telemetered sea lion movement.…”

“…Out of the 45 LHX tagged animals, 20 mortalities were detected between November 2005-December 2018 (Horning and Mellish, 2014;Horning et al, 2017b;Bishop et al, 2019). Two of the detected mortalities yielded insufficient data to determine the proximate cause of mortality and were excluded from analysis.…”

“…Instead, predation risk in marine systems is derived in terms of non-consumptive effects, through indirect measures of behavioral changes and changes in space use for prey, relative to predator distributions or space-use (Wirsing et al, 2008;Hammerschlag et al, 2015;Breed et al, 2017). However, with the development of novel biotelemetry devices, recent efforts have begun to address consumptive predator-prey dynamics in marine ecosystems Mellish, 2012, 2014;Bishop et al, 2019;Seitz et al, 2019). Internal VHF devices and mark-recapture efforts have been used to successfully monitor survival of marine animals for periods beyond 1 year, but detection range, regional coverage, and determination of mortality locations are still limited using these methods (Horning and Hill, 2005).…”

Demystifying individual heterogeneity

“…Empirical research shows that there are countless processes underlying both fixed and dynamic condition (e.g. Bishop et al, 2019; Bowen et al, 2015; Gould et al, 2018; Paoli et al, 2020), although some of these processes contribute more often to one type of individual heterogeity than the other. For example, among‐individual variation in genotypes will generally produce fixed condition in vital rates; however, mutations in the genome may lead to dynamic condition.…”