Ontogeny of dive performance in pup and juvenile Steller sea lions in Alaska

Abstract

Development of competent diving ability is critical to obtaining nutritional independence in marine mammals such as Steller sea lions (SSLs), Eumetopias jubatus (Schreber, 1776). We studied diving performance in pup (75) and juvenile (36) SSLs using satellite data recorders. In general, dives by SSLs were brief and shallow. Overall, 82.3% of dives were <2 min long and 86.9% of dives were <10 m deep. Long (>5 min) and deep dives (>100 m) constituted only 2.49% and 0.77%, respectively, of total dives. We used linear mixed-effects models to investigate the relationships between the response variables maximum-daily-depth, time-at-depth, mean-dive-duration, dive rate, and time-at-sea and the predictor variables age, sex, population (eastern and western Alaska populations), time-of-day, and month-of-year. All response variables except dive rate were positively related (P < 0.05) to age. Dive rate declined (P < 0.001) with age. Time-of-day, month, population, sex, and some first-order interactions were all significantly (P < 0.05) related to some measure of diving performance. With large samples we were able to identify significant relationships between the response variables and the predictor variables, even though the total amount of variation explained by the models was low, because most dives were short and shallow regardless of age, sex, population, time-of-day, or month-of-year. Depths and durations of dives by juvenile animals increased throughout the range of ages studied and were similar to or greater than those previously reported for juveniles and adult females. We expect maximum depths and durations to continue to increase with age until body mass plateaus at about 10 years of age. Therefore, we expect older animals to be more efficient foragers, as they would have greater aerobic dive limits as well as more experience locating and capturing prey.

Steller sea lion foraging response to seasonal changes in prey availability

“…Depths and durations of their dives are similar to adult females (Pitcher et al 2005) and juveniles and subadults dive to similar depths and durations (Rehberg and Burns 2008). A total of 111 individuals was fitted with satellite dive recorders (SDR, Wildlife Computers) that relayed point location estimates, on-land/ at-sea cycles and summarized dive characteristics (Pitcher et al 2005). For each individual, we linked the on-land/at-sea data to location estimates and used this information to identify which ones were present within the Frederick Sound study area during 6 wk windows centered around scat collections (median dates).…”

“…1). This area encompasses a variety of marine habitats (depth ranges between 5 and 480 m) that are typical of areas used by foraging sea lions in SE Alaska (Pitcher et al 2005). Most trips (> 90%) made by juvenile sea lions are <15 km from haulouts (Raum-Suryan et al 2004), while adult females appear to forage within 15 km during summer and may travel considerably farther during winter (Merrick & Loughlin 1997).…”

“…Although these tagging results apply only to juveniles, studies of the ontogeny of dive performance show that by 1.5 yr of age, juveniles are capable of sustained periods of long dives. Depths and durations of their dives are similar to adult females (Pitcher et al 2005) and juveniles and subadults dive to similar depths and durations (Rehberg and Burns 2008). A total of 111 individuals was fitted with satellite dive recorders (SDR, Wildlife Computers) that relayed point location estimates, on-land/ at-sea cycles and summarized dive characteristics (Pitcher et al 2005).…”

“…Individuals reached greater depths during December (maximum daily depth = 99 m) than May-July (85 m). Juvenile Steller sea lions are capable of diving deeper than these depths; their deepest dives reached 200 m by 1 yr of age and exceeded 400 m by 3 yr of age (Pitcher et al 2005).…”

A novel presence-only validation technique for improved Steller sea lion Eumetopias jubatus critical habitat descriptions

“…However, telemetry data on Steller sea lions have been collected at only a few sites and for a relatively small number of juveniles and adult females, and provides an incomplete picture of seasonal changes in distribution because tag deployment and duration of attachment is influenced by the timing of moult (Raum-Suryan et al 2004, Pitcher et al 2005. Given the evidence of site and season-specific foraging (Loughlin et al 1998, Raum-Suryan et al 2002, Gende & Sigler 2006, ecological signals relevant to identifying sea lion habitat may be obscured if telemetry data are combined from different sites or generalized from a single season.…”