The duration of offspring care is critical to female fitness and population resilience by allowing flexibility in life history strategies in a variable environment.  Yet, for many mammals capable of extended periods of maternal care, estimates of the duration of offspring dependency are not available and the relative importance of flexibility of this trait on fitness and population viability has rarely been examined.  We used data from 4,447 Steller sea lions Eumetopias jubatus from the Gulf of Alaska and multistate hidden Markov mark-recapture models to estimate age-specific weaning probabilities. Maternal care beyond age 1 was common: weaning was later for animals from Southeast Alaska (SEAK) and Prince William Sound (PWS, weaning probabilities: 0.538–0.648 / 0.784–0.873 by age 1 / 2) compared to animals born to the west (0.714–0.855 / 0.798–0.938).  SEAK/PWS animals were also smaller than those born farther west, suggesting a possible link.  Females weaned slightly earlier (+0.080 at age 1 and 2) compared with males in SEAK only.  Poor survival for weaned versus unweaned yearlings occurred in southern SEAK (female survival probabilities: 0.609 versus 0.792) and the central Gulf (0.667 versus 0.901), suggesting poor conditions for juveniles in these areas.  First-year survival increased with neonatal body mass (NBM) linearly in the Gulf and non-linearly in SEAK.  The probability of weaning at age 1 increased linearly with NBM for SEAK animals only. Rookeries where juveniles weaned at earlier ages had lower adult female survival, but age at weaning was unrelated to population trends.  Our results suggest the time to weaning may be optimized for different habitats based on long-term average conditions (e.g., prey dynamics), that may also shape body size, with limited short-term plasticity.  An apparent trade-off of adult survival in favor of juvenile survival and large offspring size in the endangered Gulf of Alaska population requires further study.

Mark-resighting data; capture histories are included.

ch = capture history; S = seen suckling that occasion, u = seen that occasion but not suckling, 0 = not seen that occasion

sx (sex) = (M)ale and (F)emale

mass = mass at marking adjusted for age

SEAK: Southeast Alaska

occasions were 2001 (first occasion) + 5 occasions per year 2002-2018

nr (natal rookery) = (F)orrester, (H)azy, (W)hite Sisters, Gra(V)es Rocks

GOA: Gulf of Alaska

occasions were 2000 (first occasion) + 10 occasions per year 2001-2016

nr (natal rookery) = (J) PWS, (X) Sugarloaf, (T) Marmot, (A) Ugamak