Boyd Porter

MSc 1997

Dr Andrew Trites

Winter ecology of Steller sea lions (Eumetopias jubatus) in Alaska.

The goal of this study was to describe aspects of the winter ecology of Steller sea lions (Eumetopias jubatus) from behavioural observations made at a winter haulout between January and April 1996 in Southeast Alaska (Timbered Island, 133° 48′ W, 55° 41′ 45″ N). My major hypotheses were: 1) Mothers suckle male offspring more than female offspring, and spend less time on shore to provide for the higher metabolic needs of males. 2) Mothers with yearlings have higher demands than those with pups and should spend more time nursing and more time at sea foraging. Another important goal was to describe the weaning process.

Mature females and their dependent young (pups age < 1 y and yearlings 1-2 y) predominated on the winter haulout while, numbers of mature male and sub-adult males remained low. There was considerable variability in numbers of animals on shore from one day to the next. A Drop-one Stepwise regression showed numbers on shore were correlated with weather and sea conditions. The ranked order of important variables included: wave height, air temperature, wind’ speed, wind direction, sky cover, barometric pressure, ocean swell, and tide height. Haul-out patterns were negatively correlated with poor weather and rough sea conditions. No diurnal haul-out pattern was detected. Two measures of maternal investment were made: the time spent suckling by different sex and age class (pups and yearlings), and the time their mothers spent on land and at sea foraging.

Differences between the average length of male pup-suckling bouts (x = 37.2 ± 2.8 min. SE, n=12) were not significantly longer than female suckling time (x = 35.9 ± 2.3 min., n=12). However, yearlings suckled significantly longer (x = 44.0 ± 2.8 min.SE, n=5) than pups (x = 36.5 ± 1.8 min., n=24). Males spent an average of 33.9% (± 1.1 SE) of the time they were with their mothers onshore suckling compared to 29.9% (± 4.4) by female offspring, but the difference was not statistically significant, and the proportions did not change during the 3 month study period.

Average trips to sea by mothers with yearlings lasted 2.3 d (x = 59.7 ± 4.7 h SE, n=6) and were significantly longer than the 1.9 d average trips of mothers with pups (x = 48.0 ± 6.3 h, n=18). Mothers with male pups (< 1 y) were away for an average of 2.2 d (x = 52.5 ± 15.2 h SE, n=12) compared to 1.8 d (x = 43.5 ± 12.6 h, n=12) for those with female pups. Mothers spent an average of 35.2 h (± 7.0) on land with male offspring and 34.5 h (± 6.0) with females before departing on foraging trips. Mature females (n=30) showed little seasonal variation in trip duration. Shore visits averaged 19.4 h between trips to sea (n=30, SE=0.90, range 11.2-29.2h). Differences in time spent on shore between offspring genders and age classes were not statistically significant. Lactating females spent an average of 14.2 ± 1.1% SE (n=30) of their time on shore while immature animals spent three times as long (40.2 ± 2.1%). Average rotation time (feeding/resting cycle) from sea to the haulout by mature females was 3.0 days and was consistent through the season. Weaning of pups and juveniles was not observed between January and April. Haul-out patterns during the winter were influenced more by disturbance, sea and weather conditions than by endogenous rhythms.


Winter ecology of Steller sea lions (Eumetopias jubatus) in Alaska. Porter, B. 1997. Masters thesis, University of British Columbia, Vancouver BC. 84 pages (PDF)

Insights into the Timing of Weaning and the Attendance Patterns of Lactating Steller Sea Lions (Eumetopias jubatus) in Alaska During Winter, Spring and Summer. Trites, A.W., B.P. Porter, V.B. Deecke, A.P. Coombs, M.L. Marcotte and D.A.S. Rosen. 2006. Aquatic Mammals 32:85-97. (PDF)

Suckling attempts during winter by two non-filial Steller sea lion pups (Eumetopias jubatus). Porter, B.T. and Trites, A.W. 2004. Mammalia 63:23-26. (PDF)

Attendance patterns of Steller sea (Eumetopias jubatus) lions and their young during winter. Trites, A.W. and B.T. Porter. 2002.   (PDF)