Attendance Patterns of Steller Sea Lions and Their Young During Winter

Steller sea lions are born during the summer (late May to early July). Most pups wean before their first birthday but some nurse for longer. The first winter following birth has generally been thought to be a critical stage in the life history of young Steller sea lions and may be the key to understanding the population decline that has occurred in most parts of Alaska. Unfortunately little is known about the life history of Steller sea lions during winter.

Timbered island off the coast of Southeastern Alaska

Boyd Porter and Andrew Trites documented the attendance patterns (time at sea and time on shore) of lactating Steller sea lions and their young during winter. Armed with binoculars and many thermoses of coffee, they watched mothers with pups (7-9 months old) and yearlings (19-21 months old) from Jan 22 to Apr 1 at a non-breeding haulout site (Timbered Island) in Southeast Alaska.

Trites and Porter discovered that the winter attendance cycle of lactating females (consisting of one trip to sea and one visit on land) was longer (3 days) than that of their pups and yearlings (~2 days). This difference suggests that dependent pups and yearlings do not accompany their mothers on foraging trips. Remaining near shore probably reduces the risk of predation, as well as the energetic cost of a pup accompanying its mother. It may also help explain why they do not appear to dive as deeply as their mothers if they are staying closer to shore in shallow water.

Trites and Porter also observed that pups and yearlings tended to stay on or near the haulout while their mothers were away and showed no signs of weaning or consuming solid food during winter. This suggests that weaning does not occur during winter (as previously thought), but later in the spring - April to June.

Complete details about this study are contained in Trites and Porter (2002).

3 April 2002