Field studies of Steller sea lions (Eumetopias jubatus) at Marmot Island, Alaska, 1979 through 1994

Abstract

The Alaska Department of Fish and Game and the National Marine Fisheries Service conducted field studies of Steller sea lions (Eumetopias jubatus) on Marmot Island, Alaska, from 1979 through 1994. Marmot Island is one of 15 trend sites and four trend rookeries used to identify population trends in the central Gulf of Alaska. Prior to the 1970s, Marmot Island was one of the largest Steller sea lion rookeries in Alaska. Since the 1970s, numbers of non-pup (adults, sub-adults, and juvenile) sea lions observed on Marmot Island during the breeding season declined dramatically: 87.1% since 1979. From 1976 to 1994, non-pups declined 76.9% at the 14 other trend sites and 79.1% at the three other trend rookeries in the central Gulf of Alaska. The proportion of juvenile sea lions on Marmot Island also declined, from 15 to 35% of all nonpups during 1979 and 1983, respectively, to 5% or less during 1987 through 1994. The decline in pup numbers were commensurate with those for non-pups. From 1979 to 1994, pup numbers declined 88.1% at Marmot island and 84.4% at the other three trend rookeries in the central Gulf of Alaska. From 1991 to 1994, pup production declined by 50.1% at Marmot Island and by 40-50% at two of the other central Gulf rookeries. Since 1979, four rookery sites on Marmot Island have either become haulouts or have been abandoned entirely.

Non-pup counts were made from 12 through 29 June on Marmot Island since counts made during these dates were likely to be 90% or more of the maximum number of sea lions counted during the breeding season. The hours of 1100 to 1400 ADT or 0900 to 1700 ADT were optimal for observing 90% and 80%, respectively, of daily maximum counts. Optimal dates for counting pups were 29 June through 9 July. Beach 3Z, a rookery beach occupied by relatively few sea lions during the breeding season, was the preferred haul-out site from December through March or April. How and when sea lions make the fall and spring transitions between breeding season and non-breeding season distribution could not be determined.

During June and July of 1987 and 1988, 751 pups (390 females, 361 males) were branded and tagged on Marmot Island. Through December 1994, 151 (20.1%: 78 females and 73 males) of these animals were relighted on 204 occasions. Most resightings (116 of 204: 56.9%) were from Marmot Island; an additional 33 resightings (16.2%) were from within a radius of about 75 km. The resighting most distant from Marmot Island, approximately 1,700 km, was at Loretta Island, British Columbia, Canada. All resightings except five were of live animals. The observed numbers of resightings were significantly below the number expected to be alive (based on life tables) at any given age. Nine branded females (2.3%) were observed with pups during 1993 or 1994: five were on Marmot Island and three were on the Sugarloaf Island rookery, about 75 km north of Marmot Island. One female with a pup was seen during March 1994 on Latax Rocks, approximately 70 km north of Marmot Island. It is not known on which rookery that pup was born.


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