Fur seal investigations, 1992

Abstract

This report is a collection of papers resulting from northern fur seal (Callorhinus ursinus) research conducted by the National Marine Mammal Laboratory in 1992 on the Pribilof Islands (10 July - 1 September) in the Bering Sea, and San Miguel Island (20 May - 1 November) off southern California.

Since 1991, harem bull counts increased 15.5% and 39.8% on St. Paul and St. George Islands, respectively. The total number of pups born on St. Paul Island (182,468 ± 19,263) was estimated from shear sampling surveys conducted during 17 August -1 September and was not significantly different from the 1989 and 1990 estimates. The total number of pups born on St. George Island (25,160 ± 1,730) was estimated from surveys conducted during 14-17 August and was not significantly different from the 1990 estimates.

A tagging project was initiated in 1987 to examine variability in population dynamics. The 1992 studies based on this tagged population support past findings that there is a tendency for males to be resighted near their natal rookery; that the date of first sighting is significantly related to age (P < 0.001) (older males arrive earlier in the season than younger ones); and a significant relationship exists between male pup weight and weight at age 2 (P = 0.001, n = 20) and age 3 (P = 0.006, n = 81), but not at  age 4 (P = 0.07, n = 78) or age 5 (P = 0.18, n = 15).

The estimated incidence of entanglement in net debris among female fur seals on St. Paul Island was 0.062% for 1991 and 0.036% for 1992 with a 2-year mean of 0.046%. This is a reduced level of entanglement compared with a similar study in 1985 which showed entanglement rates of 0.06%-0.23% and a mean of 0.14%. The proportion of juvenile males observed entangled in 1992 was 0.29%. The frequency of occurrence of trawl webbing among entangling debris in 1992 was about one-half that observed prior to 1988. The proportion of seals entangled in other types of debris did not change. The 1992 studies on male entanglement confirm earlier estimates that the annual survival of seals entangled in small debris is about one-half that of nonentangled seals. Seals from which debris was removed had significantly (P < .05) higher survival rates than those which remained entangled.

The maximum number of territorial bulls on San Miguel Island was 72 on 8 July. Live pup counts on 29 July and 5 August totaled 1,124 and 965, respectively. on 11 August, 274 dead pups were counted. One hundred and fifty pups were tagged on 27 September and another 150 were tagged on 1 November. Mean pup weights for 1992 were significantly lower (P < 0.001) than mean pup weights for all non-El Niño years between 1977 and 1991, but significantly greater (P < 0.001) than those of the          El Niño years of 1976 and 1983. This indicates that while the health of preweaned pups was compromised during the 1991-1992 El Niño, it was not as detrimental as previous warm water events.


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