Since the 1930s northern fur seals (Callorhinus ursinus) have been observed entangled (usually around their necks and shoulders) in various objects (Scheffer, 1950). Just after the Second World War, seals were observed caught in rubber rings thought to be of military origin. At that time less than 0.004% of the commercially harvested juvenile males were observed to be entangled. The incidence of entanglement among young males taken in the commercial harvest increased, especially in the late 1960s when commercial fishing effort increased and synthetic materials became important in the construction of nets (Fowler 1987). During the late 1970s and early 1980s the entanglement rate on St. Paul Island averaged about 0.4% among juvenile males seals observed on land.

Entanglement of northern fur seals in marine debris has been studied using several methods since the 1960s by the National Marine Fisheries Service (NMFS) in cooperation with the Japanese National Research Institute of Far Seas Fisheries (NRIFSF) and the Aleut communities of St. Paul and St. George Islands. Surveys of entanglement among juvenile male fur seals were conducted in conjunction with the commercial harvest from 1967 through 1985 (Scordino and Fisher 1983, Scordino 1985) and using research roundups after the cessation of the commercial harvest (Bengtson et al. 1985, Fowler 1987, Fowler and Baba 1991, Fowler et al. 1992). Studies from 1988 to 1992 indicated a decline in the rate of entanglement among both juvenile males (Fowler and Ragen 1990, Fowler et al. 1992) and females (Kiyota and Fowler 1992) on St. Paul Island.

In 1996, the NMFS, the St. Paul and St. George Islands Tribal Councils, and the Pribilof Islands Stewardship Program, monitored juvenile and adult male fur seal entanglement using a combination of research roundups and surveys during the subsistence harvest. By conducting surveys in conjunction with the subsistence harvests, seals are disturbed less often.

The objective of this study, which was begun in 1995, is to determine current trends in the rate of observed on-land entanglement of northern fur seals in marine debris on St. Paul and St. George Islands. This information is being collected in order to provide: 1) a continuing index of entanglement rates, 2) a comparison of entanglement rates on St. Paul and St. George Islands, 3) a means of indirectly assessing the relative amount of entangling debris within the habitat of the fur seal, and 4) an assessment of the proportion of debris types associated with different fisheries that are impacting fur seals.

Twenty-three subsistence harvest surveys and 30 research roundups were conducted on St. Paul Island (53 total) and 26 research roundups and 9 harvest surveys (35 total) were conducted on St. George Island during July and early August of 1996. Observers counted 38,311 seals and 10,763 seals (all age classes combined) on St. Paul and St. George Islands, respectively. Samples included 24,701 juvenile males (2-4 years old) on St. Paul Island and 6,057 juvenile males on St. George island. Seventy-one entangled juvenile and adult male seals were captured, examined and the debris was removed (56 on St. Paul Island and 15 on St. George Island). Forty-three seals with scars indicating evidence of previous entanglement were also observed during surveys. Twenty-one of these seals were adult males, some of which had fresh, open wounds indicating their debris was removed or lost during 1996. An additional 47 male seals, 14 female seals, 25 seals of unknown sex and approximately 7 pups were captured and disentangled during other research activities from late June through November.

As in previous years, entangling debris consisted primarily of pieces of trawl net, plastic packing bands and loops of synthetic or natural twine. No seals entangled in monofiliment gillnet were observed during male entanglement surveys in 1996. Differences in the relative percentage of entangling debris were observed between age classes of seals (Figure 1). Packing bands were the most common (68.4%) debris type in which adult males were entangled on St. Paul Island. On St. George Island, only three adult males were observed entangled, one in each of the three major debris types. Trawl net comprised the largest proportion of entangling debris among juveniles on both islands (43.2%, and 50.0% on St. Paul and St. George Island respectively), followed by packing bands (32.4% on St. Paul and 25.0% on St. George). The observed incidence of loops of twine on St. George Island decreased from 29.4% in 1995 to 15.8% in 1996. As in 1995, more entanglement in packing bands was observed on St. Paul (44.6%) relative to St. George Island (26.7%) for all age classes combined.

The rate of entanglement was estimated for both adult and juvenile male fur seals as the ratio of all entanglement sightings (initial and subsequent sightings for juveniles) to the total number of animals observed in each age class. The rate of entanglement for juvenile males was calculated as 0.23% (56/24,701) on St. Paul Island and 0.21% (13/6,057) on St. George Island. Among adult males, the rate of entanglement was calculated as 0.14% (19/13,610) on St. Paul Island and 0.06% (3/4,706) on St. George Island. The incidence of entanglement among juvenile males on St. Paul Island is within the range of entanglement rates observed from 1988 to 1992 (Figure 2) and in 1995 (Robson et al. In press). The difference in the juvenile entanglement rate on St. George Island between 1995 and 1996 may be due to a buildup of entangled seals on prior to the initiation of an organized effort to capture and remove debris from entangled seals in 1995.

Fowler et al. (1993) attributed decline in the rate of entanglement on St. Paul Island from a mean rate of 0.4% between 1976 and 1985 to current levels to a reduction in the fraction of seals entangled in trawl net fragments. The observed decline in the rate of entanglement among northern fur seals may reflect a reduction in the amount of debris introduced into the marine environment. Consistent with the data for debris on northern fur seals, the abundance of trawl webbing debris observed on sampled beaches of several Alaskan islands has also declined in recent years (Johnson 1990). Information on fur seal entanglement and incidence of debris on beaches may indicate progress in reducing marine pollution resulting from national and international educational and regulatory efforts in recent decades such as the Convention for the Prevention of Pollution from Ships (MARPOL 73/78). 

References

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