The Steller sea lion (Eumetopias jubatus), an iconic and endangered species of the Pacific north coast, is a facing a new and poorly understood threat.
In a draft biological opinion , released 2 August by the US National Marine Fisheries Service (NMFS), scientists report that the adult population of Steller sea lions around the western-most Aleutian islands plunged 45% from 2000 to 2009 while the birth rate of new pups dropped 43%.
The results are worrying enough, the document suggests, to merit a ban on some commercial fishing in the area to prevent malnutrition in the marine mammal’s most critical habitat.
“We need to mitigate the potential for competition for food in areas where the sea lion numbers are declining,” says Jim Balsiger, regional administrator for Alaska fisheries, in a NMFS-issued news alert. No definitive cause of the population decline is identified, but the sea lions’ food supply is considered crucial to recovery. Steller sea lions are primarily night feeders, diving down as far as 400 metres below the surface to feast on a wide range of fish species. Adult males can weigh in at over 1000 kilograms.
The NMFS is proposing closing the mackerel and cod fishery year round in the habitat region showing the most serious population decline around the western-most Aleutians. Just east of these islands, the agency proposes more limited fishing seasons and fishing proximity to rookeries.
The biological opinion and remedial plans will be tabled next week at the North Pacific Fishery Management Council, a federally sanctioned group that advises on fishing limits in the region. Typically, the fishing season being considered for change begins in January.
Majestic foragers of the icy waters, Stellers have faced growing threats from massive fishing operations that extract huge amounts of the fish biomass from nearby seas. Some studies have shown that certain killer whales – transients that roam far – are also a sea lion threat.
During field work this spring, NMFS researchers estimated that there are only about 1,000 adult Stellers in the western-most Aleutian zone. This is based on extrapolation from the 225 pups counted at rookeries there.
Stellers have been the subject of long studies and battles between biologists and the fishing industry, who can be expected to sharply question any new limitations at the upcoming meeting. [see Nature 436, 14-16 (7 July 2005)]
The opinion published earlier this week was expected to be released last year, but was delayed repeatedly in behind-the-scenes political battles similar to those in the past, sources say.
Historically, most biologists support the idea that past fishing prompted the earlier collapse of the Steller’s populations in western Alaska. In eastern Alaska, the Steller’s population is not endangered, and has been growing in some regions.
The biological opinion, which arises from the US Endangered Species Act protections, is likely to spark further debate. Already US Senators from Alaska and Washington are urging the federal government to go slow on any new closures. The tight timeframe to meet all federal regulatory requirements to change the fishing season means tough decisions must be made by autumn.
– by Rex Dalton
(Photo credit: National Marine Mammal Laboratory)