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NOAA Technical Memorandum NMFS-AFSC-356

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Estuarine distribution, nutritional and thiaminase content of eulachon (Thaleichthys pacificus) in Southeast Alaska, with implications for Steller sea lions

Abstract

Eulachon (Thaleichthys pacificus) are small, pelagic fish whose spawning habits and marine distribution are not well understood.  Eulachon are a nutrient-rich, seasonally important source of food for predators, including Steller sea lions (Eumetopias jubatus) in southeast Alaska.  Echo integrated-trawl surveys were conducted from 3 April to 6 May 2006 to estimate eulachon population structure and biomass, identify fish movement, location, and acoustic targets. The presence of Steller sea lions and other marine predators attracted to pre-spawning aggregations of eulachon were used to fine-tune the timing and location of our survey sites.

We identified the distribution of pre-spawning adults, sub-adult, and juvenile eulachon in five estuaries of northern southeast Alaska during the 2006 spring spawning season and analyzed their nutritional and thiaminase content. Mean eulachon weight and nutritional energy varied between sites, from a low of 10.68 g body weight ± 2.93 g with a total energy of 94.4 kJ/fish in Berners Bay, to a high of 33.22 g body weight ± 5.99 g with a total energy of 291.9 kJ/fish in Lutak Inlet. Of the five estuaries surveyed, juvenile and sub-adult eulachon were found only in Taku Inlet, Berners Bay, and Chilkat Inlet and were concentrated at a distinct depth of 50-80 m throughout the estuaries. Adult eulachon were found in three of the five estuaries: Lutak Inlet, Taku Inlet, and Berners Bay with no fish identified with our fishing gear or acoustics in Taiya Inlet.

Eulachon school biomass and the duration these schools spend in the estuaries correlated with sea lion abundance. Sea lion numbers were directly proportional to eulachon biomass. Spawning eulachon move quickly into their freshwater spawning river, with estuary size directly proportional to the time spent in the estuary. Thiaminase, a potentially harmful enzyme, was found in eulachon at all five sites, with a mean activity range of 5.0-7.0 umols·g-1·min-1. The ingestion of thiaminase laced eulachon by Steller sea lions does not appear to affect the health of sea lions in our study area because they were not ingested long enough for the thiaminase to take effect. The role that thiaminase plays in the health of marine predators is not well understood and warrants further study and could play a role in the junk food theory that was used to explain Steller sea lion declines in other areas especially in populations with low dietary diversity.

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