Spawning migration characteristics and ecology of Eulachon (Thaleichthys pacificus)

Abstract

Eulachon Thaleichthys pacificus has experienced dramatic reductions in their distribution and abundance along the west coast of North America. This prompted the listing of this species as "Threatened" under the U.S. Endangered Species Act (ESA) of 1973 for populations found in the southern portions of their range, but not in Alaska. Key gaps in knowledge of Eulachon spawning ecology exist that impede population monitoring efforts and habitat protection. Currently, many monitoring efforts ignore estuaries as possible spawning habitat leading to inaccurate estimates of population abundance and trends. Furthermore, estuaries are not designated as critical spawning habitat for Eulachon. This is important because a critical spawning habitat designation under the ESA provides for a regulatory framework on which to focus conservation and restoration efforts for some of the most imperiled aquatic habitats in North America. I hypothesized that Eulachon spawn in estuaries based on limited observations in other research and the close phylogenetic relationship between Eulachon and other smelts (Osmeridae) that can tolerate salinity. To test this hypothesis, I first studied the effects of salinity on the fertilization and hatching success of Eulachon in a laboratory setting to determine salinity tolerance. Second, I investigated estuary spawning in the Twentymile and Antler rivers, Alaska using radio telemetry and substrate surveys to confirm spawning areas. Third, I examined the relationship between adult spawning run intensity and the environmental variables of tide height, water discharge, and day or night to better inform future population monitoring efforts. My findings in the laboratory indicated that Eulachon can fertilize eggs and produce viable offspring in brackish water. These results were confirmed by egg areas observed in the estuaries of the Twentymile and Antler rivers. Furthermore, spawning run intensity increased in association with spring tides, but there were no clear relationships between spawning run strength and freshwater discharge or day and night. Based on the results of my work, I recommend changes to population monitoring study design and designation of critical spawning habitat to include estuaries. Future research should focus on determining the lower limits of Eulachon spawning habitat to further improve population monitoring and habitat protection.