Life history and age at maturity of an anadromous smelt, the eulachon Thaleichthys pacificus (Richardson)

Abstract

Trace element and fork length (LF) frequency analyses of eulachon Thaleichthys pacificus otoliths were used to determine age at maturity and repeat spawning potential, two aspects of eulachon life history that are not known but are important for successful management of this species. The LF‐frequency analysis for ocean caught and spawning eulachon was used to estimate age at maturation. Two size classes of eulachon were caught in the ocean and spawning eulachon were consistently the largest fish indicating that spawners from mid‐coast of British Columbia were 3 years old. Laser‐ablation inductively‐coupled plasma mass spectrometry (LA‐ICP‐MS) was also used to reconstruct the Ba:Ca and Sr:Ca molar ratios deposited spatially into the otolith to estimate spawner age for five populations of eulachon. Age at maturation differed among populations examined. Based on the seasonal fluctuations in Ba:Ca molar ratios caused by seasonal upwelling of deep waters, it was determined that more southerly populations spawned at a younger age than the northern populations examined. Southern populations of eulachon, Columbia River, Washington, U.S.A., spawn after 2 years. Eulachon from the Fraser, Kemano and Skeena Rivers in British Columbia, Canada, generally mature after 3 years. Some Skeena River eulachon and most of the eulachon from the Copper River, Alaska, U.S.A., matured after 4 years. In contrast to the Ba:Ca molar ratios in the otolith, Sr:Ca molar ratios maintained a relatively flat profile over the life of the eulachon. The lack of a change in Sr:Ca ratios within the otolith, the single size class of spawners across all systems and the single age class within most populations strongly suggest that eulachon in the present study are semelparous.