Quantifying Within‐Season Repeat Spawning by Rainbow Smelt with Implications for Comparing Sex Ratios and Survey Catches among Runs

Abstract

AbstractAnadromous Rainbow Smelt Osmerus mordax provide an important nearshore prey base, but they have experienced severe population declines during the last century. Population estimates from spawning run surveys have not been calculated because within‐season repeat spawning, a behavior qualitatively described for this species based on previous studies, has not been quantified. Our objective was to determine repeat spawning behavior at two spawning sites by using passive integrated transponder technology. A preliminary laboratory tag retention and mortality study was performed to determine the effect of tagging on study results, and it showed that tagging did not cause significant mortality and tags were well retained. In our field studies, continuous monitoring of tagged fish showed that Rainbow Smelt made movements to the spawning grounds against the tide and at low tide, allowing the fish to be at the spawning grounds during the darkest nighttime hours. This is in contrast to past assumptions that Rainbow Smelt only spawn during nighttime high tide. We found that repeat spawning behavior was a predominantly male behavior, consistent with past studies. The behavior of males did vary between the two sites, although at each site the behavior was consistent among study years. In contrast, the behavior of females was consistent both among study years and between the sites. This finding indicates that future assessments of spawning populations of Rainbow Smelt could be improved by using female catch numbers, similar to other species that display skewed sex ratios.