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

Abstract

Anadromous Rainbow SmeltOsmerus 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.

Evaluating the Effectiveness of Fyke‐Net Sampling to Characterize an Anadromous Rainbow Smelt Population in the Fore River, Massachusetts, USA

“…Corrected sex ratios using the repeat spawning estimates from PIT tagging for the Fore River estimated a sex ratio of 2.91 in 2011 and 1.31 in 2012; given the fairly consistent annual sex ratios in the survey net for each year of our study, sex ratios in the Fore River appear to be stable during this period (Enterline et al 2020).…”

“…Variability in the survey net catches at lower abundances suggests that when fewer fish are present, it is easier to avoid capture in the net (Hubert et al 2012); at higher abundances avoidance is reduced. A long-term survey that assesses Rainbow Smelt abundance MANAGEMENT BRIEF by CPUE should only use the nighttime fishing period to calculate effort, as we did here, because Rainbow Smelt almost exclusively migrate at night (Enterline et al 2020). Tidal height and streamflow likely influence the capture efficiency of the survey net, but the correlations were not significant and the small number of experimental netsampling events precluded multivariate modeling of the influence of environmental conditions.…”

“…The differences in the numbers of Rainbow Smelt that were caught in 10-mm length bins were minor between the two fyke-net designs and only differed significantly between the nets for females in 1 of the 4 years (Figure 4). Both sexes display repeat spawning, and while males showed no correlation between age and the number of repeat spawning events, female repeat spawning events declined with age, which may lower the likelihood of encountering larger (older) females in the survey net (Enterline et al 2020).…”

“…The differences between the nets in the sex ratios of Rainbow Smelt were not generally significant, with the exception of the overall sex ratios for the survey net in 2011 and 2012, where the ratios of males to females were smaller than was predicted by the experimental net (Table 2, top panel). Evaluating potential sex ratio biases was important because repeat spawning behavior has been reported in multiple locations (Murawski et al 1980;Enterline et al 2020). Enterline et al (2020) PIT-tagged Rainbow Smelt at two geographically distinct locations (including the Fore River) and observed repeat spawning at a consistently higher rate for males (up to ninefold, with an average of 1.72 and 1.46 in 2011 and 2012, respectively) than for females (up to fourfold, with an average of 0.28 and 0.78 in 2011 and 2012, respectively).…”

“…Evaluating potential sex ratio biases was important because repeat spawning behavior has been reported in multiple locations (Murawski et al 1980;Enterline et al 2020). Enterline et al (2020) PIT-tagged Rainbow Smelt at two geographically distinct locations (including the Fore River) and observed repeat spawning at a consistently higher rate for males (up to ninefold, with an average of 1.72 and 1.46 in 2011 and 2012, respectively) than for females (up to fourfold, with an average of 0.28 and 0.78 in 2011 and 2012, respectively).…”