Counting eggs before they hatch: Comparing metrics of spawning seasonality in Blueback Herring

Abstract Objective Year-class success is shaped in part by the population’s reproductive phenology—its seasonal timing of spawning. Quantifying phenology may take several approaches representing varying levels of effort, expense, and expertise. The objective of this study was to assess different measures of reproductive activity so that we could develop an accurate representation of the phenology of anadromous Blueback Herring Alosa aestivalis in the Connecticut River. Using data collected on the spawning grounds over 4 years, we compared the seasonal timing of spawning based on three metrics derived from nondestructive sampling (abundance of all adults, abundance of females, and abundance of “spawner” females in reproductive condition) and two metrics derived from destructive sampling (abundance of females weighted by their gonadosomatic index and abundance of spawner females weighted by their gonadosomatic index). Methods As part of an ongoing monitoring program, we collected Blueback Herring along the Connecticut River in northern Connecticut and southern Massachusetts in 2019, 2021, 2022, and 2023 via boat electrofishing every week during the reproductive season at four sites. The field crew sexed and measured all captured individuals and performed a nondestructive assessment of reproductive state; a subsample was then euthanized to obtain ovary weights. We quantified weekly CPUE of all individuals, females, and spawner females and weekly patterns of ovary mass to estimate the abundance of female reproductive tissue. Results Blueback Herring abundance varied among years and between regions, but the timing of migration was relatively consistent among years and between regions. Abundance was an order of magnitude higher at the two southern sites. Females arrived on the spawning grounds later than males such that we observed seasonal changes in sex ratio, yet the proportion of females that were spawners declined as the season progressed. Ovary size also declined as the season progressed, with females tapering their reproductive output. Summarizing the reproductive phenology as the mean week of the season weighted by different metrics, we found that estimates based on ovary mass depicted a significantly earlier season than estimates based on abundance of all individuals or females alone but were similar to estimates based on abundance of spawner females. Conclusions Research and monitoring that seek to quantify temporal patterns in the spawning season should consider the gain in accuracy that arises when weighting abundance by ovary size.