Temporal declines in fecundity: A study of southern Gulf of St. Lawrence Atlantic herring (Clupea harengus) and implications for potential reproductive output.

Individuals must reproduce to survive and thrive from generation to generation. In fish, the fecundity of individuals and estimates of total reproductive output are critical for evaluating reproductive success and understanding population dynamics. Estimating fecundity is an onerous task; therefore, many populations lack contemporary estimates of fecundity and size-fecundity relationships. However, reproductive dynamics are not static in time; therefore, it is important to develop contemporary fecundity estimates to better inform conservation and management action. To highlight the importance of contemporary fecundity estimates, we examined the fecundity of southern Gulf of St. Lawrence (sGSL) spring and fall spawning Atlantic herring in 2022, developed size-fecundity models, and compared these to historical fecundity estimates and models. Our results suggest that the average fecundity of sGSL spring and fall herring has undergone a substantial temporal decline of approximately 47% and 58%, respectively, since the 1970s and 1980s. The size-fecundity relationships for fall spawning herring have shifted, with fish of a given size exhibiting lower fecundity in 2022 compared to the 1970s. Alternatively, the size-fecundity relationships for spring spawning herring have remained relatively static. Furthermore, simulations highlighted a substantial reduction in potential reproductive output in 2022 compared to 1970 of approximately 32% and 68% for spring and fall spawners, respectively, based on a fixed number of mature females, which may have negative implications for stock rebuilding. Overall, our study provides support for periodic estimates of fecundity in fish populations to better understand temporal changes in reproductive and population dynamics.

The influence of thermal cues on the reproductive phenology of Silver Shiner, Notropis photogenis.

Reproductive phenology and the length of the growing season vary in response to interannual environmental variability, with implications for population dynamics of freshwater fishes. Understanding the reproductive phenology of imperilled species in relation to environmental conditions is needed to better evaluate potential responses to changing environmental conditions, estimate future population dynamics and develop comprehensive recovery strategies. We examined Silver Shiner, a species listed as "Threatened" under Canada's Species at Risk Act, during spring 2018 and 2019 to better understand the reproductive phenology of the species at the northern edge of its range in Canada. The initiation of Silver Shiner spawning occurred on the descending limb of the hydrograph and was completed before the onset of the extended period of low summer flow. In addition, both the initiation and cessation of spawning occurred in response to a cumulative growing degree day base 5 (GDD5 ) cue, with logistic regression models indicating a 50% probability the population initiated and ceased spawning when GDD5 reached 68°C•days and 368°C•days, respectively. Logistic regression incorporating GDD5 effectively predicted spawning initiation and cessation, providing useful models for examining the impacts of alterations to the thermal regime on reproductive phenology and improving the ability to evaluate changes in the larval growth period. Furthermore, the models can facilitate the development of real-time estimates of spawning activity, and therefore ensure that disturbance to the species is minimized during the sensitive reproductive period.