Somatic and reproductive energetic condition of Spring– and Fall– spawning Atlantic herring (Clupea harengus) in the Northwest Atlantic Ocean

Fish species may delay maturity, skip spawning, or decrease reproductive investment to prioritize their own fitness during adverse environmental or feeding conditions. Atlantic herring (Clupea harengus), a commercially valuable species and important prey item for larger fish, birds, and marine mammals, is currently overfished in the Northwest Atlantic Ocean. Recent research suggests that the species may also be declining in energetic condition from the few existing historical records, but comparisons in the region are complicated by the presence of spring– and fall–spawning contingents. From 2021 – 2023, we collected Atlantic herring from fishery–dependent and –independent sources and performed proximate composition analysis (water, lipid, protein, and ash) on fish body and gonad compartments, to quantify the varying day of the year energetic patterns of each contingent. Our results indicate energy density (ED) is highly variable with season, location, sex, contingent, and an individual’s point of maturation. Energetic information is also presented in conjunction with measurements of age, mean oocyte diameter, and potential fecundity to further investigate life history differences between spring and fall spawners. We then visualized annual ED patterns of each contingent using a generalized additive model (GAM). Including an interaction between day of year and spawning contingent increased the explanatory power of the GAM by 10.8% and revealed contingent-specific energetic patterns. Fall spawners attained an annual ED maximum that was nearly 20% higher compared to spring spawners. If future monitoring efforts are unable to distinguish between spring and fall spawners, our results suggest that there will be less variation between contingents during the spring compared to the fall, when energetic differences are more pronounced.