Intra‐annual energy density cycles of spring‐ and fall‐spawning <scp>Atlantic</scp> herring <i>Clupea harengus</i> reveal different reproductive allocation tactics

Atlantic herring Clupea harengus are total spawners that exhibit a large degree of reproductive plasticity and have substantial intra-annual variation in their energetic condition. Recent research suggests that the species may be declining in energetic condition in the northwest Atlantic Ocean from the few historical records, but comparisons in the region are complicated by the presence of spring- and fall-spawning contingents. Proximate composition analysis (water, lipid, protein and ash) results of Atlantic herring somatic tissue and gonads revealed sex-specific energy allocation patterns and generated strong predictive relationships used to estimate the energy density (ED; kJ g^-1 wet weight) of 1104 individuals collected between 2021 and 2023 by fishery-dependent and -independent sources. Generalized additive models showed non-linear somatic ED cycles that varied by spawning contingent and day of year. The best model included an interaction between day of year and spawning contingent, and explained 64.6% of the deviance, 12.2% more than the model without spawning contingent. Gulf of Maine and Georges Bank Atlantic herring ED was less variable in the spring compared to the fall, when there was a smaller energetic offset between contingents. Estimates of contingent-specific seasonal somatic ED cycles provide context for interpreting values across studies, evaluating long-term energetic condition trends and assessing possible environmental influences on the energy dynamics of these two spawning contingents that are managed as one stock complex.