Egg and larval production of Eastern Baltic cod in captivity over one spawning season.

Eastern Baltic cod (Gadus morhua) has experienced a dramatic decline in biomass, distribution, size and condition over recent decades, related to a combination of fishing pressure and environmental stressors. Reduced parental size and condition are known to impact reproductive success, affecting egg quality, fertilization and hatching success and larval survival. However, how these reproductive traits vary throughout the spawning season at the population level remains poorly understood for this stock. To address this knowledge gap, this study investigated egg and larval characteristics of captive Eastern Baltic cod over an entire spawning season. Fish from three different size classes were kept in three separate tanks and allowed to spawn naturally. Eggs were collected daily from April to August, with peak spawning observed in early to mid-June. We assessed egg and larval size, deformities, thiamine concentration, daily survival and hatching success over time. Our findings revealed high variability in these reproductive traits both across the spawning season and between size groups, without consistent temporal trends. Notably, smaller fish produced the largest eggs with the highest total thiamine concentrations, which contrasts with previous studies linking larger egg size with larger females. At the same time, these eggs showed the lowest survival and hatching rates. The only consistent pattern across all tanks was a positive relationship between egg and larval size, though unrelated to survival. These results suggest higher than expected complexity of reproductive dynamics in Eastern Baltic cod and raise the question whether the stock has undergone a shift in reproductive strategy, possibly reflecting an increased allocation towards egg quality over quantity during the recent period of stock decline.