Egg Size and Viability and Seasonal Offspring Production of Young Atlantic Cod

Experiments monitoring the egg and larval production of captive northwest Atlantic cod Gadus morhua indicated that first-time spawners perform poorly compared to second-time spawners. They breed for a shorter period, produce fewer egg batches, exhibit lower fecundity, and produce smaller eggs with lower fertilization and hatching rates; moreover, their larvae are less likely to hatch in environmental conditions favorable for survival. Batch-average egg diameters of first-time spawners underwent a continuous rapid decline between initial and terminal batches, whereas second-time spawners commonly exhibited a parabolic curve (slight increase followed by a decrease). Seasonal composite hatching rates of all eggs spawned by first- and second-time spawners were 13 and 62%, respectively. Larval production per maternal gram was an order of magnitude greater for second-time than for first-time spawners (200 versus 20 larvae/g). Batch-average egg diameter declined 11% and larval dry weight declined 29% from initial to terminal batches when data were pooled over both spawner types. Egg diameter correlated positively with larval dry weight, yolk dry weight, and ratio of yolk to larval dry weight. Additional monitoring of individuals between the first and second breeding year confirmed results obtained on 12 cod pairs monitored during the same year at constant temperature. These results have a direct bearing on the development of mathematical models of spawner–recruitment relationships, and they suggest that conventional approaches may overestimate the reproductive potential of age-truncated populations.