Early life history affects fish size mainly by indirectly regulating the growth during each stage: a case study in a demersal fish

The complex life histories of demersal fishes are artificially separated into multiple stages along with changes in morphology and habitat. It is worth exploring whether the phenotypes expressed earlier and later during the life cycle are related or decoupled. The life stages of first year Pacific cod (Gadus macrocephalus) were tracked over different hatch years and regions to test whether the early life history had a long-lasting effect on subsequent growth. We further explored the contribution of growth in the early and subsequent life history stages to body size at the end of each stage. In addition to the accessory growth centre and the first annual ring, the other two checks on the otolith possibly related to settlement and entering deeper waters were identified in 75 Pacific cod individuals. The direct and indirect relationships among the life history stages was interpreted based on path analysis. The results showed that growth prior to the formation of the accessory growth centre had a significant effect on the absolute growth of the fish before and after settlement and migration to deep water. However, there was no or moderate evidence that early growth affected the body size at each stage, which was mainly regulated by growth during the stage. This study supports the lasting effect of early growth and clarifies that it affects size mainly by indirectly regulating staged growth. Quantifying the phenotype relationships and identifying the internal mechanisms form the basis for assessing population dynamics and understanding the processes behind the changes.