Spatial, interannual, and generational sources of trait variability in a marine population

Life‐history traits of individuals in marine populations exhibit large sources of variability. In marine fish, variation of individual size at a given age has three main components: (1) spatial, correlated with the location in which individuals are caught, (2) temporal, correlated with the time when individuals are caught, and (3) generational, correlated with the year of birth of the examined individuals. These variations, if present, have practical implications for individual fitness as well as for sampling, survey design, and population assessment. Disentangling these variations and understanding their sources is hard, given the potentially correlated nature of their effects on individual traits. This study examines the size‐at‐age relationship of the Bering Sea Pacific cod, an economically and ecologically important groundfish. We used extensive records spanning 1994 to 2016 (inclusive) of 25,213 observations of both environmental variables and catch, lengths, and ages. We found that the average size of individuals of the same age could differ up to 7 cm. Notably, we found that the cohort composition of the sampled population explained >75% of the year effect and that individuals caught in the northwest and shallower portion of the sampling area were on average 5 cm smaller than individuals caught in the southern and deeper portion. We further found that northwest movement of young cod (age 1–5) as a result of warming places individuals in areas where we predict them to have smaller size at age. Smaller and less conditioned individuals are less fecund and may not be able to perform long migrations to return to their distant spawning grounds. Both the spatial distribution and water temperature experienced by Pacific cod in the Bering Sea are changing, and this study provides a mechanism for how these changes affect Pacific cod life‐history traits and individual fitness. Life‐history traits of individuals in marine populations exhibit large sources of variability. Trait variability affects individual fitness (Webb et al. 2010) and the assumptions necessary to sample the population (Puerta et al. 2019a). Growth rate is one of such traits that results in individuals having vastly different body sizes at a given age. In turn, body size and growth rate have well‐known fitness consequences, which, for marine fish, are especially evident during early life stages (Sogard 1997, Houde 2008). Observed variability of individual growth rates originates from multiple processes, including heterogeneous environmental conditions experienced by different individuals