Thermal reaction norms for growth vary among cohorts of Pacific cod (Gadus macrocephalus)

While much effort has been directed at determining the spatial scales of adaptation in thermal reaction norms for growth, it is widely assumed that these reaction norms have high temporal stability. Water temperatures in the Gulf of Alaska in 2007 were the coldest on record since the mid-1970s and we present evidence that the thermal reaction norm for growth of age-0 Pacific cod (Gadus macrocephalus) in this cohort differed significantly from two adjacent cohorts. In addition to exhibiting higher growth potential at low temperatures, the 2007 cohort had a higher mean vertebral count, consistent with the widespread thermal effect known as “Jordan’s Rule.” Variation among cohorts in these physiological and morphological traits suggests a persistent response to environmental history (epigenetic effect). Temperature-induced phenotypic plasticity in the reaction norm for growth has significant implications for using growth rates to evaluate habitat quality and illustrates the complex responses of fishes to climate variability.