Recruitment Signals in Juvenile Cod Surveys Depend on Thermal Growth Conditions

Commercially important cod species (e.g., Atlantic cod Gadus morhua and Pacific cod Gadus macrocephalus) often use warmer surface waters and coastal areas to maximize growth in their first year but may be vulnerable to seasonal, annual and decadal changes in thermal growth conditions.  In this study, we analyzed coastal time-series from three regions (Gulf of Alaska, Newfoundland and Norway) to examine whether: 1) recruitment among regions was a function of seasonal and annual age-0 growth potential in the coastal nursery and 2) decadal scale changes in growth conditions within the region corresponded with a change in 1^st year survival.  Daily growth potential was modeled for each region using temperature data and laboratory measured growth of age-0 juvenile Atlantic and Pacific cod.  Results indicated that low winter growth potential in the sub-Arctic system (Newfoundland) corresponded with a poorer recruitment index compared to other regions with milder overwintering conditions.  However, regional warming during the summer growth period in Norway corresponded with decreased growth potential and deteriorating recruitment signals.  We conclude that models of temperature-dependent growth potential of age-0 gadids offer a mechanistic tool for understanding recruitment dynamics, both within and among regions in response to climate change.