Condition, size, and winter duration affect winter survival probability of juvenile Atlantic cod Gadus morhua in a coastal subarctic ecosystem

Juvenile Atlantic cod Gadus morhua in coastal Newfoundland, Canada, settle in nearshore habitats in 3-6 pulsed events each year, from mid-summer to early winter, creating a broad size range of age-0 fish within each year-class, with potentially different survival trajectories entering their first winter. We hypothesized that early-arriving pulses, high autumn body condition, and low winter severity would contribute to high first-winter survival of age-0 cod. To test these hypotheses, we examined juvenile cod catch data, archived samples, and winter temperature records to determine the impact of winter duration, body condition, and settlement time on winter survival from 2001 to 2019. Settlement pulse, pre-winter condition, and winter duration had an interactive effect on survival. Fish condition did not vary across pulses within a given year-class, but we observed improved winter survival during long winters when body condition was high, with highest survival among the larger, early-arriving pulses of fish. Late-arriving pulses of fish were small-sized before winter onset, but these pulses unexpectedly survived better than pulses settling earlier in the season during short winters when body condition was high. However, all settlement pulses had a survival advantage when settlement occurred earlier on average (associated with warmer autumns), suggesting that increased growth and size within a pulse can also contribute to overwintering success. Our findings challenge some assumptions of overwintering ecology (e.g. 'bigger is better', low temperature stress) but illustrate the importance of cohort effects for overwintering survival in a sub-arctic marine ecosystem in a changing climate.