High site-fidelity and low mortality of juvenile Atlantic cod (Gadus morhua) in subarctic coastal habitat during their first winter

In subarctic Newfoundland, age-0 juvenile Atlantic cod (Gadus morhua) settle into coastal habitats in several summer and fall pulses, yielding a broad length-frequency distribution prior to their first winter. The first winter is often associated with physiological and metabolic stress and has been considered a critical survival period determining cohort strength. We evaluated size-structured overwinter mortality and movement using mark-recapture and condition metrics by marking 226 cod in two batches one week apart, in October 2016. We estimated fall and overwinter mortality, and documented movement of fish recaptured in May 2017 using Cormack-Jolly-Seber models. We recaptured 30 marked juveniles. High fall mortality characterized late settling cohorts relative to earlier settling cohorts (16.6%center dot d(-1) vs. 4.5-7.7% center dot d(-1)). Overwinter mortality was unexpectedly low (0.0052 and 0.0022% center dot d(-1)). Individual condition (Fulton's K) of juvenile cod remained high throughout winter across all size groups. We expected higher mortality of juvenile cod and broad dispersal of juveniles over winter (32 weeks). In contrast, our results indicated low mortality and high site-fidelity in their first winter. This study indicates the period leading up to winter is important for survival, suggesting winter is not a survival bottleneck and may even provide a refuge compared to the rest of the year.