Overwinter survival of juvenile Atlantic cod (Gadus morhua) in a costal ecosystem

Abstract

Acknowledging poor understanding of natural mortality in juvenile fish, especially throughout first winter, I synthesized past studies that suggest that physiological and metabolic stress in the first winter result in a critical survival period that determines cohort strength. Juvenile Atlantic cod (Gadus morhua) in coastal Newfoundland settle in nearshore habitats in 3-6 pulsed events each year, creating a broad size-structured age-0 year-class with potentially different survival trajectories entering their first winter. To assess size-specific survival across winter, I classified age-0 and age-1 pulse structure using finite mixture distribution models. I evaluated juvenile cod survey data and archived juvenile cod samples with winter temperature records to determine effects of winter duration, body condition, and settlement time on overwinter survival. Higher condition entering winter and smaller size combined with earlier fall settlement all improved overwinter survival among pulses. Increased body condition in the fall, and earlier settlement timing among pulses aligned with high fall temperatures. I further investigated the role of body size and condition on overwintering success through feeding trials at ambient overwinter sea temperatures, using demersal age-0 cod collected from Newman Sound, Newfoundland. I showed that small amounts of consumed food (<1% body weight ∙ d-1) maximized winter growth and condition potential of juvenile cod in Newfoundland waters. I also used capture-mark-recapture and condition metrics to evaluate in situ winter movement and survival of age-0 cod in Newman Sound. Highest fall mortality characterized late settling juveniles relative to early settling juveniles (16.20%∙d-1 vs. 4.52-7.72%∙d-1), noting unexpectedly low overwinter mortality in the first two groups (0.0052 and 0.0022%∙d-1). My research highlights the importance of food prior to and during winter to promote energy reserves for survival in the first year of life. These combined studies demonstrate potentially low winter mortality in early settling juveniles and emphasize the critical importance of the period leading up to winter for survivorship, with increased resources before and during winter reducing a survival bottleneck. These findings advance the understanding of overwinter survival and recruitment in sub-arctic marine ecosystems in a changing climate and challenge many assumptions of high overwinter mortality in sub-arctic cod populations.