Seasonality and water masses shape energetics of a high Arctic fish: quantifying energy density and condition indices

Seasonal life history decisions and environmental factors shape energetics over annual cycles. We quantified seasonal energy density of polar cod Boreogadus saida , a central predator and prey in the Arctic marine food web. We collected individuals from Atlantic and polar waters in the Barents Sea, to assess the impact of seasonality and water masses on energy density (kJ g -1 wet weight (WW)). Additionally, we compared energy density to 4 condition indices (Fulton’s, residual and hepatosomatic indices, and percent dry weight) to test their accuracy as energetic proxies for polar cod. Energy density varied seasonally and was lower in Atlantic waters. In polar waters, energy density was lowest in April/May, whereas seasonal variation was weaker in Atlantic waters. Energy density was highest in December in both water masses, although not much higher than March in polar water. Differences between water masses were seasonal, greatest in March (Atlantic: 4.07 ± 0.53 kJ g -1 WW; polar: 5.77 ± 0.62 kJ g -1 WW) and negligible in December (5.66 ± 0.83 and 5.86 ± 0.62 kJ g -1 WW, respectively). Percent dry weight best represented energy density, while Fulton’s and residual (derived from WW) indices were poorly correlated to energy density. Our results show that seasonality and environmental factors shape energetics, and that energy proxies must be carefully selected to accurately represent the energetic status of an organism. Our seasonal resolution highlights potential ‘critical windows’, when polar cod may be most vulnerable to stressors, whilst our spatial comparison provides insight into potential impacts of ongoing Atlantification.