Liver condition of 0 and 1-group cod (Gadus morhua) in the Barents Sea

Liver condition is hypothesized to be a better condition index for juvenile cod than other somatic condition indices, since cod store most of their excess energy in their liver. It is suggested that population changes in liver condition can be used as a proxy for overwinter mortality. Here, we have collected 0- and 1-group cod from the same cohort in the Barents Sea before and at the end of the winter season. We have taken measurements for somatic- and liver-based condition indices, as well as enumerating nematode endoparasites found in each individual cod. The average wet weight of sampled cod increased by 85% (approx. 0.4% day(-1)) from 6.0 to 10.1 g from September to February, while the average liver wet weight decreased by 15%. The relative water content of the liver also increased significantly over the winter, resulting in a 42% reduction in average dry liver weight from 0.116 to 0.067 g over the 4-5-month period between samplings. About 30% of the 1-group cod surviving the winter had less than 20% of the liver energy content of same-sized 0-group cod. The liver condition was also 5-10% lower in cod with high nematode endoparasite load than the cod without nematode endoparasites. The significance of the loss of liver energy content is discussed in relation to the marked seasonality combined with climate change and the challenges this imposes on the survival of 0-group cod in high-latitude environments during their first winter.