Seasonal variation in geographic distribution of North East Arctic (NEA) cod – survey coverage in a warmer Barents Sea

Shifts in the geographic distribution of fish influence coverage of a stock in scientific surveys, and variation in climate and abundance of the stock are potential factors influencing this. During the winter survey for demersal fish in the Barents Sea in 2011, 6-7-year-old North East Arctic (NEA) cod (2004/2005 year-classes) appeared in unexpectedly high numbers. We investigated the geographic distribution of NEA cod during winter and autumn, using new survey data from the autumn ecosystem survey. The results provide new insight into the seasonal age-specific geographic distribution of NEA cod, which is used to evaluate how well the winter survey has covered different age groups. We found that the winter survey seems to have consistently under-covered 1-3-year-old cod. Older cod gradually reappeared in the survey area, fully returning as 6-7 years old. The autumn ecosystem survey covered all age groups and year-classes. The 2004/2005 year-classes were more widely distributed than the weaker 2003 year-class, and were closer to the survey area border in winter as 1-4-year-olds. The relationship between geographic distribution and survey coverage seems to explain the deviation in the survey indices. The unexpected results for ages 6 and 7 in the 2011 winter survey are probably caused by under-coverage of younger ages in the preceding years, and this could have been exaggerated by the combination of high temperatures and abundant year-classes. These findings have implications for the winter survey as a source for abundance indices to the stock assessment.