Decreasing length at age in a rapidly expanding population of northern rock sole in the eastern Bering Sea and its effect on management advice

The highly productive eastern Bering Sea shelf has undergone a dramatic increase in the size of flatfish stocks during the last 20 years. This has been highlighted by the northern rock sole stock, which has increased well over 15 times from approximately 175×10 6 kg in 1975 to nearly 3×10 9 kg in 1994. Annual bottom trawl surveys and an age-structured population model have been used to track the increase. A new evaluation of the data has shown significant decreases in length-at-age (and resultant weight-at-age) as the population increased and expanded westward toward the shelf edge. The downward trend in length-at-age primarily affected year classes between 1979 and 1987. A small increase has occurred subsequently. Potential density-dependent relationships are examined with the significant regression of mean length at age-3 versus population changes. These differences in growth were incorporated into a stock assessment model to examine their effect on estimates of recruitment variability, fluctuations in stock biomass and yield recommendations.