Decreasing weight-at-age of Atlantic herring (Clupea harengus) from the Baltic Sea between 1986 and 1996:: a statistical analysis

Observations during the 1990s indicate that individual growth rate of Atlantic herring (Clupea harengus) decreased by 30% to 50% in the Baltic proper. There are several possible explanations, however, no statistical analysis has been performed to define and describe the changes in herring growth rates. Tn this study we compared mean weight-at-age and length-specific weight (i.e. condition) data collected in September-October 1986-1996. Results indicated an increase in weight-at-age between 1986-1989. Thereafter, weight-at-age and fish condition declined in all parts of the Baltic proper. The decreases were evident in 0 through to 8 year old fish. Moreover, the declines were larger in the northern (51%) than in the south-western (42%) part of the Baltic proper and for younger (1-4 years) fish compared to the older fish (5-8 years). Possible causes of the decrease in weight-at-age and condition are discussed. However, the decreases seem to be well correlated with a drastic increase in the number of pelagic fish in the Baltic proper and a consequent reduction in food availability. (C) 2000 International Council for the Exploration of the Sea.