CHANGES IN NORTHERN BALTIC ZOOPLANKTON AND HERRING NUTRITION FROM 1980S TO 1990S - TOP-DOWN AND BOTTOM-UP PROCESSES AT WORK

During the stagnation period of the Baltic Sea the mean weight-at-age of Baltic herring decreased by 50% (between 1977 and 1992). This has u sually been attributed to a top-down process, i.e. to the simultaneous collapse of cod stocks and their predation. We present long-term data for 1980 to 1993 showing that bottom-up effects may also have played a role: along with the decline of salinity, the biomass proportion of zooplankton taxa preferred by herring (larger than 20 mu g ind.(-1) in wet weight) significantly declined. To support our hypothesis we pres ent a study in which Baltic herring feeding and selective predation we re investigated during 1985, a time of good growth and high weight-at- age, and 1991, when herring growth and weight-at-age were poor. In thi s study, herring stomachs and simultaneously taken plankton samples we re analysed from trawl surveys conducted in the northern Baltic proper during the peak of the herring feeding season in late summer. During both 1985 and 1991, herring selectively preyed on the larger zooplankt on categories, especially neritic copepods. However, in 1991, a smalle r proportion of the prey in herring stomachs consisted of neritic cope pods, apparently because their share in plankton had decreased. Conseq uently, and despite an increase in total zooplankton biomass, the esti mated carbon content of the food eaten by herring was lower, and the a verage stomach fullness index (on a scale of 0 to 5) decreased from 3. 9 in 1985 to 1.9 in 1991. Also, the amount of mesenteric fat on herrin g stomachs declined from 4.2 to 3.2 (scale 0 to 5), indicating a longe r-term failure in feeding success. We suggest that, in addition to pos sible top-down effects (a release of cod predation), bottom-up process es mediated via changes in mesozooplankton species composition have al so influenced herring growth and that both of these processes are affe cted by the same environmental factor-the Baltic salinity level.