No abstract

We used stable nitrogen isotopes to describe the pelagic food-web structure of three coastal Baltic Sea areas, each of which was sampled twice. Two of the areas were influenced by 15 N-rich nutrient discharges from a sewage treatment plant. Analyses were made of particulate organic matter (Ͻ35 m, mainly phytoplankton), zooplankton, mysids (Mysis mixta and M. relicta), sprat (Sprattus sprattus), smelt (Osmerus eperlanus), four size classes of herring (Clupea harengus), and pikeperch (Stizostedion lucioperca). Discharges from the sewage treatment plant significantly increased ␦ 15 N values in the whole food web, from phytoplankton to piscivorous fish. Based on nitrogen isotopic compositions, consistent trophic food-web structures were observed on both occasions and in all three areas. The results indicate that zooplankton and mysids may have more complex diets than assumed before. Apparent trophic fractionation, i.e., differences in ␦ 15 N between a consumer and its assumed food, averaged 2.4‰ with a standard error of Ϯ0.5‰. Differences between areas in fish ␦ 15 N show young-of-the-year herring, sprat, smelt, and pikeperch to be relatively non-migratory.

The trophic isotope fractionation of mysids, in response to a change in the isotopic composition in their diet, was examined in the laboratory. Field-caught Mysis mixta and Neomysis integer had similar δ13C values, ranging between -23.1 and -21.5‰. They also had similar δ15N values, and for both species, this value increased with body size. Experimental starvation did not change the isotopic composition. To test for metabolic fractionation of isotopes, the mysids were fed different diets, newly hatched Artemia and detrital Enteromorpha. The isotopic composition was monitored in abdominal muscle tissue, exoskeleton, and feces over 12 weeks. The δ13C composition in muscle tissue had not reached an isotopic equilibrium with the diet at the end of the experiment, while the exoskeleton came into equilibrium with the food in 2-3 weeks. Muscle was enriched in 15N relatively to the food, +3.6‰ for Artemia and +2.7‰ for Enteromorpha. The isotopic composition in muscle, exuviae, and feces may form a basis for diet reconstruction of mysids. The feces δ13C and δ15N values mirror the diet over the last few hours, exuviae δ13C values represent nutrients metabolized 2-3 weeks ago, and muscle tissue integrates the isotopic signal over a relatively long period.

Seals and fish-eating birds have increased in the Baltic Sea and there is concern that they compete with fisheries. Using data from around year 2010, we compare consumption of different fish species by seals and birds to the catch in the commercial and recreational fishery. When applicable this is done at the geographical resolution of ICES subdivisions. Predation by birds and mammals likely has limited impact on the populations of the commercially most important species (herring, sprat, and cod). In the central and southern Baltic, seals and birds consume about as much flatfish as is caught by the fishery and competition is possible. Birds and seals consume 2-3 times as much coastal fish as is caught in the fishery. Many of these species are important to the fishery (e.g. perch and whitefish) and competition between wildlife and the fishery is likely, at least locally. Estimated wildlife consumption of pike, sea trout and pikeperch varies among ICES subdivisions and the degree of competition for these species may differ among areas. Competition between wildlife and fisheries need to be addressed in basic ecosystem research, management and conservation. This requires improved quantitative data on wildlife diets, abundances and fish production.

We studied the seasonal dynamics of zooplanktivory by the major zooplanktivores (sprat Sprattus sprattus, herring Clupea harengus and the mysid shrimp Mysis mrxta) in 3 coastal areas of the northern Baltic Sea proper from the beginning of July through the end of October, 1985. The 3 areas are within 30 km of each other and dffer in nutrient loading and primary productivity. Consumption rates were obtained by combining abundance estimates of the planktivores (from Bongo nets, gill nets and acoustics) with diet analysis and bioenergetics models. Both the dominating planktivore groups and total planktivory rates changed over the study period. Sprat and yearling herring were the major zooplanktivores in July and August whereas young-of-year herring and M. mixta were more important in September and October. Planktivory rates increased from low levels at the beginning of July to a peak in August coinciding with a late summer decline in crustacean zooplankton biomass. Planktivory rates were lower than estimated zooplankton production rates in July and early August when zooplankton biomass was increasing and similar to or higher than production in the autumn when zooplankton biomass declined. Both clupeids and mysids consistently selected prey in the order cladocerans (Bosmina longispina maritima and Pleopis polyphemoides) > Eurytemora affinis hirundoides copepods > Acartia copepods. The selected species represented a smaller proportion of total zooplankton biomass and decreased earlier in the season in the least productive area, indicating a larger and earlier effect of planktivory in that area compared to the most productive area.