Environmentally driven predator-prey overlaps determine the aggregate diet of the cod Gadus morhua in the Baltic Sea

Aquatic ecosystems are environmentally heterogeneous with features such as fronts or clines of temperature and salinity. This heterogeneity varies over time and is likely to cause changes in predator-prey overlaps, which will affect the diet composition of the predators. We investigated how inflows of oxygenated and saline deep water alternating with stagnation periods affect the consumption rates of the herring Clupea harengus L. and the sprat Sprattus sprattus L. by the cod Gadus morhua in the Bornholm basin of the Baltic Sea. We developed conceptual models for the effect of predator-prey overlaps on the aggregate diet of the predator population to test the hypothesis that the effects of inflows on the aggregate diet are mediated by changes in cod-clupeid overlaps. After estimating salinity and oxygen thresholds of the spatial distributions of cod and clupeids and calculating cod-clupeid overlaps from March 1958 to 2004, we applied the models relating cod-clupeid overlaps to observed numbers and masses of herring and sprat in cod stomachs. Our study indicated that (1) the ratio of consumed herring to sprat increases faster than proportionally to the ratio of cod-herring to cod-sprat overlaps, and (2) the ratio of consumed herring to sprat increases slower than proportionally to the ratio of herring to sprat in the sea. The latter is most pronounced at the low ratios of overlaps that occur during stagnation periods, which destabilise the clupeid populations, and may have contributed to the drastic increase in the sprat abundance during the late 1980s.