Climate, zooplankton and pelagic fish growth in the Central Baltic Sea

Oceanographic conditions in the brackish central Baltic Sea are strongly linked to atmospheric forcing and the unusual period of persistently strong westerlies that, since the late 1980s, have resulted in an increase in average water temperatures and decreasing salinity. These changes in temperature and salinity resulted in a change in the dominance of the mesozooplankton community from Pseudocalanus sp. to Temora longicornis and Acartia spp. Similar to the copepod community, the central Baltic fish community shifted from cod ( Gadus morhua ), dominant during the 1980s, to sprat ( Sprattus sprattus ), dominant during the 1990s. Further, the commercially important pelagic fish species herring ( Clupea harengus ) and sprat exhibited reductions in growth. Using Principal Component and Correlation Analyses we investigated the temporal variability in the importance of the food supply as well as competition on condition of central Baltic pelagic fish species. Our results indicate that herring condition results from a combined effect of changes in the food environment and increased competition with sprat, while sprat condition appeared to be primarily determined by intra-specific competition

Food consumption by clupeids in the Central Baltic: evidence for top-down control?

Considerable changes have taken place in the pelagic ecosystem of the Central Baltic Sea during the last decade. Owing to a combination of high fishing pressure and unfavourable hydrographic conditions, the cod (Gadus morhua) stock as the top predator in the system was reduced from a high level in the early 1980s to its lowest size on record in the early 1990s. The preferred prey species sprat (Sprattus sprattus) showed a significant increase in population size since the late 1980s to the highest level on record in recent years, while the herring (Clupea harengus), another important planktivore in the system, did not show such a response. We investigate whether fluctuations in clupeid stock size cascade down to the trophic level of mesozooplankton, based on stomach content data and daily ration estimates in combination with stock sizes estimated from Multispecies Virtual Population Analysis. Estimates of daily consumption by the populations of the two species for 1978–1990 were compared with standing stocks of mesozooplankton species. No evidence was found for food limitation as might be expected if clupeids exert a strong top-down control on mesozooplankton. Also no influence on interannual variability of mesozooplankton abundance was detected. However, predation did contribute to the seasonal development of two copepod species (Pseudocalanus elongatus and Temora longicornis)

Analysis of instance hardness for the maximally diverse grouping problem and the iterated maxima search heuristic

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Ramifications of generalized Feller theory

Generalized Feller theory provides an important analog to Feller theory beyond locally compact state spaces. This is very useful for solutions of certain stochastic partial differential equations, Markovian lifts of fractional processes, or infinite dimensional affine and polynomial processes which appear prominently in the theory of signature stochastic differential equations. We extend several folklore results related to generalized Feller processes, in particular on their construction and path properties, and provide the often quite sophisticated proofs in full detail. We also introduce the new concept of extended Feller processes and compare them with standard and generalized ones. A key example relates generalized Feller semigroups of algebra homomorphisms via the method of characteristics to transport equations and continuous semiflows on weighted spaces, i.e. a remarkably generic way to treat differential equations on weighted spaces. We also provide a counterexample, which shows that no condition of the basic definition of generalized Feller semigroups can be dropped

Spatial and temporal variability in the stomach content of herring and sprat in the Bornholm Basin

Herring and sprat stomachs and zooplankton samples were collected on 8 sites in the Bornholm Basin in May/June 1999 at different times of the day. Sprat showed a distinct increase in stomach content from early morning to the afternoon, whereas the stomach content of herring remained rather constant. Both herring and sprat showed a clear dominance of mesozooplankton in food composition with only small contributions of makrozooplankton and ichthyo-plankton. Both clupeid species showed similar preferences in selection of food items with a dominance of the copepods P. elongatus and T. longicornis. Compared to herring, other copepod species, especially Acartia spp. contributed to a larger extent to the food of sprat. Cladocerans played a less important role as food for both species. Spatial differences in stomach content have been revealed between the sampling sites. These resulted from differences in the distribution of food organisms in the water column as well as a different vertical distribution of the predators at different times of day. The share of juvenile copepods in herring stomachs was bigger than in sprat stomachs. Generally, selective feeding seemed to be stronger developed in sprat than in herring