With its narrow connection to the North Sea, strong currents, a large number of river estuaries and a bottom profile marked by ridges, basins and troughs, the Baltic represents an inland sea with highly different water qualities. The fact that these morphological and hydrographic conditions can also influence the fate of fish stocks has now been shown by a team of fisheries biologists from GEOMAR Helmholtz Centre for Ocean Research Kiel and the National Institute of Aquatic Resources (DTU Aqua) at the Technical University of Denmark. For their publication in the international journal Progress in Oceanography, they investigated the densities of cod, plaice and flounder eggs, which determine the position of the eggs within the different salinity layers and thus their dissemination throughout the Baltic Sea. "Our study is an example of how observations of nature, knowledge of the natural processes, and modeling in a bio-physical reference system combine to form a grand picture," emphasizes author Dr. Christoph Petereit. "We have combined a variety of methods in order to learn more of the whereabouts of the eggs and larvae of important fish species."
From January to March 2011, Petereit went on weekly trips to the Baltic with a commercial fisherman to catch animals in spawning condition and to fertilize eggs. Also, on four scientific expeditions with the research vessel ALKOR he collected eggs and sperm. Using glass columns with a precisely calibrated profile of different layers of salt water, the fisheries biologist then determined their density. Their diameter and dry weight were also measured.
Based on this information, the scientists calculated distribution paths of eggs and young fish larvae using a hydrodynamic model. "Our computer program simulated the direction and speed of currents which vary greatly throughout the seasons," said oceanographer Hans-Harald Hinrichsen. "In addition, it realistically models the temperature, salinity
|Contact: Maike Nicolai|
Helmholtz Centre for Ocean Research Kiel (GEOMAR)