Varied Salt Content of Baltic Sea Impacts Fish Stocks

The Baltic Sea is very unique because of its connection to the North Seas, strong currents, river estuaries and unique topography along the sea floor. In a new study, fishery biologists from GEOMAR Helmholtz Centre for Ocean Research Kiel and the Institute of Aquatic Resources at the Technical University of Denmark have shown that these "morphological and hydrographic conditions" also influence the fate of fish stock.

In the study, published in Progress in Oceanography, the researchers collected data on the densities of cod, plaice and flounder eggs found in different salinity layers. This allowed them to study how the fish would disseminate 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," said author and fishery biologist 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."

Over three months at the beginning of 2011, Petereit took weekly trips to the Baltic with fishermen to capture animals while they were spawning. In addition, he collected eggs and sperm with the research vessel ALKOR. Peteriet was then able to precisely calculate the density, diameter and dry weight of the eggs, according to a press release announcing the findings.

Using this information, the scientists were able to model the distribution pattern of eggs and young fish larvae.

"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 and oxygen at different depths for the entire Baltic Sea."

Using the model, the researchers could calculate how far the eggs were transported and whether they could survive the water conditions through which they travelled.

"Because of their different density, the eggs and the initial larval stages of different species are distributed on different levels in the water column of the Baltic," Petereit said.

Farther east in the Baltic, the water is less salty, and therefore less dense, so the eggs sink to the ocean floor where they do not survive. These density differences also ensure that the fish stocks do not mix.

In fact, genetic analyses of cod have shown that the eastern and western stocks are two separate groups. Petereit added, "Our work provides a hypothesis for why there can't be any exchange between the two stocks."

By showing distinct niches in the water where fish thrive, researchers have shown surprising lack of interdependence of the fish. For example, overfishing of one species does not benefit other species by freeing up habitat space. This conclusion has implications for fishery management, as each species must be looked after individually and not taken as a collective fish population.