Poor Baltic Sea conditions linked to low flounder numbers

Poor hydrological conditions in the Baltic Sea, likely influenced by human-caused climate change, are probably behind the low number of flounder, ERR reports, citing a fisheries scientist. Fishers have also attributed the decline to the spread of the invasive round goby.

Taavi Suitsberg, chairman of the Läänemaa Coastal Fisheries Society and a professional fisherman, said fishers believe round gobies are reducing local fish populations by consuming their food and reproducing more quickly.

However, Suitsberg said the increase in round goby numbers was a mixed development because the species is also a food fish for larger fish. He added that he could not say precisely why flounder numbers were falling, and that the explanation involving the invasive species had also reached fishers through scientists.

Redik Eschbaum, head of the coastal fisheries research group at the University of Tartu’s Estonian Marine Institute, said aquatic ecosystems were so complex that it was difficult to identify one exact cause for a species’ decline.

Eschbaum said the problem could not be directly attributed to a lack of food, as researchers had not found emaciated flounder. Flounder numbers had also been low before the round goby became widespread, which pointed to the condition of the Baltic Sea.

Cod is in an even worse position than flounder, Eschbaum said. Like flounder, cod spawning depends on inflows of saltwater, and catching cod is completely prohibited in the Baltic Sea.

The eggs of both species are pelagic, meaning they float freely in the water rather than attaching to underwater plants or other objects. The fish spawn in deeper, saltier layers of water, where the eggs also develop.

Eschbaum said two factors may have made spawning more difficult. Decomposing algal matter on the seabed may have reduced oxygen levels, while changing wind directions linked to climate change may have altered the timing and scale of saltwater inflows.

“The inflow of saltwater is no longer as extensive and does not happen at the time it used to. But fish cannot be expected to anticipate that,” Eschbaum said.

The Baltic Sea has historically been considered to have two flounder races, deepwater-spawning and coastal-spawning flounder. Later, scientists established that they were two entirely separate fish species. Coastal-spawning flounder are faring somewhat better, but they also need saltier water.

Eschbaum said flounder numbers were not expected to increase in the near future. However, he said the fish could not be classified as endangered. Flounder is Estonia’s fish of the year.