According to Taavi Suitsberg, chairman of the NGO Lääne County Coastal Fisheries Association and a professional fisherman, fishers believe the decline in flounder numbers is due to the invasive round goby, which competes with native fish species for food and reproduces more quickly.

At the same time, Suitsberg said the growing number of gobies is a double-edged sword for fishers. Although they are said to be crowding out flounder species, gobies are themselves edible fish and serve as food for larger fish.

However, Suitsberg acknowledged that he could not say exactly why flounder numbers are declining and that fishers had also learned about the possible impact of the invasive species from scientists.

Redik Eschbaum, head of the coastal fisheries working group at the University of Tartu's Estonian Marine Institute's Department of Fish Biology and Fisheries, said aquatic ecosystems are so complex that it is difficult to pinpoint a single cause for a species' decline.

However, Eschbaum confirmed that a lack of food cannot be the direct cause, as researchers have not found emaciated flounder. Flounder numbers were also already low before the round goby became widespread, all of which points to hydrological conditions as the cause.

According to Eschbaum, cod are in an even worse situation than flounder. Like flounder, their spawning depends on inflows of saltwater. Cod fishing is banned altogether in the Baltic Sea.

One thing flounder and cod have in common is pelagic eggs, meaning their eggs float freely in the water rather than sticking to underwater plants or objects. These species spawn in deeper, saltier layers of water where their eggs also develop.

Eschbaum said there are two possible reasons why spawning has become more difficult for these species. First, decomposing algae on the seabed may be leaving too little oxygen there.

Another possibility is that climate change has altered wind patterns. "Saltwater inflows are no longer as extensive and don't occur at the same time of year as they used to. But the fish have no way of anticipating that," Eschbaum said.

Eschbaum said that historically, two types of flounder were distinguished in the Baltic Sea: deepwater-spawning flounder and coastal-spawning flounder. It was later discovered, however, that they are two entirely separate species.

Although Baltic Sea flounder are a good example of rapid evolution, the two species have adapted differently. Coastal-spawning flounder, for example, are faring somewhat better, although they too would benefit from saltier water.

Although Eschbaum said flounder numbers are not expected to increase in the near future, the species cannot be considered endangered.

--

Follow ERR News on Facebook and X and never miss an update!