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Flounders in the Gulf of Finland: Decline caused by the near disappearance of one species

Over the past 40 years, there has been a dramatic decline in fishery landings of an iconic Baltic Sea fish: the flounder. In the 1980s, the landings of the flounder fishery in the Gulf of Finland dropped by 90 per cent, a trend that was later confirmed by fishery-independent surveys. Morphologically very similar species can disappear before we even notice they are there.

There are two cryptic (i.e. morphologically very similar) species of flounder in the Baltic Sea: the European flounder (Platichthys flesus), which spawn pelagic eggs in high-salinity offshore basins, and the recently described Baltic flounder (Platichthys solemdali), the only endemic fish of the Baltic Sea.

The latter lays demersal eggs and is well adapted to the low salinities of the coastal waters of the Gulf of Finland and northern Baltic Proper. It was long assumed that only the demersal-spawning species occurs in the Gulf of Finland, where salinity is too low for P. flesus's reproduction.

Researchers from the University of Helsinki developed a simple genetic test to distinguish the two species. By analyzing the DNA from flounders' ear bones collected over the past four decades, they discovered that European flounders were in fact once the most abundant species in the Gulf of Finland. However, they have almost completely disappeared.

"We discovered that the decline in fishery landings closely mirrors in time the near-complete disappearance of the pelagic-spawning European flounder -- a species that was not supposed to occur here. This species -- not the Baltic flounder -- dominated local assemblages until three decades ago, but has since disappeared as a result of worsening environmental conditions," says researcher Paolo Momigliano from the University of Helsinki.

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