Facebook post by University of Helsinki

It was long assumed that only Baltic flounders reside in the Gulf of Finland (northern Baltic Sea). Now researchers have discovered that European flounders were in fact once the most abundant species there. However, they have almost completely disappeared. European flounders have been mistaken for Baltic flounders for decades. Researchers only discovered a few years ago that they are, in fact, two species. Dynamics of fish stocks can be more complex than what they seem due to mixed populations of undiscovered species. Sometimes such cryptic species can disappear before we even notice they are there.
Disappearance of one undiscovered species caused the decline of flounders | University of Helsinki helsinki.fi

Eutrophication and climate change likely explain the near-complete disappearance of European flounders from the Finnish coast.