Investigations on fish in the sea are often made through international cooperation because fish does not bother with country boarders. On the Swedish west coast and in part of the North Sea Sweden is part of a scientific survey called the International Bottom Trawl Survey (IBTS) and has been for almost five decades.
Twice a year, in January-February and in August-September, The Department of Aquatic Resources at SLU (SLU Aqua) investigates the abundance and species composition of the fish community close to the sea bottom by trawling a number of hauls in different areas and at different depths. On board the research vessel the catch is sorted and weighed by species and a number of the individuals are length measured. For the commercial species a number of individuals are also aged, but that is made later at the laboratory in Lysekil.
In a normal year about 70 species of fish are found and also some invertebrates are noted. The focus is on benthic species, but the trawl also catch some pelagic species on the way down and up.
The purpose of the survey is long-term monitoring of demersal fish to provide data on commercial species for stock assessment calculations made by the International Council for the Exploration of the Sea (ICES) and for examining changes in fish distribution and abundance.
Now the biodiversity of fish in the open sea of Kattegat, Skagerrak and part of the North Sea is also entering the SBDI portal. Some data is already there but more will come.