Piotr Margońskii and Krystyna Maciejewska
Sea Fisheries Institute,  Kołłątaja 1, 81-332 Gdynia, Poland

Key words: Mysis mixta, Neomysis integer, southern Baltic Sea, distribution, abundance, biomass.

Abstract.
The distribution, abundance and biomass of two Mysidacea species (Mysis mixta and Neomysis integer) is described. M. mixta was the most abundant in the Słupsk Furrow and at the shallow water stations along the Polish coastline. It was almost absent in the deep waters of the Bornholm Basin. Occasionally, M. mixta was observed in the Gdańsk Deep. Neomysis integer was less abundant than M. mixta. It was found mainly in shallow waters south of Bornholm Island, on the northern edge of the Słupsk Bank and farther to the east, very close to the coastline.
The values calculated from MIK net samples were lower than those from Bongo nets at the same stations. This was probably caused by a filtered water volume about 15 times greater in the case of the MIK nets. The greater volume of water decreased the influence of the patchy distribution of mysids and increased the probability of their successful catch.
A significant increase was observed, in comparison to the 1993-1994 data, in the abundance and biomass of both species of Mysidacea during June 1995 in the Słupsk Furrow, and during the August 1996 cruise at the stations in the Gdańsk Basin. This might be explained by the movement of large water masses and the improvement of oxygen conditions in the Słupsk Furrow and the Gdańsk Basin