3 sites in the Bornholm Basin: station 16 (59° 09'N, 15° 40'E), station IBY-5 (55° 14'N, 15° 59'E), station 27 (55° 00'N, 16° 00'E); 1 site in the Gdansk Basin: station G-2 (54° 50'N, 19° 20'E); 1 site in the Gotland Basin: station Gt-1 (55° 36'N, 18° 26'E)

Dissolved oxygen parameters in the water column; Zooplankton wet weight biomass; Salinity of the water column; Temperature of the water column; Zooplankton taxonomy-related abundance per unit volume of the water column

The aim of the project was to assess factors influencing the recruitment of the Baltic fish utilized by fisheries. In the period 1991-1993, five deep water stations were sampled randomly, 2-3 times per year. The sampling periods are as follows: 20-25 Mar 1991, 25-29 May 1991, 05-09 Aug 1991, 28 May-04 Jun 1992, 08-12 Jul 1992, 06-11 Oct 1992, 02-07 Apr 1993, 23- 28 Jul 1993. The mesozooplankton abundance and species composition were determined within the range of individual biomass from 63 ng to 16 microg (approximately 50-500 microm of length). The following depth strata, corresponding to the average location of a hydrological phenomenon were sampled: 0-15m (euphotic layer, above the seasonal thermocline); 15-30m (layer of seasonal thermocline), 30-60m (lower part of seasonal thermocline, and lower part of isohaline layer); 60-90m (halocline layer) or 60-75m at the station no. 27 (upper part of halocline and near bottom layer); 90-104m only at the station G-2 (near bottom layer of oxygen deficit below halocline). Samples from the mentioned depth strata were obtained by integration of material from 4 discrete levels sampled with a single 5 or 9 litre bathymeter, e.g. sample from 0-15m depth-strata was obtained by mixing water portions from 0, 5, 10 and 15m, while 15-30m sample was obtained by mixing samples from 15, 20, 25 and 30m depth, 30-60m sample by mixing samples from 30, 40, 50 and 60m, etc. Such an approach gave the total examined water volume of 20 to 36l per 15 or 30m depth-strata. The water was filtered through 40 microm gauze and plankton concentrated in this way was preserved with 4% formalin buffered with borax. Samples, when necessary split into subsamples, were examined under a dissecting microscope, identified (if possible to species level), staged, counted, measured and the biomass of individual specimens was calculated by means of algorithms for volumes of solid figures. Results were grouped into 8 body volume (biomass) groups on the assumption that volume of 16 microm**3 equals the weight of 1 microg. The specimens whose biomass exceeded 16 microg were not considered here, as they were either inadequately sampled or were too large to be consumed by early cod larvae.

National Marine Fisheries Research Institute
Kollataja 1
Gdynia

81-332
Poland