Bulletin of the Sea Fisheries Institute 1 (155) 2002 - CEEMaR

18
NATALIA KRASOVSKAJA
% Late spawning
100
80
60
40
20
%
%
0
100
80
60
40
20
0
100
80
60
40
20
0
I II III I II III I II III I II III I II
Average spawning
I II III I II III I II III I II III I II
Early spawning
I II III I II III I II III I II III I II
February Mach April May June
age group2 age group 3 age group >3
Fig. 10. Changes of herring age composition in the Vistula Lagoon by 10-day periods within the early,
average and late spawning periods.
herring at the spawning grounds was observed later due to the late ice breakup in the Vistula
Lagoon (Table 2). Further, a rapid increase in air temperature caused intensive water warming;
thus the mean water temperature in the 1951-1954 spawning period (13.3 o C) exceeded the
average level (10.3 o C). According to the current results, such thermal conditions cause mass
herring spawning to begin immediately and occur continuously, despite herring schools replacing
each other. The ripe stage dominates throughout the period which herring are present in the
lagoon. In opposition to the first half of the 1950s, the average interval between the first spawn-