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

12
NATALIA KRASOVSKAJA
7
6
A
Salinity, psu
5
4
3
2
Krasnoflotskoje
Tolkmicko
Baltiysk strait
1
0
I II III IV V VI VII VIII IX X XI XII
4.5
4
B
3.5
3
2.5
2
1.5
1
1950-1964
1960-1970
1970-1980
1980-1990
1990-2000
0.5
III IV V
Months
Fig. 4. Salinity conditions in the Vistula Lagoon. Monthly means for 1950-1964 (Anon 1971) (A)
and inter-annual changes in spring by decades (B).
Inter annual variability of spawning in relation to environmental conditions
and the state of stocks
The timing and duration of herring spawning and the hydrological conditions in the Vistula
Lagoon are remarkably variable (Table 2). Range of variables, standard deviation values and
coefficients of variation are presented in Table 3. The presence of herring schools in the lagoon
varied from 34 to 104 days throughout the period under consideration. The spawning event
extends from February 12 to June 25 in different years (March 24-May 27, on average). Within
this period, spawning and mass spawning can take place during 57 (28-90) and 35 (13-63)
days, respectively. The difference in the spawning season exceeds two months from year to
year with April 25 being the boundary between the end of the earliest spawning and onset of the
latest one. Mean water temperature within the spawning period varied from 7.7 to 14.1°C and
can rise rather quickly from about 1°C when the first herring enter the lagoon to more than
19°C during the last spawning waves. It should be noted that Baltic herring are very tolerant to
low salinity. In the Vistula Lagoon, herring spawn at a value of 1.23-4.93 psu. The data presented
are characterized by high CV, in the range of 11.6-38.7%, with minimum values at the end of
the herring spawning period (Table 3). Water temperature and salinity as well as the time when
the first herring entered the lagoon had a significant impact on the timing and duration of
spawning (Table 4).