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

22
NATALIA KRASOVSKAJA
mid summer at average water temperatures of 6-7 o C (Oulasvirta et al. 1985). In the Bothnian
Bay located in the northernmost part of the Baltic Sea, spawning begins one month later and
sometimes takes place in July. The major spawning event occurs at water temperatures ranging
from 3-4 to 15-16 o C (6-7 o C, on average). Berenbeim and Evtjuchova (1986) studied herring
reproduction timing in different areas of the Baltic Sea under the assumption that they were
functions of latitude. They argued it was possible to forecast the onset of spawning in the
northern spawning grounds based on results obtained earlier in the southern areas. Hence, the
above-mentioned shift to an earlier spawning time in the Vistula Lagoon must be reflected in
other spawning areas of the Baltic Sea.
The current results suggested that repeated spawning in the Vistula Lagoon occurs in the
same locations at different times. This concurs with observations made by Aneer (1983) in the
northern Baltic proper, by Oulasvirta (1985) in the Gulf of Finland and by Kornilovs (1994) in
the Gulf of Riga.
The spawning pattern causes year class variations particularly under such erratic conditions
as those observed in the Vistula Lagoon in spring (Anon. 1971, Sergeeva 1983). It is
likely that conditions within a protracted spawning period are so variable that occasionally
only part of the season provides for high embryonic and larval survival. Thus, a prolonged and
discrete spawning period ensures that either multiple cohorts or a successful one, whose development
coincides with favorable conditions, contribute to recruitment (Cushing 1982).
Historical data on herring fishery in the Vistula Lagoon have shown the vulnerability of
the spawning stock to heavy exploitation during the breeding period, and examples of clupeid
collapse and the following stock recovery are well documented (Lasker 1985, Jakobsson 1985,
Ojaveer 1988), although the reasons for such events are not always evident. High fishery mortality
in the Vistula Lagoon in the 1950s caused recruitment failure by reducing the mass spawning
time and the capture of a significant part of the spawners in the premature stages just before
breeding. These types of changes in the spawning run coupled with high fishery pressure probably
led to the decreased survival of embryonic stages (Birjukov and Shapiro 1971, Shapiro
1975), and caused a drastic decline of spring spawning coastal herring in the southeastern
Baltic.
In consideration of the importance of this spawning area to herring and the real probability
of overfishing during the spawning season, the catch value and number of pound nets in the
Russian part of the Vistula Lagoon have been limited since the mid 1970s.
REFERENCES
Alekseev, F. and E. Alekseeva 1996. Assessment of gonads maturity stages and study of sex cycles,
fecundity, eggs production and maturation rate of marine commercial fishes. Methodical manual.
Tr. AtlantNIRO, Kaliningrad: 1-75 [in Russian].
Aneer, G., G. Florell, V. Kautsky, S. Nellbring S. and L. Sjostedt 1983. In-situ observations of Baltic
herring (Clupea harengus membras) spawning behaviour in the Asko-Landsort area, northern
Baltic proper. Mar. Biol. 74: 105-110.
Aneer, G. 1989. Herring (Clupea harengus L.) spawning and spawning ground characteristics in the
Baltic Sea. Fish. Res. 8: 169-195.
Anon. 1965. Manual of sampling and statistical methods for fishing biology. FAO Manuals in Fisheries
Science. N 1, Fasc. 9. Sec. 4: 37-40.