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

4
NATALIA KRASOVSKAJA
30%. However, its role in herring reproduction is evidently more important due to the close fryrecruitment
relationship (Birjukov and Shapiro 1971, Krasovskaya 1992) and the high density
of larvae which in some years exceeds 9x10 10 ind. The proportion of herring belonging to other
populations is low. Otolith studies indicated that only about 2% (0.2 - 5.4%) of fishes caught in
the Vistula Lagoon are open sea herring. The percentage of herring infested by Anisakis nematode
larvae, noted in the lagoon from 1985 (Gaevskaja and Krasovskaya 1986), varies within
the range of 0.0-5.3% (1.5%, on average). According to Reimer (1970), Grabda (1974) and
Friess (1977), this infestation is considered as a natural tag of herring feeding on euphausids in
the North Sea, Kattegat, Skagerrak areas and spawning in the southwestern Baltic Sea (socalled
Rugen herring). Therefore, the spring spawning coastal herring of the southeastern Baltic
make up at least 90% of the annual catch in the Vistula Lagoon. In general, herring catch
fluctuations in the Vistula Lagoon from the 1950s correspond well with reports by Seletskaya
(1962) and Popiel and Strzy¿ewska (1971) for the southeastern part of the Baltic Sea. The
literature which attempts to define the status of this spawning stock as a distinct population is
not in consensus. Birjukov (1970), Elwertowski (1982) and Ojaveer (1988) considered this to
be the same population as that of coastal herring in the Gulf of Gdansk. Strzyzewska (1969,
1979), meanwhile, examined some of the morphological structures of Baltic herring and noted
significant differences between the populations in the Vistula Lagoon and in other spawning
grounds in the southern Baltic Sea. Considering the close relationship between fry abundance
and spawning stock recruitment of herring in the lagoon, Shapiro and Shemina (1975) concluded
that it comprised one distinct population, although the high variation of herring morphological
features was emphasized (Gaevskaja and Shapiro 1981). However, unlike the summarized
fisheries assessment for ICES Sub-divisions 25-29 and 32 (including the Gulf of Riga)
performed by the ICES Working Group from 1990, the spawning stock state in the Vistula
Lagoon may characterize the general situation of the spring spawning coastal herring of the
southeastern Baltic. The assessment unit 25-29 and 32 was created in 1990 as a combination of
several stock components with different growth rates and patterns of year-class fluctuation. In
accordance with the 1997 and 1998 Working Group on the assessment of Baltic Pelagic Stocks,
the perception of this herring stock development was quite different when considering number
and biomass (Anon. 1997, 1998). It was noted that the declining trend of stock biomass was
caused by drastic changes in the mean weight of herring. In terms of number, the stock size
estimation was even higher than the 1982-1991 average level. The most recent assessment
revealed that both the biomass and abundance of herring (ICES Sub-divisions 25-29, 32) is at
a historically low level (Anon 2000, 2001). The results obtained in the Vistula Lagoon have
shown that besides the mean weight-at-age decrease, a sharp decline of herring spawning stock
began in 1994-1995 owing to fair recruitment (Krasovskaya 1999).
The aim of this work is to review the long term monitoring data which characterizes the
Baltic herring spawning activity in the Vistula Lagoon from 1950 to 2000 and which provides
a rather precise picture of its seasonal and inter-annual variability.
MATERIALS AND METHODS
The Vistula Lagoon (54°15’-54°43’ N, 20°13’-19°25’E) is a semi-enclosed, shallow, brackishwater
basin situated in the southeastern part of the Baltic Sea (Fig. 1). The lagoon is divided by
the Russian – Polish border. The total area of the lagoon is 838 km 2 (Russian part – 473 km 2 ).