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

6
NATALIA KRASOVSKAJA
Two sets of herring stock assessment data which included the Vistula Lagoon were available
only from the 1970s for SD 25-29 and 32 (and the Gulf of Riga) (Anon. 2001) and for the
Vistula Lagoon spawning stock, in particular (Shemina 1977, Krasovskaya 1992); however,
there were no comparable estimations for the whole study period from the 1950s. Thus, variations
in total catch and catch per pound net in the Vistula Lagoon were used as an index of
herring stock dynamics. Daily information on catches and the number of pound nets, as well as
sample data collected during the spawning period since the 1950s were used. Two or three
random samples per week of 100 fish each, i.e. 10-25 samples per spawning season, were
examined. The total length and total weight of the fishes was measured and recorded, and the
sexual maturity stage was determined. Otoliths were used for reading age. Differentiating the
herring of different origin was based on typical features of the otolith structure. The timing of
herring spawning was defined with the relatively small error of about 3-5 days. The date of
spawning completion was the most difficult to determine. Data provided by local fishermen
who catch eel with eel traps in the Vistula Lagoon and herring samples obtained from the eel
traps were used to determine the latest date of the herring’s presence in the lagoon.
The staging of maturity was based on a six-point scale (Alekseev and Alekseeva 1996).
As mentioned, the Vistula Lagoon is a spawning ground, therefore there are adult herring in
maturing (III), mature (IV), ripe (V) and spent (VI) maturity stages. In this case, stages III and
IV are considered to be pre-spawning and stage V is running, when flowing roe and milt occurs.
These maturity stages correspond approximately to the maturing (IV), mature (V), ripe
(VI) and spent (VII) stages of the Maier scale (Anon. 1965).
The onset of spawning was determined when the proportion of fishes in stages V and VI
(VI and VII in the Maier scale) constituted more than 30% of the catches. Mass spawning was
defined as the period when the percentage of these stages exceeded 70%, and, additionally, the
catches per pound net approached the mean seasonal value. Specimens in stage V were noted
throughout the period when herring were present in the lagoon, but these single events were not
considered to indicate the onset of spawning. The problem of age/length at first maturity is not
discussed in the current work because the Vistula Lagoon is a spawning ground only and all
herring entering the basin are either in the pre-spawning and running stages of maturity.
The dates of the Vistula Lagoon’s release from ice, the average daily water temperature
and salinity values recorded by the Kaliningrad hydrometeorological observatory were used.
Water temperature data presented characterize the thermal conditions for the Baltiysk Strait
and the central part of the basin (Krasnoflotskoje). The classification of winters from the 1950s
to the 1970s by degree of severity was taken from Antonov (1964), Birjukov (1970), Sergeeva
(1983) and Anon. (1994). In more recent years, it was based on the mean monthly air temperature
in Kaliningrad during the winter season (Table 1). The results obtained correspond well
with those reported by Kalejs and Ojaveer (1989).
In order to compare spawning timing in different environmental conditions, the following
variables were grouped: first spawning wave dates (early, average, late); mean water temperature
in spring in the Baltyisk Strait (warm, average, cold); water salinity in the Vistula Lagoon
(high, average, low). The values x ± σ/2 as the borderlines of intervals were used to distinguish
the above mentioned groups of observations after the verification of the assumption of normality
with Chi-square goodness of fit test was performed (Table 2).