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

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16 NATALIA KRASOVSKAJA % 100 80 60 40 20 0 I II III I II III I II III I II III I II February March April May June early spawning late spawning average spawning Fig. 7. Percentage of herring at maturity stages III, IV (IV and V of Maier scale) in the Vistula Lagoon. Changes by 10-day periods within the early, average and late spawning periods. Fig. 8. Time interval between the first herring entering the Lagoon and the start of mass spawning in relation to thermal conditions – mean water temperature and temperature difference between both mentioned periods (1970-2000). In general, herring arrive at spawning areas in “waves”. Older fish begin maturation earlier and reach the high stages of maturity more rapidly (Iles 1964); this is the reason for the trend of larger and older specimens dominating at the beginning of the spawning period. Changes in maturity stage ratios, CPUE and age composition of the schools during the late, early and average spawning season in the Vistula Lagoon are presented in Figs. 7, 9-10. Typically, three “waves” are observed. The first one consists of age groups 3 and older, three-year-old fishes
Spawning of the Baltic herring in the Vistula Lagoon .... 17 25 Catch per pound net by 5-day periods [t] 20 15 10 5 0 20 40 60 80 100 Cumulative catch [%] early spawning average spawning late spawning Fig. 9. CPUE dynamics within early, average and late herring spawning period in the Vistula Lagoon by 5-day periods in relation to the cumulative catch. constitute the bulk of the second one and they often dominate throughout the spawning period. Age group 2 (recruits) spawning is usually observed later when water temperatures are higher (Tables 3, 4; Fig. 10). The available data suggest that the fish of the consecutive “waves” stay in the lagoon no longer than 5-10 days before migrating to the Baltic Sea immediately after spawning. In the case of early spawning onset, more than three “waves” can be detected. Less abundant schools gradually enter the lagoon at intervals of a few days, but the succession of age composition changes remains the same. In the case of late spawning, herring schools replace each other uninterruptedly (Fig. 9). As spawning progresses, the fishes undergo final maturation prior to entering the lagoon where spawning occurs continuously and the ripe stage dominates (Fig. 8). The size of fish within age groups usually decreases towards the end of breeding. Mean length values at ages 2 to 6 fall by 3.7-5.9% and mean weight values by 19.4 - 22.4%. Based on the long reproduction period and spatial distribution of herring schools as well as early larvae concentrations, it is assumed that repeated spawning occurs in the same locations at different times. Long term trend in herring spawning time in the Vistula Lagoon The trend towards more frequent mild winters and the early occurrence of spring, especially in the last decade, should be mentioned when studying the period since the 1950s. These climate changes effectively advance reproductive processes through both physiological and environmental conditions. A large proportion of fish spend the over-wintering season in stage III (IV on the Maier scale) (Iles 1964, Butskaja and Kuznietsov 1977). Higher temperature accelerate gonad growth and the achievement of a higher maturity stage. At the same time, mild winters cause ice to clear earlier and water warming to commence in the inshore spawning areas. A distinct shift towards an earlier herring spawning time (Figs. 11 and 12) at lower temperatures was observed in the Vistula Lagoon in spite of the positive trend of mean water temperature in spring (Figs. 13, Tab. 5). The first half of the 1950s differ significantly from among the years under consideration. The annual catches in this period were at the highest level (Fig. 2, Table 5). The arrival of
Page 1 and 2: ISSN 1429-2335 Bulletin of the Sea
Page 3 and 4: BULLETIN OF THE SEA FISHERIES INSTI
Page 5 and 6: Spawning of the Baltic herring in t
Page 7 and 8: Table 1. Data characterizing the he
Page 13 and 14: Table 4. Timing (date) and duration
Page 15: Spawning of the Baltic herring in t
Page 27 and 28: 28 KORDIAN TRELLA Studies of the fe
Page 29 and 30: 30 KORDIAN TRELLA Okhotsk Magadan S
Page 31 and 32: 32 KORDIAN TRELLA Table 1. Walleye
Page 33 and 34: 34 KORDIAN TRELLA DISCUSSION Walley
Page 35 and 36: 36 KORDIAN TRELLA REFERENCES Bailey
Page 39 and 40: Evaluation of the chemical composit
Page 47 and 48: Length correction of larval and ear
Page 49 and 50: Length correction of larval and ear
Page 51 and 52: 54 SVAJUNAS STANKUS There is little
Page 53 and 54: 56 SVAJUNAS STANKUS 60 50 Length [c
Page 55 and 56: 58 SVAJUNAS STANKUS Commercial sign
Page 57 and 58: 60 SVAJUNAS STANKUS Repechka, R., S
Page 59 and 60: 62 NORBERT WOLNOMIEJSKI and IRENA G
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WSKAZÓWKI DLA AUTORÓW INFORMACJE
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Bulletin of the Sea Fisheries Institute 1 (155) 2002 - CEEMaR

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