ICES CM 3/R:8 Distribution and size structure of smelt (Osmerus eperlanus L.) larvae in the Lithuanian part of the Curonian Lagoon during their migration to the Baltic Sea Vida ŽILIUKIENĖ Institute of Ecology of Vilnius University, Akademijos 2, 26 Vilnius, Lithuania. Fax: (+37 5) 2729257, E-mail: ziliukas@ekoi.lt INTRODUCTION At present, more and more attention is paid to the studies of fish larvae, which not only provide better understanding of fish life during their initial stages of development, but also allow to evaluate better fish resources, their spawning conditions and forecast their catching limits. Since fish larvae are one of the most sensitive organisms to various environmental changes, the influence of abiotic and biotic factors on their abundance and species diversity is much greater than on adult fish. Constant ichthyological studies in the Lithuanian part of the Curonian Lagoon have been carried out since 949. However, the fish larvae in this water body have been investigated insufficiently. The studies were mostly concentrated on nutrition of fish larvae. Only in 996 the first comprehensive data of smelt larvae abundance, distribution in different parts of the Curonian Lagoon were obtained. This paper generalises the data collected during the period from 996 till 2 on the abundance, distribution and size structure of smelt larvae in the Lithuanian part of the Curonian Lagoon. MATERIAL AND METHODS Smelt larvae were sampled twice a year depending on weather conditions at the end of May or at the beginning of June and at the beginning of July from 996 to 2. Thirtythree stations were chosen in the aquatic part of the Curonian Lagoon (Fig. ). The studies were carried out every year at stations No. 3 and 6. The depth of the lagoon at the stations No. 4, 23 24, 29 and 32 was about 4 m; at the stations No. 3, 7, and 3, the depth was about 3 m; at the remaining stations the depth was about 2 m. The nearest stations were situated at the distance of 5 m off the shore.
5 km Baltic Sea Juodkrantė 22 2 Dreverna 9 8 Pervalka 29 27 33 Preila 32 26 3 Nida 25 24 Curonian Lagoon 28 23 2 5 4 7 3 Ventė 8 9 5 6 7 4 6 3 Kintai Atmata Vytinė 2 Skirvytė Figure. Location of smelt larvae sampling stations in the Curonian Lagoon. Smelt larvae were caught at the depth of.5 m with two ichthyoplankton nets of the Gulf Olympia type, with the entry opening measuring 9 cm in diameter and with the 5-µ mesh size. The amount of filtered water with one net made 4.5 m 3. Samples were preserved in 4 % formaldehyde solution. In total, 68 samples were taken and over 6 larvae were measured. Statistical analysis of the data obtained was carried out based on Statistica 5. and Excel program packages.
RESULTS At the end of May and at the beginning of June 996-2 the share of smelt larvae in the ichthyoplankton of the Curonian Lagoon was 25%, on average. The highest proportion (62.7 %) of smelt larvae in the ichthyoplankton was determined in 996 (Fig.2). Since 997, their ratio had sharply decreased. At the beginning of July the share of smelt larvae in the ichthyoplankton of the Curonian Lagoon was 4 %, on average. 7 6 5 4 % 996 997 998 999 2 Years Figure 2. Proportion of smelt larvae (% according to abundance, N) in the ichthyoplankton of the Curonian Lagoon at the end of May and at the beginning of June in different years. The density of smelt larvae at the end of May and at the beginning of June 996 2 at different stations varied from to 28. ind./m 3. The acquired data revealed that most of the migrating smelt larvae reach at this time the part of the Curonian Lagoon among Ventė, Preila, Pervalka and Kintai. Their highest abundance was found at stations situated further south than Kintai. Figure 3 shows that the highest abundance of smelt larvae in 996 was found at station No. 5 (2.2 ind/m 3 ); in 997, 999 and, it was highest at station No. 3 (28., 6.7 and.3 ind/m 3 respectively); in 998, it was highest at station No. 3 and 6 (.3 ind/m 3 ); and in 2, the highest abundance of smelt larvae was recorded at station No. 6 (.75 ind/m 3 ). The lowest abundance of smelt larvae was caught in the water areas to the south of Ventė Cape and to the north of Dreverna. Horizontal
distribution of smelt larvae revealed that they migrate mainly from their spawning grounds to the Baltic Sea through the eastern part of the Curonian Lagoon. 996 5 28 999 6 4 8 2 8 6 4 2 7 6 5 4 3 2 6 7 8 9 3 4 5 6 2 2 3 6 7 23 24 25 26 27 28 997 6 3 6 4,4 25 5,2,8,6,4 5,2 3 4 5 7 3 6 2 3 6 998 5 26 2 6 5,4,2,8,6,4,2 2 3 6 7 8 9 2 22,8,7,6,5,4,3,2, 2 3 6 7 26 27 28 29 3 32 33 Figure 3. The density of smelt larvae at different stations of the Curonian Lagoon at the end of May and at the beginning of June 996-2 While observing the dynamics of the densities of smelt larvae at stationary stations No. 3 and 6 from 996 till 2, it was found out that numerous years were followed by sparse years (Fig. 4). Especially high densities of smelt larvae were recorded on June
3, 997. It should be noted that not a single smelt larvae was caught at control stations on June 2,. The many-year dynamics of densities of smelt larvae at stationary stations No. 3 and 6 was very similar to that in the whole aquatic part. Trend line shows a decrease in the density of smelt larvae. If the numerous year 997 is excluded, trend line remains the same, thus we can talk about the decrease of smelt larvae in the Lithuanian part of the Curonian Lagoon. 25 5 trend line 5 996 997 998 999 2 Years Figure 4. Average density of smelt larvae at stacionary stations No. 3 and 6 at the end of May and at the beginning of June in different years. At the beginning of July, smelt larvae were found only on several control stations, where their density did not exceed.2 ind./m 3. A significant decrease in smelt larvae can be observed in the Curonian Lagoon during this month, as the majority of them have already migrated to the Baltic Sea. The caches by beach-seine showed, that in July the highest concentration of smelt is in the northern part of Lithuanian coastal zone of the Baltic Sea near Monciškės. At the end of May and at the beginning of June, the length of smelt larvae in the Curonian Lagoon varied from 8. to 27. mm. The shortest mean length of smelt larvae was determined in 996, while in it was the longest. At the beginning of July, the length of smelt larvae varied from 5. to 34. mm.
Figure 5 shows that larvae caught later were not always longer than the larvae caught earlier. For example, on 28 May 996 the average length of larvae was 4.3 mm, whereas the average length of larvae caught on 26 May 998 was 8.5 mm. It is known that smelt spawn when the water temperature reaches +4 6 C. Water of the Curonian Lagoon reaches this temperature from the end of March till the beginning of April. In 996, the temperature of 4 C was registered only on April. Thus, smelt began to spawn much later than usual, i.e. only on 7 April. It determined the small size of smelt larvae in 996. 26 22 Length (l) mm 8 4 6 998 5 26 996 5 28 999 6 997 6 3 6 4 2 6 5 Date Max Min Mean+SD Mean-SD Mean Figure 5. Length of smelt larvae at the end of May and at the beginning of June in different years. CONCLUSIONS. At the end of May and at the beginning of June 996-2 the share of smelt larvae in the ichthyoplankton of the Curonian Lagoon was 25%, on average. The highest proportion (62.7 %) of smelt larvae in the ichthyoplankton was determined in 996 Since 997, their ratio had sharply decreased. At the the beginning of July the share of smelt larvae in the ichthyoplankton of the Curonian Lagoon was 4 %, on average.
2. The density of smelt larvae at the end of May and at the beginning of June 996 2 at different stations varied from to 28. ind./m 3. At that time the highest assemblages of smelt larvae in the aquatic part of the Curonian Lagoon were found between Ventė and Kintai. Smelt larvae migrate to the Baltic Sea from their spawning grounds mainly through the eastern part of the Curonian Lagoon. The highest density of smelt larvae was recorded on June 3, 997. Not a single smelt larva was caught at study stations on June 2,. Trend line shows a decrease in the density of smelt larvae. 3. At the beginning of July, smelt larvae were found only at several control stations, where their density did not exceed.2 ind./m 3. A significant decrease in smelt larvae can be observed in the Curonian Lagoon during this month, as the majority of them have already migrated to the Baltic Sea. 4. At the end of May and at the beginning of June, the length of smelt larvae in the Curonian Lagoon varied from 8. to 27. mm. The shortest mean length of smelt larvae was determined in 996, while in it was the longest. At the beginning of July, the length of smelt larvae varied from 5. to 34. mm. The temperature of water and the beginning of the spawning period influenced the length of smelt larvae.