Bulletin of the Sea Fisheries Institute 2 (156) 2002 - CEEMaR

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BULLETIN OF THE SEA FISHERIES INSTITUTE 2 (156) 2002 The occurrence, food and size structure of smelt (Osmerus eperlanus L.) larvae in the Lithuanian part of the Curonian Lagoon Vida Žiliukiene Institute of Ecology, Akademijos 2, 2600 Vilnius, Lithuania Abstract. Smelt larvae were sampled twice a year either at the end of May or at the beginning of June and at the beginning of July from 1996 to 2000 in the Lithuanian part of the Curonian Lagoon. The share of smelt larvae in the ichthyoplankton of the Curonian Lagoon reached 18%, on average, at the end of May and at the beginning of June. Their density varied at different stations from 0 to 28.0 ind./m 3 . The highest smelt larva density was noted on June 3, 1997. At the end of May and the beginning of June, they mainly concentrated in the eastern part of the lagoon between Vente and Kintai (Fig. 1). At the beginning of July, smelt larvae were found only at several control stations, where their density did not exceed 0.2 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. In 1998 the density of smelt larvae was lower at a depth of 0.5 m than at 1.5 m. Studies carried out repeatedly in 1999 did not confirm this tendency. Smelt larvae feed on zooplankton, mainly on copepods. However, correlation coefficients did not show a strong relationship between the density of smelt larvae and the biomass of zooplankton. At the end of May and at the beginning of June, the lengths of smelt larvae varied from 8.5 to 23.0 mm. At the beginning of July, the lengths of smelt larvae varied from 15.0 to 34.0 mm. The shortest mean length of smelt larvae was determined in 1996, while in 1998 it was the longest. Key words: the Curonian Lagoon, smelt, larvae, density, distribution, length, zooplankton, food, spawning INTRODUCTION The Curonian Lagoon is the largest inland reservoir in Lithuania. It covers an area about 1610 km 2 (413 km 2 belong to Lithuania). The lagoon is connected with the Baltic Sea in the north by a narrow strait, and Lithuania’s largest river, the Nemunas, flows into it from the south. Thus, the Curonian Lagoon is mainly a freshwater basin. The water salinity in the northern part of it can sometimes reach about 3 PSU but only when northerly and northwesterly winds prevail. The Curonian Lagoon is comparatively shallow water body; the deeper areas reach up to 5.8 m and the mean depth is 3.8 m (Kunskas 1978). Most of the pollution carried in by the Nemunas does not reach the Baltic Sea and stays in the Curonian Lagoon. Therefore, various ecological processes such as eutrophication, oxygen deficit formation, etc. are present in it.
34 VIDA ŽILIUKIENE The fish fauna in the Curonian Lagoon is composed of co-dominating freshwater and migratory fish species as well as marine fishes. In total, 36 fish species have been recorded in the Curonian Lagoon waters. From 15 to 20 fish species are usually found in commercial and experimental catches (Repeèka et al. 1998). One of the main commercial fish species in the Curonian Lagoon is smelt. The first reliable data regarding the fish resources and fishery in the Curonian Lagoon date from the end of the nineteenth century (Benecke 1881). Regular ichthyologic investigations in the lagoon have been carried out since 1949 (Maniukas 1959, Gaigalas et al. 1978, Gerulaitis et al. 1996, Repeèka 1995, Gaigalas 2001). Precise knowledge of the larval period of fish, however, is still lacking. The international programme “Assesmment and Monitoring of Costal Fish Resources” was begun in 1994 in the Curonian Lagoon to investigate ichthyoplankton (Repeèka 1996, Repeèka et al. 1996). In 1996, the first data about smelt larva density and distribution in the Lithuania part of the Curonian Lagoon were obtained (Žiliukiene 1998). The aim of this article was to generalize the data from smelt larva studies in the Curonian Lagoon, to analyze in more detail smelt density dynamics, their horizontal and vertical distribution, size structure, relationship with zooplankton. Smelt resources were also evaluated based on of research data and commercial catches. MATERIAL AND METHODS Smelt larvae were sampled twice a year either at the end of May or at the beginning of June and at the beginning of July when weather conditions were favorable (sunny, average waves, southeastern wind) from 1996 to 2000 in the Lithuanian part of the Curonian Lagoon. Twenty-eight stations were selected for the studies (Fig. 1). The depth of the Curonian Lagoon at stations 4, 23 and 24 was about 4 m, at stations 3, 7 and 11 – about 3 m, at other stations – about 2 m. The studies were carried out every year at stations 13 and 16. Gulf samplers are considered to be among the best for ichthyoplankton sampling (Aneer et al. 1992). The Gulf Olympia (Hildén and Urho 1988, Aneer et al. 1992) was used to catch smelt larvae. Two ichthyoplankton nets with 19 cm mouth opening diameter were attached at the same (0.5 m) or different (0.5 and 1.5 m) depths in front of the motorboat in order to prevent the effects of turbulent streams on the catches. The amount of water filtered through one ichthyoplankton net while towing at a speed of 0.55 m/s for 5 minutes was 4.5 m 3 . The Lithuanian part of the Curonian Lagoon is very shallow (1.5-2 m depths prevail), in places it is ranked with underwater vegetation and there are big stones on the bottom, therefore higher towing speeds at some investigated stations are dangerous. This towing speed was used for catching ichthyoplankton in the Curonian Lagoon in 1995 (Repeèka 1996) and for catching fish larvae and juveniles in the Nemunas River (Stakenas and Svecevièius 1998). The mesh size of ichthyoplankton nets varied from 90 to 500 µ (Aneer et al. 1992). It was attempted to catch fish larvae in the Curonian Lagoon with 300 µ mesh size nets. These ichthyoplankton nets appeared to be inefficient – large amounts of zooplankton were caught with it (Repeèka 1996). Therefore 500 µ mesh size nets were used for catching smelt larvae. Two samples were taken at every station. Samples from the same depth were joined together. In all, 84 samples were obtained. The extent of the annual investigations is presented in Table 1.
Page 1 and 2: ISSN 1429-2335 Bulletin of the Sea
Page 3 and 4: 4 WIES£AW BLADY, JAN NETZEL and JA
Page 9 and 10: 20 23 26 29 32 35 38 41 44 47 50 53
Page 11 and 12: 12 WIES£AW BLADY, JAN NETZEL and J
Page 13 and 14: 14 WIES£AW BLADY, JAN NETZEL and J
Page 15 and 16: 16 JÓZEF SOSIÑSKI and KORDIAN TRE
Page 31: 32 JÓZEF SOSIÑSKI and KORDIAN TRE
Page 35 and 36: 36 VIDA ŽILIUKIENE Table 1. The ex
Page 37 and 38: 38 VIDA ŽILIUKIENE 1997.06.03 1999
Page 39 and 40: 40 VIDA ŽILIUKIENE Frequency [%] F
Page 41 and 42: 42 VIDA ŽILIUKIENE REFERENCES Anne
Page 43 and 44: BULLETIN OF THE SEA FISHERIES INSTI
Page 45 and 46: Morphomechanic changes during embry
Page 55 and 56: BULLETIN OF THE SEA FISHERIES INSTI
Page 57 and 58: Variability of sprat peak spawning
Page 67 and 68: 70 JÓZEF SOSIÑSKI AND ANDRZEJ KOM
Page 77 and 78: WSKAZÓWKI DLA AUTORÓW INFORMACJE

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