Feeding of 0+ smelt Osmerus eperlanus in Lake Peipsi

Abstract

INTRODUCTION Smelt Osmerus eperlanus (L.) is the main zooplanktivorous fish species in Lake Peipsi and is so an important predator on zooplankton and large invertebrates in this lake. Smelt is zooplanktivorous at younger ages, gradually shifting to larger invertebrates during growth, and the oldest and largest smelts are piscivorous (Karjalainen et al., 1997; Vinni et al., 2004). When smelt start feeding, the number of suitable food organisms available is critical for fish survival. Rotifers and crustacean zooplankton (cladocerans and copepods) are important food items during the first summer because the mouth width seems to be the critical determinant of the ability of smelt to handle large food items (Strelnikova & Ivanova, 1983; Naesje et al., 1987). Phytoplankton can also be important as an initial food item for newly-hatched smelt larvae, as found in Norwegian Lake Mjosa by Naesje et al. (1987). Later, from mid-summer to autumn, age-0 smelt in Lake Mjosa feed mainly on copepods and cladocerans, with no strong preference for either of these groups (Naesje et al., 1987). In the Kuronian Lagoon (coastal waters of the Baltic Sea), young smelt prefer cladocerans from June to September, copepods in September, and cladocerans after that (Vashkevichiute, 1959). In Finnish lakes, age-0 smelt feed mainly on copepods, sometimes on cladocerans, most often in proportion to their availability in the plankton (Sterligova et al., 1992; Karjalainen et al., 1997). Earlier research on smelt feeding in Lake Peipsi and in Lake Pihkva (Tikhomirova, 1974) showed that smelt larvae feed mainly on cladocerans (Bosmina, Chydorus, Daphnia cucullata, and Diaphanosoma brachyurum, occasionally Leptodora and Sida) and copepods (Diaptomus, Cyclops, Mesocyclops) during the spring-summer period. In June and July, chironomid larvae and adult insects become more important, and smelt of 6.9 cm and longer feed mainly on fish larvae. The results of Vashkevichiute (1959) from the Kuronian Lagoon also showed that chironomids are the main smelt food in mid-summer. Chironomids are often consumed by older smelt, from the second summer onwards. Data on the feeding behaviour of the fish in Lake Peipsi are scarce, and far from sufficient for understanding the functioning of the food web in this lake. The aim of the present study was to analyse the diet of smelt in Lake Peipsi. The food eaten was also compared with the food available in the lake. STUDY SITE, MATERIAL, AND METHODS Lake Peipsi (3558 [km.sup.2]) is located in eastern Estonia, on the border of Estonia and Russia. It consists of three parts: lakes Peipsi s.s., Lammijarv, and Pihkva. The present paper deals with Lake Peipsi s.s. It is a moderately eutrophic polymictic lake with a mean depth of 8.3 m, average concentrations of total phosphorus 40 mg P [ms.up.-3] and total nitrogen 700 mg N [m.sup.-3] (Noges et al., 2008). Descriptions of the phyto- and zooplankton of Lake Peipsi are given by Laugaste et al. (2001) and Haberman (2001), respectively. Lake Peipsi provides a great variety of biotopes with a diverse trophic state. As a result of this the flora and fauna of this lake are quite rich both in the number of species and in their abundance. Lake Peipsi can be considered a large water body of quite high productivity. The biomass of phytoplankton has fluctuated from 1 to 125 g [m.sup.-3] and the mean biomass of zooplankton (in summer) is 3.1 g [m.sup.-3]. The fish catches have usually been 8000-11 000 t or 22-31 kg [ha.sup.-1] [yr.sup.-1] since the 1930s. The main commercial fishes in the lake are smelt, pikeperch, perch, bream, pike, ruffe, roach, and until the early 1990s also vendace. Smelt samples were collected in summer and autumn 2003 with a bottom trawl (height 3 m, width 18 m, 10 mm knot-to-knot mesh size at cod-end) once a month from July to November. The samples were collected at 58[degrees]42.227N, 27[degrees] 17.653E after previous monitor trawling. …