FORAGING AND GROWTH OF BURBOT LARVAE, Lota lota (L FORAGING AND GROWTH OF BURBOT LARVAE, Lota lota (L.), REARED UNDER DIFFERENT CONDITIONS OF THE ENVIRONMENT OF ILLUMINATED NET CAGES Grażyna Furgała-Selezniow1, Roman Kujawa1, Dariusz Kucharczyk1, Andrzej Mamcarz1, Daniel Żarski1, Katarzyna Targońska1, Marcin Kuciński2, Małgorzata Jankun2, Andrzej Skrzypczak3 1)Department of Lake and River Fisheries, University of Warmia and Mazury (UWM) in Olsztyn, Poland e-mail: graszka@uwm.edu.pl 2) Department of Ichthyology, UWM in Olsztyn, Poland 3) Department of Tourism and Recreation, UWM in Olsztyn, Poland Introduction Burbot (Lota lota), an exceptional gadoid fish in that it spends its entire life cycle in freshwater, has recently gained more interest owing to environmental and economic aspects (Kujawa et al. 2002). Because it is an endangered species (Paragamian et al. 2008), efforts have been undertaken to produce stocking material for burbot restoration (e.g. Kujawa et al. 2002, Paragamian et al. 2011). This species is also an interesting candidate for aquaculture production (Wocher et al. 2013). In order to improve the rearing success of this species, suitable rearing techniques for burbot larvae have to be developed. This study examines the effect of different environmental conditions on the growth, foraging behaviour and food selection of burbot larvae reared in illuminated cages. Material and Methods Fish rearing in illuminated net cages was carried out in mesotrophic Lake Maróz (north-eastern Poland), in two successive years. The experiment lasted for six weeks. The cages were stocked with 20 DPH old burbot larvae, at a density 1250 indiv. m-3. Each year, two rearing variants were tested: one composed of two cages with hiding places for larvae and the other one consisting of two cages where larvae had nowhere to hide. The zooplankton accessibility to reared burbot was defined and the fish growth was determined. The fish food was examined under a microscope for the species structure and number of individuals. Food selection according to the zooplankton groups (Rotifera, Cladocera, Copepoda) and length classes was expressed by the Strauss linear selectivity index (L). The results of rearing were compared between the variants. Figure 2. Mean number of prey in alimentary tracts of Lota lota larvae reared in two successive years in illuminated cages; vertical bars represent SD; I - year I, II - year II; W - cage without hiding places, H - cage with hiding places. Table 1. The mean values of Strauss food selection index (L) of burbot (Lota lota) juveniles reared in illuminated cages in Lake Maróz in the year II of experiment in relation to the length classes of zooplankton. Days of rearing Plankton length classes (mm) Cages without hiding plasces Cages with hidnig places t1 SD L 7 0.0-0.5 0.6-1.0 1.1-1.5 4.59* 5.13* -47.53* ±17.16 ±17.02 ±3.50 +14.40* +15.95* -30.35* 2.02 14.46 -73.99 ±10.55 ±10.86 ±2.41 +3.89 +28.67* -32.56* 14 -4.35* 7.97* -28.42* ±18.79 ±18.30 ±2.26 -14.93* +26.64* -11.71* -8.32* 8.78* -14.50* ±17.44 ±20.94 ±3.93 -25.78* +36.08* -10.30* 21 >2.0 -7.58* 6.79* 2.56* 1.16 ±24.15 ±23.29 ±2.05 ±1.10 -31.63* +30.37* +1.02* +0.24 -10.58* 10.67* -0.07 1.00 ±18.54 ±18.37 ±1.33 ±0.11 -35.81* +35.81* -0.02 +0.02 28 +1.23 -1.09 -5.48* ±12.36 ±12.37 ±0.31 +2.78 -2.47 -0.31* -2.25* 2.37* -6.34* ±14.63 ±14.65 ±0.27 -6.01* +6.33* -0.32* 35 1.6-2.0 -4.09* 4.27* -1.81 -4.10* 2.32* ±18.12 ±13.92 ±1.05 ±0.35 ±7.72 -13.52* +10.86* -0.35 -0.26* +3.27* -3.97* 4.41* -1.94 -2.37* 1.59 ±22.20 ±14.09 ±1.16 ±0.54 ±18.62 -16.10* +11.34* -0.41 -0.23* +5.40 42 -5.08* 5.59* -8.89* 0.41 2.11* ±15.90 ±15.31 ±0.69 ±1.22 ±0.34 -14.74* +15.64* -1.12* +0.09 +0.13* -6.21* 5.93* 1.31 -7.04* 1.82 ±17.77 ±3.24 ±0.10 ±1.80 -20.56* +19.26* +0.77 -0.13* +0.66 Results The species composition of zooplankton in the lake was similar in the two years of the experiment. Organisms shorter than 0.5 mm prevailed in the plankton (Fig. 1). The mean number of prey found in the alimentary tract of burbot larvae increased during the course of the study: from about 40 specimens at the beginning of the experiment up to over 300 in cages with hiding places (Fig. 2). Figure 1. Percentage composition of zooplankton according to the length classes in illuminted cages in Lake Maróz in the two following years (I & II) of experiment. Indexed (*) t test statistics values are statistically significant (p<0.05). Indexed (*) L values differed significantly from zero (p<0.05). Critical value t = │2.05│(df 29; α=0.05) Discussion The method of rearing fish larvae in illuminated net cages is based on the positive phototaxy of crustacean plankton. Successful rearing needs good access to food (Paragamian et al. 2011). The concentration of plankton in cages during the experiment, usually exceeding 400 indiv. dm-3, seemed to be sufficient for burbot larvae, as there were no starving larvae and fish cannibals recorded. Big inter-specimen variation in the amount of ingested food was also observed by other authors, e.g. Žiliukienė (2005) for Abramis brama (L.) larvae. During our experiment, burbot larvae tended to select medium-size prey irrespective of their number in the environment. References demonstrate similar tendencies for other fish species (e.g. Ponton, Müller 1990 for whitefish, Coregonus sp.). The mean number of ingested zooplankton individuals was slightly higher in cages with hiding places, especially in the second year of the experiment (Fig. 2). Also, the mean final body weight (i.e. in the second year) of fish reared up in cages with hiding places was higher than in cages where larvae could not hide (175.4 mg versus 156.7 mg). Our analysis of the values of the Strauss food selectivity index shows that at the beginning of the first year of experiment, in both rearing variants, burbot larvae preferred copepods, most numerous in the environment at that time; as they growth, larvae changed their food interest into cladocerans. In the second year, the fish more often ate copepods, irrespective of their quantities in the environment. During the whole study, the reared burbot larvae did not eat rotifers, even when they were numerous in cages. The smallest planktonic organisms measuring up to 0.5 mm in length, like rotifers, were typically neglected by fish. In contrast, the 0.6-1.0 mm group was most frequently selected (Table 1). There were also considerable individual differences between particular burbot specimens in their food preferences as well as in the amount of consumed food organisms. References Kujawa R., Kucharczyk D., Mamcarz A. 2002. Miętus. Wyd. IRS. 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