Introduction
The intensive freshwater aquaculture using the recirculating aquaculture systems (RAS) is new approach for getting more efficient, stable, profitable and high-quality marketable fish production which is currently mainly supported by the traditional pond and water through culture methods
. The using of polyculture in the RAS with similar requirements on environment and with different behavior, swimming and feeding activity, offers great potential to more effectively use capacity of this kind of aquaculture
. The aim of this study was to optimize the intensive largemouth bass culture in the RAS and to monitor the impact of the initial density on the efficiency of its production, as well as to compare the impact of the biculture of largemouth bass and pikeperch on their production efficiency with the monoculture of both species.
Material and methods
The first 140days experiment assessed the effect of initial fish density in three levels: low: 23kg.m-3; medium: 35 kg.m-3; high: 46 kg.m-3.
In total, 3303 pcs. of experimental fish [ with total length (TL) = 217.8– 225.2 ± 18.2–19.7 mm and body weight (BW) = 139.9–157.1 ± 37.2–43.6 g] were stocked in nine tanks connected to large scale experimental RAS.
After 8 hours of light, the fish were kept for another 4 hours at a reduced light intensity (until 7:00 pm) of 70– 80 lux in order to offer the largemouth bass an optimal light period and support fish sufficiently by feed and maintain all routines in RAS. Experimental fish of all groups in all tanks were fed 50% by hand and 50% by belt feeder from 8:00 am to 7:00 pm with actual calculated daily feeding rate (DFR) at 28day interval. At the beginning of the experiment, DFR was set at 1.5% of fish biomass and DFR was reduced to 0.75% of fish biomass from the middle of the experiment (from 72nd day of this experiment).
Fish were fed commercial floating feed R-2 Europa F 15 (Skretting, Norway) with a size of 2.0 mm (at the beginning of the experiment) to 3 mm (at the end of the experiment). Such pelleted feed was collected, and all non-consumed pellets were counted.
The second experiment tested the effect of mixed culture of largemouth bass and pikeperch compared to monoculture of both species during 60days intensive aquaculture. Same aged fingerlings of largemouth bass (TL = 103.8 ± 3.5 mm; W = 12.5 ± 2.5 g) and pikeperch (TL = 121.7 ± 5.5 mm; W = 12.2 ± 1.9 g) , which were produced by a combination pond and RAS culture according Policar et al. (
) . In total, 700 fish were stocked in each tank (in monoculture only one species a in biculture both species with ratio 50:50) with initial density 1.17 fish and 14.5–14.8 g per liter. Before the experiment, all experimental fish of both species were size sorted and only fish with minimum size differences (9.0–16.0 g) in individual body weight were selected for this study. At the end of the experiment, all the fish were weighted again, and the frequency of the fish at same 1 g interval with final size difference (11.1–75.0 g) was again calculated.
Results
In the first 140days experiment, all three densities provided same high specific growth rate (SGR = 0.22–0.24 %.d-1 ). Feed conversion ratio (FCR) was the lowest for low (1.39 ± 0.21 g.g-1), the highest (1.61 ± 0.08 g.g-1 ) for
medium density. High density did not have any different FCR (1.50 ± 0.17 g.g-1) compared to other tested densities. Analogous results as for SGR were also obtained for survival rate (97–100%). The second 60days experiment tested the effect and comparison groups of largemouth bass and pikeperch ( Sander lucioperca) within monoculture and biculture on production efficiency.
The higher FC was evaluated higher in both groups (monoculture and biculture) of largemouth bass (1.09) compared to groups of pikeperch (0.74–0.78). SGR were the lowest for both groups (monoculture and biculture) of largemouth bass (1.20–1.28%.d-1 ). In contrast, the highest SGR was in the group of biculture pikeperch (1.88 %.d-1 ). The similar results such as SGR were also assessed for FCR, where the higher FCR were in both groups of largemouth bass (1.44–1.48 g.g-1 ) than in the biculture of pikeperch (0.73 g.g-1). All four tested groups had a high survival rate (98.61–99.99 %) (Fig. 1).
Discussion
The optimal fish density as high as possible is very important fact of successful intensive aquaculture in many species (Ronald et al. 2014). Higher density of intensively cultured juvenile Eurasian perch (0.5–15 g) from 400 to 10 000 fish.m-3
can increase growth rate and decrease growth heterogeneity. This positive correlation between fish density and growth rate was valid until 10–16 g perch juveniles. In bigger perch juveniles up to 16 grams, increasing density from 20 to 60 kg.m-3 decreased growth rate
. This positive relationship can have effect on higher survival rate of cultured fish. The results show that fish survival rate during the experiment was significantly lower for the pikeperch than for the largemouth bass. There may be several reasons for these differences in survival of both tested species, which have the slightly different physiological processes, their tolerance and adaptation to conditions of intensive farming
.
Acknowledgements
The study was financially supported by NAZV project QK 22020144.
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