Nursing of Pike-Perch (Sander lucioperca) in Recirculating Aquaculture System (RAS) Provides Growth Advantage in Juvenile Growth Phase

SIMPLE SUMMARY: High flesh quality, along with a good potential for intensive farming in a Recirculating Aquaculture System (RAS), make the pike-perch (Sander lucioperca) a valuable fish species both from the industry and the research point of view. Although high electricity demands have been attributed to it, fish farming in a RAS is recognized as the key strategy for sustainable fish farming, given its minimal usage of natural resources as well as its independence of outdoor weather conditions. Although the older juvenile stages of pike-perch are well adapted to indoor rearing conditions, the larval and early juvenile stages seem to be particularly sensitive to indoor rearing. Switching to the indoor production of pike-perch larvae would support the continuous, weather-independent, production of fish throughout the year. Hence, our study aimed to identify the pros and cons of rearing pike-perch larvae and early juveniles (nursing period) in indoor versus outdoor systems. For this purpose, we compared the effects of pike-perch nursing in both systems on the later growth and survival of the fish. Our results show that, although nursing indoors increases fish mortality, it provides a basis for improved fish growth later in life. This can eventually shorten the period for the production of market-size fish, and, along with the advantages related to the year-round production of larvae, provide higher yields to fish farmers in the long run. ABSTRACT: This study aimed to estimate the efficacy of two pike-perch juvenile production technologies: exclusive Recirculating Aquaculture System (RAS) culture (the RAS group) and pond larviculture with a transfer to the RAS at the 42nd day post-hatch (DPH). Both direct weaning on dry feed (the Pond-D group) and 10-day gradual weaning using bloodworms (the Pond-B group) after transfer to the RAS were evaluated in pond-nursed fry. Their survival and morphometric indices were monitored after the RAS habituation period (first 10 days), after the 18-day post-habituation period and after an additional 30 days of on-grow. Our results indicate a negative allometric growth of the pond-nursed fish during the nursing period, which was slower (p < 0.0001) in comparison to the RAS-nursed fry (16.3 ± 0.4 vs. 17.8 ± 0.7%/day). After transfer, these fish grew faster than the RAS-nursed fry (7.7 ± 0.1, 4.9 ± 0.5 and 6.1 ± 0.6 during habituation, 8.5 ± 0.6, 9.3 ± 0.5 and 6.7 ± 0.1%/day during post-habituation period, in the Pond-B, Pond-D and RAS groups, respectively). However, four weeks afterwards, the RAS-nursed fry were again superior in terms of growth (4.0 ± 0.1, 3.6 ± 0.2 and 4.6 ± 0.2%/day, for the Pond-B, Pond-D and RAS groups, respectively), and this was accompanied by a significantly lower feed conversion ratio in this group. Although the survival of the RAS-nursed fry during the nursing period was lower in comparison to the pond-reared fry (11.3 vs. 67.3%), the RAS seems to provide a long-term growth advantage.