The effect of live and inert feed treatment with lactobacilli on weaning success in intensively reared pike-perch larvae

Weaning to inert diet in intensively reared pike-perch larvae is confronted with significant fish loss, which prevents successful commercialization of pike-perch production. Achievement of satisfactory feed quality and effective assimilation of nutrients by larval fish is the major challenge in larval production process. Aim of this study was to evaluate whether treatment of live and inert feed with lactobacilli could alleviate growth retardation associated with early weaning of pike-perch reared in recirculating system. Weaning started on 18th day post-hatching (DPH) either as sudden weaning (SW) or by co-administration of Anemia for six days (gradual weaning, GW). Prior to administration to fish, Anemia was treated with Lactobacillus salivarius BGHO1/Lb. reuteri BGGO6-55, while inert feed was treated with Lb. paracasei subsp. paracasei BGHN14/Lb. rhamnosus BGT10. Treatment with lactobacilli slightly raised neutral lipid level in Anemia nauplii, but significantly reduced their content in dry feed. Fish were sampled on the 24th DPH. Survival, morphometric indices, skeleton differentiation, digestive enzyme activity and opportunistic pathogenic bacteria level were assessed in whole fish specimens. GW fish were presented with better survival, body growth and phospholipase A2 (PLA2) activity. Alongside, Vibrio spp. growth was suppressed in these fish and skeleton development was improved, according to Alizarin Red staining and ColIA1/Sparc mRNA expression data. Lactobacilli application in GW fish correlated with an increase of survival, condition factor and growth rate, according to trypsin and chymotrypsin activities, indicating better utilization of dietary proteins for muscle building. In SW fish, lactobacilli elevated chymotrypsin activity, PLA2 to lipase activity ratio and improved survival and ossification, as evident from Alizarin Red staining and ColIA1/Sparc mRNA expression. This indicated improved fatty acid absorption and control of metamorphosis process. Furthermore, lactobacilli suppressed Vibrio spp. growth in SW fish. Aside from demonstrating the ability of lactobacilli to aid weaning in pike-perch larvae, this study indicated that different types of food treatment may direct fish growth in a predictable manner, allowing further cost-effective improvements of larval pike-perch rearing in intensive system