A scientific contribution towards sustainable aquaculture

The present PhD project has been made possible thanks to the support and collaboration of the major rainbow trout (Oncorhynchus mykiss, Walb.) farm in Europe. It was initially based on the original research proposal aimed at examining rainbow trout aquaculture in order to find alternative raw materials to be utilized in the formulation of feed for all the stages of breeding of this important salmonid. However, in 2017 the farm which funded the research project began rearing marine fish species in the Adriatic Sea and it was, therefore, decided to widen the scope of the study to include European gilthead sea bream (Sparus aurata, L.). During the course of the PhD triennium various activities relating to the above two species were carried out. With regard to rainbow trout, the first of the tree main studies focused on the effects on broodstocks of a diet which included materials of vegetable origin; the second assessed the supplementation of an innovative emulsifier into the diet of juveniles to improve the assimilation of lipids; the last one examined the qualitative traits of by products obtained by rainbow trout processing for their possible reutilization. In particular, a trial was carried out employing 4-year-old broodstocks of rainbow trout – both females and males – fed on a diet which totally substituted fish meal with vegetable meal and partially substituted (50%) fish oil with linseed oil. The effects of this diet, compared to a control diet containing fish meal and fish oil, were evaluated in terms of reproductive performances: quality of eggs, hatching and fingerling survival rate and mean body weight. Based on the results of this trial, it was possible to assume that feedstuffs containing vegetable protein and fat administered to broodstocks, negatively affected the quality of gametes and the progeny of rainbow trout. The second trial was performed in order to investigate the effects on the growth performance and feed conversion rate of diets supplemented, at two different doses, with a new emulsifier aimed at increasing lipid assimilation in rainbow trout juveniles. After 90 days of experimental feeding, the overall results of rainbow trout fed on the supplemented diets, can be considered satisfactory and within the normal range for the species size. Concerning the histology of the intestinal tract, a lower degree of irritation was observed in the trout fed on the experimental diets compared to those fed on the diets without the additive. The emulsifier supplemented in the feeds, resulted in beneficial effects together with a better welfare status of the rainbow trout. The research went on to consider the qualitative traits of by-products obtained by rainbow trout processing, including muscle meat and skin. This issue has become more pressing worldwide due to the ever-increasing amount of waste and wastewater. Recently, attention has been focused on the possibility of extracting precious nutrients from rendered fish proteins. In this study, the proximate composition and the fatty acid profile of rendered rainbow trout were evaluated and compared with the traits of the trimmed fillet. The results showed that rendered fish from rainbow trout processing still contains valuable nutrients, which could be successfully considered as possible feedstuff and also employed in various sectors and in innovative ways as, for example, in the production of finger food and fish burgers. Concerning sea bream, our research assessed the effects on the growth of juveniles of a partial substitution of fish meal with insect meal in their diet. Furthermore, the possibility of eliminating the use of antibiotics in sea bream rearing was also examined. In the last few years, insect meal has become one of the most studied sources of protein feedstuffs as an alternative to fish meal. A growing trial was performed in order to evaluate the efficacy of the inclusion of chironomid meal in the feed of gilthead sea bream juveniles. To this aim, samples of chironomid midges at larval stage were collected from aquatic environments, converted into meal and analysed from a qualitative point of view. Two experimental feeds with a different percentage of replacement were tested and compared to a control diet where the chironomid meal was absent. The experimental diets resulted in excellent palatability and led to satisfactory growth performances. The research also considered the possibility of rearing gilthead sea bream without the use of antibiotics. In recent years, the issue of antimicrobial resistance has become of prime importance at an international level. The possibility of rearing antibiotic free fish is the challenge we face in the twenty-first century. In this context, the company involved in this Eureka project, has decided to work towards the improvement of rearing techniques, biosafety and the management of environmental parameters, in order to reach high standards of animal welfare, from breeding to transportation and subsequent stages, in the hope that this will result in a successful battle against antibiotic-resistance. To this goal, a trial was performed monitoring the production cycle of gilthead sea bream that were farmed adopting an antibiotic free protocol in an offshore cage plant. Their growth performance, health status and flesh quality were then compared with conspecific wild fish. The antibiotic free sea bream were fed on a diet that contained not only raw materials of aquatic origin – which constituted the main source of essential fatty acids of the omega 3 series – but also those of vegetable origin, in full respect of environmental sustainability. There was a strong similarity between the morphometric parameters and somatic indices of the two fish groups of different origin. Moreover, the antibiotic free sea bream displayed a very low lipid fraction similar to that of the fish captured in the Adriatic Sea, which classified them in the category of lean fish. Data concerning the omega 3 content, demonstrated that the antibiotic free samples could be defined either as “Rich in omega 3”, or as an “Omega-3 source”.