IDENTIFICATION OF HISTOLOGICAL INDICATORS OF LARVAL/JUVENILE QUALITY IN MARINE FISH SPECIES

This PhD activity was conducted within the framework of the EU Horizon2020 funded project PerformFISH, which aims at increasing the sustainability and the competitiveness of Mediterranean gilthead sea bream (GSB, Sparus aurata) and European sea bass (ESB, Dicentrarchus labrax) farming. One of the main specific objectives is the improvement of predictability, quality and sustainability of the hatchery phase; in fact, the production of good quality larvae and juveniles is fundamental for the success of marine aquaculture, as the poor quality of juveniles affects growth performance, survival, technical viability and susceptibility to disease and infection both in the hatchery phase and in the subsequent grow-on stages. In this light, the aim of the thesis was to identify histological indicators that can be supportive for farmers in order to promptly recognize fish health problems and correct sub-optimal managerial practices to avoid losses. For this purpose, based both on a literature review, and on the previous experience of the research group, an original Multiparametric Semi-quantitative Scoring System (MSSS) specific for the histological evaluation of larval GSB and ESB was designed. It allows to obtain a complete histopathological view of a larval batch, also thanks application of an algorithm which synthesizes MSSS descriptors in a single Cumulative Histological Index (CHI). The construction of the MSSS is here explained in detail, with particular attention to the descriptors, the causes of their alteration in rearing conditions, and the criteria for the score assignment. The MSSS was applied on many GSB and ESB samples from 3 European farms, to test its applicability in a hatchery context and the ability to highlight the problems. It emerged that this tool could be a valid support for commercial hatcheries, both as internal quality control and or for the control of incoming fish. Finally, the MSSS was also used for larval evaluation during a thermal imprinting pilot test, which involved the application of 3 different thermal regimes during the autotrophic phase of GSB and ESB. This set had the dual purpose of verifying whether the water temperature applied in the early stages of embryonic development can affect the following development stages, and whether the MSSS can be effective also as a research tool in a controlled experimental setting. Although the MSSS proved to be valid for this purpose, and the differences found in the first stage evaluated are possible effects of the thermal regimes, it was not possible to define whether the differences found in the subsequent stages are equally attributable to these.