Distinguishing the impacts of rearing volume versus the stocking density on the skeletal quality of Sparus aurata during the larval and pre-ongrowing rearing phases

Abstract

Recent profitability issues within the European aquaculture industry coupled with a pervasive push towards a blue economy focusing on long-term environmental and economic sustainability has resulted in the necessity for the aquaculture industry to revamp their practices and focus on cutting internal costs while simultaneously improving product quality. Due to the undeniably negative impacts of skeletal deformities in farmed fish (welfare issues, economic losses, and consumer perception), ameliorating the morphological quality by reducing the development of skeletal deformities is of paramount importance. Feasibility and straightforward solutions which can be quickly adapted and applied in commercial farms offers an ideal opportunity for European farmers to improve the quality of their fish. Until now, previous studies have primarily focused on comparing morphological quality based on different rearing strategies such as low stocking densities in large volumes versus high stocking densities in smaller volumes. The work done in this thesis attempts to reconcile the effects of the different rearing strategies in order to individuate which between available swimming space (volume) and crowding effects (stocking density) is the main driver in eliciting skeletal deformities in gilthead seabream. This thesis consists of work investigating the stocking density and rearing volumes and their respective impacts on skeletal elements and deformities in the commercial species (Gilthead seabream, Sparus aurata). In case study 1 (CS1) seabream were reared from eggs up to early juveniles (60 dph) (hatchery phase) and in case study 2 (CS2) from ~8.7g up to the sub-adult (~55g) stage (pre-ongrowing phase) in commercial-scale aquaculture conditions. The design envisaged to test the effects at an industrial scale of larger (1000 L) and smaller (500 L) tank volumes on seabream, stocked at 3 different densities (1. low density; 2. intermediate density; and 3. high density). Performative data was taken from fish from both of the case studies prior to the fish being anatomically assessed for skeletal quality by means of visual inspection on whole mount stained (CS1) and radiographed specimens (CS2). Recorded data was analyzed by various multivariate statistical analyses to investigate differences among the conditions. The results from the thesis indicated that high stocking density is the primary driver inciting stunted growth and a greater presence of skeletal anomalies in the vertebral column and head in both the hatchery and pre-ongrowing phases. Furthermore, increased tank volume/swimming space did reduce the presence of anomalies affecting the jaws in both studies. Several potential hypotheses to explain the results found are presented in the discussion sections.