The effects of stocking densities versus tank volume on the skeleton of gilthead seabream Sparus aurata in the hatchery and preongrowing phases of the production cycle

INTRODUCTION: Gilthead sea bream (Sparus aurata) production is one of the main aquaculture industries in the Mediterranean, producing 258,754 tones of seafood in 2019 (FAO, 2021). However, in recent years uncertainty regarding the profitability and the economic losses have been inevitable for many production facilities due to rapid market expansion in the 1980s followed by an oversupply establishing a lower market value in the last two decades (Llorente et al., 2020). Therefore, a focus on increasing the production value rather than increasing production quantity would be a sustainable solution to improve profitability and adjust for long-term environmental and economic goals in the EU (Llorente et al., 2020). The coupled application of Large Volumes (O/ =30-60m3) and low densities (< 16 larvae/L) has been demonstrated (Koumoundourous et al., 2004; Boglione et al., 2009; Prestinicola et al., 2013) augment the survival rate and the morphological quality of gilthead sea bream and other Sparids. However, the separate effects of density or volume, decoupled from each other has not been investigated. This knowledge will help farmers to produce subadults of higher quality to be ongrown by modulating only one of these two factors, without any need for extra economic investment, hightech solutions, or new tanks. The aim of this study was to individuate which between ‘large volume’ and ‘low density’ is the main driver in attaining high quality gilthead sea bream during both the hatchery (from eggs to juveniles) and the preongrowing (Waverage up to ~55 g). The experimental design envisaged to test the effects at a commercial scale of A) larger and smaller tank volumes on seabream, stocked at the same density; and B) higher and lower stocking densities on seabream maintained in the same tank volume. The experimented tank volumes were smaller, and the densities higher than those tested in previous studies. The choice of the experimental tank volumes (500 vs 1000 L) was based on the ubiquity of these tanks in almost every Mediterranean farm. The densities we utilized were those indicated as interesting to be tested by API (Italian Association of Fish farmers). MATERIALS & METHOD: Experimental rearing were conducted in the EcoAqua facilities at the University of Las Palmas, Gran Canaria (Spain) for the hatchery phase and at the Intituto Portugues do Mar e Atmosfera facilities in Olhão Portugal for the preongrowing phase. 3 different densities (Low Density (LD): 25 eggs/L and 5kg/m3; Medium Density (MD): 125 eggs/L and 10kg/ m3; High Density (HD): 250 eggs/L and 20kg/m3) were utilized for the hatchery and ongrowing phases respectively. Two tank volumes were tested for each density condition, in all the trials: 500L tanks (small volume) and 1000L tanks (large volume). Natural seawater was pumped into the systems and all of the rearing parameters were maintained the same for all of the conditions, save the volume or the density. Additionally, oxygen was maintained at above 70% SAT for both trials. Seabream were reared for approximately 2 months in each trial. Juveniles from the hatchery phase were whole-mount stained with Alizarin red while the sub-adults from the preongrowing cycle were radiographed. Monitoring of skeletal anomalies was done for both studies using an adapted alphanumeric code to account for skeletal elements affected and region of body in which the anomaly was located Prestinicola et al. (2013). Data was expressed in a raw matrix in order to calculate the frequencies of anomaly types found over the total amount of anomalies and a binary matrix to calculate the frequencies of individuals affected by every anomaly types. All statistical analyses and graphs were done using Python and Past 4.02 (Hammer et al., 2001). RESULTS: Strikingly the environmental parameters of varying degrees in density and volume elicited similar responses in both early juveniles and subadults. Both experimental cycles enhanced significant greater lengths, reduced opercular, jaw, and vertebral axis anomalies in LD reared seabream, while larger volumes reduced the incidence of jaw anomalies. This outcome highlights the predominant effect of environmental drivers on skeletal plasticity in this species, regardless of notable differences in genetic origin, life-stage, and ontogenetic phases (Fig. 1). The possible hypotheses (behavioral, chemo-physical, physiological, etc…) that can be formulated to explain this primary, more positive effect of low stocking density rather than the larger tank volume, are largely discussed. ACKNOWLEDGEMENT: This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie grant agreement No. 766347, BioMedAqu, ETN 766347.