Distinguishing the Effects of Water Volumes versus Stocking Densities on the Skeletal Quality during the Pre-Ongrowing Phase of Gilthead Seabream (Sparus aurata)

Abstract

Gilthead seabream (Sparus aurata) production is a highly valued aquaculture industry in Europe. The presence of skeletal deformities in farmed gilthead seabream represents a major bottleneck for the industry leading to economic losses, negative impacts on the consumers’ perception of aquaculture, and animal welfare issues for the fish. Although past work has primarily focused on the hatchery phase to reduce the incidence of skeletal anomalies, this work targets the successive pre-ongrowing phase in which more severe anomalies affecting the external shape often arise. This work aimed to test the effects of: (i) larger and smaller tank volumes, stocked at the same density; and (ii) higher and lower stocking densities maintained in the same water volume, on the skeleton of gilthead seabream fingerlings reared for ~63 days at a pilot scale. Experimental rearing was conducted with gilthead seabream juveniles (~6.7 ± 2.5 g), which were selected as ‘non-deformed’ based on external inspection, stocked at three different densities (Low Density (LD): 5 kg/m3; Medium Density (MD): 10 kg/m3; High Density (HD): 20 kg/m3) in both 500 L and 1000 L tanks. Gilthead seabream were sampled for growth performance and radiographed to assess the skeletal elements at the beginning and end of the experimental trial. Results revealed that (i) LD fish were significantly longer than HD fish, although there were no differences in final weights, regardless of the water volume; (ii) an increase in the prevalence of seabream exhibiting cranial and vertebral axis anomalies was found to be associated with increased density. These results suggest that farmers can significantly reduce the presence of some cranial and axis anomalies affecting pre-ongrown gilthead seabream by reducing the stocking density.

Skeletal anomalies in gilthead seabream (Sparus aurata) larvae reared in different densities and water volumes

“…Similarly, in zebrafish (reared from 30 to 90 dph), the SL decreased with the increasing rearing densities (Martini et al, 2021). Significantly greater TL, but not weight, was also found in the fish reared in low‐density conditions (5 kg/m 3 ) from a recent experiment on gilthead seabream subadults (Dellacqua et al, 2023). Another study rearing adult gilthead seabream (~271 g) for 9 weeks at three density conditions (5, 10, and 20 kg/m 3 ) revealed the same inverse relationship between increasing density and fish size (both weights and length, in this latter case) (Araújo‐Luna et al, 2018).…”

“…Similarly, in zebrafish (reared from 30 to 90 dph), the SL decreased with the increasing rearing densities (Martini et al, 2021). Significantly greater TL, but not weight, was also found in the fish reared in low-density conditions (5 kg/m 3 ) from a recent experiment on gilthead seabream subadults (Dellacqua et al, 2023).…”

“…Concerning reared commercial species, a similar approach has been applied to gilthead seabream (Sparus aurata) subadults (Dellacqua et al, 2023). In that work, sub-adults obtained from different spawns which had been previously selected for the absence of externally detectable skeletal anomalies, were reared from 6.7 g up to 54.9 g at 3 different densities (LD: 5, MD: 10, and HD: 20 kg/m 3 ) in both 500 and 1000-L water volumes, for $63 days.…”

“…Additionally, a follow‐up study on the effects of tank volume in mitigating the development of skeletal anomalies in medaka, testing a density of 40 fish/L in 3 versus 6 L tanks, confirmed the results found in zebrafish with another model fish species: that increased swimming space did not mitigate the negative effects of HD on the skeleton of medaka (Di Biagio, 2022). Concerning reared commercial species, a similar approach has been applied to gilthead seabream ( Sparus aurata ) sub‐adults (Dellacqua et al, 2023). In that work, sub‐adults obtained from different spawns which had been previously selected for the absence of externally detectable skeletal anomalies, were reared from 6.7 g up to 54.9 g at 3 different densities (LD: 5, MD: 10, and HD: 20 kg/m 3 ) in both 500 and 1000‐L water volumes, for ~63 days.…”

“…In this scenario, this study aims at verifying if the same positive effects of low rearing density and mitigating effects of greater available water volume on the development of skeletal anomalies are detectable in gilthead seabream reared during the hatchery (larval rearing) phase from eggs up to 60 dph, in two different water volumes (1000 vs. 500 L), and stocked at three different initial densities. This approach foresaw: (i) that the tested densities were those considered of interest to fish farmers (Italian Fish Farmers Association (API)); (ii) that the water volumes were identical to those previously used in the experimental rearing during the pre‐ongrowing phase in the same species (Dellacqua et al, 2023) and were significantly larger than tanks or aquaria often used for experimental studies on model fish (1–3 L Di Biagio et al, 2022; Martini et al, 2021), and commercial species like gilthead seabream (170–200 L Dominguez et al, 2021; Dominguez et al, 2022; Izquierdo et al, 2017; Izquierdo et al, 2019; Sivagurunathan et al, 2022; Tseng et al, 2021). Furthermore, these large tanks required a very large number of fish (initial number of eggs = 615,385) to be subjected to the experimental trial.…”