Scale regeneration in reared Gilthead seabream: Revisiting the use of scales in identifying aquaculture escapees

Abstract

The high frequency of regenerated scales (degree of scale regeneration, SRD) in reared fish has been used in the past for the identification of aquaculture escapees in the wild stocks. In the present study, we examined the evolution of the SRD in Gilthead seabream (Sparus aurata) throughout the entire rearing process (hatchery and on‐growing, 2.0–21.9 cm standard length, SL). For this purpose, nine fish samples were randomly taken from different populations and farm sites, at different time points (58–393 days post‐hatching, dph). Results demonstrated that scales are lost continuously, from the onset of squamation (11.2 ± 5.7% mean SRD, 58 dph, 2.0 cm SL) to the end of the on‐growing period (98.7 ± 3%–99.8 ± 0.9% mean SRD, 314–393 dph, 21.7–21.9 cm SL). At the transition between the hatchery period and the on‐growing in sea cages, fish presented a wide distribution of SRD, ranging from 40% to 92% (122 dph, 5.1 cm SL). Despite the different sample origins, a uniform relationship between SRD and SL was observed, with a clear inflection point (2.7 cm SL) and two periods with different rates of scale loss. Our findings are discussed in relation to the use of regenerated scales for the identification of aquaculture escapees in wild stocks and the need to revisit the different suggested critical SRD thresholds.

Stock Discrimination of Gilthead Seabream (Sparus aurata Linnaeus, 1758) through the Examination of Otolith Morphology and Genetic Structure

“…In addition, Geladakis et al [41] showed that wild-caught fish with SRD levels greater than 30% exhibit intermediate otolith phenotypes between wild fish with lower SRD (≤30%) and reared fish. Since scales are continuously lost during the rearing process [74], we suggest the 60% SRD as a minimum threshold, beyond which wild-caught fish have an increased possibility of including individuals that escaped from net pens near harvest sizes, thus presenting phenotypes closely similar to those of reared fish. Following our results, a positive correlation emerged between SRD levels and the magnitude of otolith shape and asymmetry variation, suggesting that SRD can not only be used as a reliable index for detecting the presence of aquaculture escapees in the wild, but also to provide insights into the timing of fish escape from aquaculture net pens.…”

“…In aquaculture facilities, friction and physical collisions among fish are intensified due to handling manipulations (e.g., counting, size-grading, and vaccination) and high population densities within sea-cages, leading to significantly high rates of scale loss [73,133,134]. Lost scales are promptly replaced by new (regenerated scales), characterized by the presence of a large regeneration nucleus lacking growth circulii [74,135,136]. Based on our results on the otolith shape and asymmetry analyses, fish with a high degree of scale regeneration (31-60 and M60 SRD groups) exhibited increased otolith shape variability in comparison to the L30 group.…”

“…Scales were photographed and examined for the presence or absence of a regenerated nucleus. The degree of scale regeneration (% regenerated scales) (SRD) was then calculated for each specimen (e.g., [74,78]). Within each geographic group, in order to assess potential temporal changes in SRD, Kruskal-Wallis and Mann-Whitney U tests were performed between the SRD of fish collected in different years.…”

“…In the present study, to minimize any effects of the possible presence of aquaculture escapees in the examined samples, the SRD was used to assign wild-caught fish into one of three predetermined groups: (a) fish with SRD lower than 30% (L30), (b) fish with SRD between 31-60% , and (c) fish with SRD higher than 60% (M60). Following Geladakis et al [41,74], wild-caught seabreams with SRD higher than 30% (50% of fish specimens, Geladakis et al [41]) had an increased possibility of including fish that escaped from net pens at various phases of the rearing process.…”

“…In the present study, we investigated the genetic population structure and spatial variation in the shape and asymmetry of otoliths in wild-caught Gilthead seabreams sourced from the Aegean and Ionian Seas (eastern Mediterranean). To control for the potential presence of aquaculture escapees among wild-caught individuals, we used the degree of scale regeneration (SRD, % of regenerated scales), which has been shown to be significantly higher in reared fish [40][41][42]73,74], as an indicator to exclude potential aquaculture escapees (as previously demonstrated by [41]) in the comparisons of otolith morphology (shape and asymmetry) and genetic composition between wild-caught fish of different geographical origins.…”