Differences in otolith shape and fluctuating‐asymmetry between reared and wild gilthead seabream (Sparus aurata Linnaeus, 1758)

Abstract

Otolith structure is a useful tool in discrimination among fish populations as it is a permanent record of the influence of endogenous and exogenous factors. In the present study we examined otolith morphology and fluctuating asymmetry (FA) for differences between wild‐caught (by bottom trawl) and reared specimens of Gilthead seabream (Sparus aurata). Based on the frequency of regenerated scales (degree of scale regeneration, SRD) on each specimen, a threshold of 30% SRD was used to assign wild‐caught fish individuals as wild (≤30% SRD, LR group) or as possible aquaculture escapees (>30% SRD, HR group). Based on the analysis of elliptic Fourier descriptors, significant differences were found in otolith shape between reared (Rr) and the wild‐caught groups (LR, HR). Reared fish had otoliths with significantly larger perimeter (OP) than wild‐caught fish. Furthermore, FA was significantly higher in the Rr than the LR group for OP and all except one shape descriptors (harmonics 2–7). The HR group exhibited intermediate levels of FA between the low and high FA levels observed in the LR and Rr groups. Results are discussed in terms of the value of combining otolith and scale morphology for the identification of escapees in wild Gilthead seabream stocks.

Plastic Responses of Gilthead Seabream Sparus aurata to Wild and Aquaculture Pressured Environments

“…Considering that the 70% level of successful classifications, this supports a hypothesis of separate fish stocks (Harbitz and Albert, 2015), and the applied image-based techniques and EFA methods in this study proved to be a reliable tool for wild-farmed origin delineation (see also Geladakis et al, 2020). Farm-associated individuals showed the lowest classification rate (31%) to its origin, with half of individuals classified as being of wild origin, as expected, due to limited shape variations between the origins.…”

“…Still, an indentation of the excisura area was found to be the main mean shape trait responsible for successful classification of individuals to their origin, when farmed (87%) and wild fish (71%) were considered, respectively. A similar farmed vs wild differentiation pattern in mean otolith shape of gilthead seabream has been reported elsewhere (Geladakis et al, 2020), suggesting that environmental conditions, including an altered diet, play the primary role in the formation of a farmed-specific otolith shape, while the genetic background was important to a lesser extent.…”

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

“…Existing literature suggests that SRD of reared fish at harvesting is >93%, and such high threshold has been used for discerning aquaculture escapees from wild fish (Izquierdo‐Gomez et al, 2017). Despite the remarkable similarity of the reported SRD levels for reared seabream at harvest size between different studies (>93%, Izquierdo‐Gomez et al, 2017; >94%, Geladakis et al, 2021; >89%, present study), our finding that SRD increases continuously during the on‐growing phase controvert the use of such a high threshold. The low SRD values at the beginning of cage farming (40%–92%) reported here clearly suggest that early escape events may release in the wild, fish with SRD values lower than the previously used threshold of 93%.…”

“…Wild‐caught samples with SRD values higher than 43% are not necessarily composed exclusively by fish escaped from aquaculture facilities. Geladakis et al (2021) used a conservative threshold of 30% SRD to exclude escapees from the comparison of otolith morphology and fluctuating asymmetry between wild and reared Gilthead seabream. They showed that wild‐caught fish with SRDs higher than this threshold presented intermediate phenotypes, between the wild fish of low SRD (0%–30%) and the reared fish.…”

“…Similar to other commercially farmed fish species, accidental escape events of reared seabreams, due to mainly technical or operational failures, are not rare (Arechavala-Lopez, Uglem et al, 2012;Glaropoulos et al, 2012). These escape events might be considered as a potential threat to the natural populations, due to genetic, physiological and morphological differences between reared and wild fish (Alarcon et al, 2004;Arechavala-Lopez et al, 2013;Carrillo et al, 2001;Fragkoulis et al, 2017;Geladakis et al, 2021;Karaiskou et al, 2009;Segvic-Bubic et al, 2014;Šegvić-Bubić et al, 2020;Zuzul et al, 2019). The ability to discriminate different origins (wild vs. farmed) in the wild represents a first step towards functional management of escapees.…”