Introduction
Over the last decade, due to the continuou s growth of the aquaculture sector, marine-origin ingredients , i.e. fish meal (FM) and fish oil (FO), have been partly substituted with sustainably produced plant raw materials. The aforementioned changes in fish feeds composition affect the micronutrients balance resulting into lower levels of bioavailable minerals in aquatic feeds compared to fish natural diet , thus inorganic mineral supplements are commonly used to meet fish requirements.
Gilthead sea bream (Sparus aurata ) farming is of great importance to European finfish mariculture accounting for the 13% of its production value. This species is marketed mainly as whole fish and more recently as fillets. Unfortunately, gilthead seabream fillets suffer from gaping, a texture deterioration phenomenon which leads to economic loss, because of the rejection by consumers due to its unappealing appearance. Factors that have proven to be strongly associated with the fish propensity to gap, include the species, harvest or slaughter history, temperature during storage (Lavety et al., 1988; Sheehan et al., 1996; Robb et al., 2000) and diet (Kousoulaki et al., 2016).
Taking all the aforementioned into account, this study aimed to evaluate the impact of organic mineral feed inclusion on growth performance and fillet quality in terms of gaping occurrence, in gilthead seabream.
Materials and methods
Three isoproteic , isolipidic and isoenergetic diets were formulated: a control diet (Diet1) containing high FM (30% ), a plant-based diet (Diet2) containing low FM (12.5%) and a plant-based diet supplemented with organic minerals (0.18%) (Diet3) . These finishing diets were fed to triplicate groups of gilthead seabream (253g initial body weight) for a 92-day period. Water temperature maintained at 19οC in an open flow water system. Fish were fed by hand to apparent satiation with the three diets. At the end of the trial, fish were slaughtered by the commercial method (ice-killing) , individually weighted, packed with ice (0oC), and shipped to a commercial fish processing unit. After mechanical scale removal by drum, fish were machine filleted, weighted, ice-packed and transferred (within two hours) to the Hellenic Centre for Marine Research (HCMR). Individual fillets were assessed for their gaping according to a recently published gaping scale for Mediterranean fish species (Kogiannou et al., 2022), pH, water holding capacity (WHC), total composition, fatty acid profile, chemical freshness (K value), and total and soluble collagen content. The SPSS version 26.0 was used for the statistical analysis . Growth, p hysical and chemical parameters are presented as mean±st.dev . and comparisons among means were made using one-wa y analysis of variance (ANOVA). The non-parametric χ2 test was applied to find statistical differences in the frequency of gap occurrence between the diets . Differences were considered significant at the level of P<0.05.
Results
Fish growth parameters and feed intake were similar for all treatments, while organic minerals inclusion had beneficial effect on fish fillet quality. Specifically, as shown in Figure 1. distribution of gaping scores frequencies were found to be statistically different among diets . In both Diet1 and 2, increased high gaping scores incidence were observed while flesh integrity remained quite intact in fish received Diet3. Additionally, fish fillet liquid losses and pH were not different in gilthead seabream fed the organic minerals supplemented diet compared to Diet1 and 2. No statistically significant differences were found also in the fillet composition of the fish fed the experimental diets however, increased levels of n6 fatty acids were observed in both plant-based diets (Diet2 and 3). Finally, t he total collagen content and collagen solubility in gilthead seabream muscle , calculated based on the hydroxyproline conten t, were found to be significantly affected by the diet composition and the organic mineral supplementation.
Conclusions
Dietary organic minerals included in low fishmeal diet for gilthead seabream significantly improved fillet quality in terms of achieving lower gaping incidence probably due to changes occurred in collagen content and solubility. The exact mechanism responsible for these observations remains to be identified.
Acknowledgement
The project is co-funded by Greece and the European Union under the Fisheries and Maritime Operational Program 2014-2020 (75% EMFF contribution, 25% National Contribution). Project title: “Imp roving of produced gilthead seabream and pagrus fillets by reduction of the gaping problem (PERFILLET)” (2019–2022).
References
Kogiannou , D., Kotsiri , M., & Grigorakis , K. (2022). A method to assess gaping in Sparidae species fillets. Aquaculture Research, 53, 689–693. https://doi. org/10.1111/are.15590
Kousoulaki , K., Mørkøre , T., Nengas , I., Berge, R. K., & Sweetman, J. (2016). Microalgae and organic minerals enhance lipid retention efficiency and fillet quality in Atlantic salmon (Salmo salar L.). Aquaculture, 451, 47-57.
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