Introduction
It is already widely accepted that fish larvae need high levels of phospholipids (PLs) in their nutrition to ensure optimal growth. Phospholipids are important structural components of cellular membranes, and during the larval stage the formation of tissues and organs is still underway (Sargent et al. 2002; Saleh et al. 2012). On the other hand, there is no solid data regarding phospholipid needs of fish in the juvenile stage or later. This may be because there is some evidence, albeit limited, of the existence of a phospholipid biosynthesis mechanism in fish (Sargent et al. 2002). Currently in Mediterranean aquaculture genetic selection programs are in place that have achieved improved growth rates in fish (Ferosekhan et al. 2022). Knowing that faster weight gain also means faster tissue growth, questions are raised about whether the current nutritional design of feeds are adequately covering the needs of fish in structural nutrients, such as phospholipids. The aim of the present study is to investigate if the increased nutritional intake of PLs will affect production and physiological characteristics of gilthead seabream juveniles (Sparus aurata). Simultaneously, different PL sources will be explored (marine, plant, animal) regarding their efficiency.
Materials and methods
Seven different diets (treatments) were formulated and produced, of which one was used as a Control diet (typical composition of commercial diets, PL 0.5%). The rest of the diets contained, in pairs, different phospholipid sources (krill-K, soy/rapeseed-SR, egg-E) in two levels (PL 0.7% and PL 1.1%). Going forward the diets will be referenced to as K0.7 and K1.1, SR0.7 and SR1.1 and E0.7 and E1.1. Twenty-one homogeneous groups of 20 juvenile gilthead seabream with initial body weight (BW) 28.0 ± 0.13 g (triplicated groups for each treatment) were used. Fish are fed a restricted ration according to water temperature and BW. Mean water temperature is 17.3 ± 0.7 oC. When the fish have tripled their initial weight, the trial will end, and they will be weighed and sampled. The growth parameters that are being investigated include: fish weight (W) and its coefficient of variation (CVw), specific growth rate (SGR), feed conversion rate (FCR) and thermal growth coefficient (TGC). For TGC, degree days (DDs) are also calculated (1538.25 for 90 days of rearing). Additionally, the physiological condition of the fish will be evaluated by analyzing blood samples (cholesterol, glucose, albumin, triglycerides, total protein). The trial is ongoing, therefore the results presented below concern the growth for the first 90 days of the experimental rearing.
Results and Discussion
Fish weight was significantly lower in the Control diet after 90 days of rearing, regardless of the phospholipid inclusion level and origin (Figure 1). This was already observed only two weeks after the beginning of the trial. No differences were observed for CVw and FCR among diets. SGR and TGC for all experimental diets were equally significantly higher than Control diet (Figure 1). The results for the first 90 days of rearing suggest that gilthead seabream juveniles have higher phospholipid needs than previously reported. It is also encouraging that despite low water temperatures, there is still an obvious improvement in growth. Regarding the efficiency of the different origins of PLs, the present absence of differences does not permit a conclusive evaluation yet. However, it seems that the inclusion of egg PLs doesn’t seem to be as efficient as the inclusion of marine and plant PLs. The limited literature data does not permit adequate comparisons with present results, especially when most studies focus on increasing the phospholipid levels of feeds that have a very low fish meal percentage. However, present results suggest that gilthead seabream juvenile growth is affected by the level of dietary phospholipids and indicate a differential effect of PL origin.
References
Ferosekhan, S., Sarih, S., Afonso, J.M., Zamorano, M.J., Fontanillas, R., Izquierdo, M., Kaushik, S., Montero, D. 2022. Selection for high growth improves reproductive performance of gilthead seabream Sparus aurata under mass spawning conditions, regardless of the dietary lipid source. Animal Reproduction Science, 241.
Saleh, R., Betancor, M.B., Roo, J., Benitez-Santana, T., Hernandez-Cruz, M.C., Moyano, F.J., Izquierdo, M.S. 2012. Optimum krill phospholipids content in microdiets for gilthead seabream (Sparus aurata) larvae. Aquaculture Nutrition, 19(4).
Sargent, J., Tocher, D., Bell, G. 2002. The Lipids. In: Fish Nutrition, Halver, J., Hardy, R. (Eds.), Academic Press, USA, pp. 181–257.