Introduction
It has already been established through research that fish larvae have high nutritional needs in phospholipids. Phospholipids (PLs) have a prominent role as structural components of cellular membranes, and at the larval stage the formation of tissues and organs is still underway (Sargent et al. 2002; Saleh et al. 2012). At the same time, not much is known about the PL needs of fish in the juvenile stage or later. This may be because there is some evidence, albeit limited, of the existence of a PL biosynthesis mechanism in fish (Sargent et al. 2002). Currently in Mediterranean aquaculture improved growth rates in fish have been achieved through the application of certain genetic selection programs (Ferosekhan et al. 2022). Knowing that faster weight gain also means faster tissue development, questions are raised about whether the needs of fish in structural nutrients, such as PL, are covered only by their biosynthetic ability. 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 (Sparus aurata) of commercial size. Simultaneously, the efficiency of PL sources of marine and plant origin is explored.
Methodology
Five different diets were formulated and produced, of which one was used as a Control diet (typical composition of commercial diets, PL 0.45%). The rest of the diets contained, in pairs, krill oil (K) and soybean oil (S) in two levels (PL 0.7% and PL 1.0%). This was to achieve a 50% and 100% increase in PL content compared to the Control diet. Going forward the diets will be referenced as K50, K100, S50 and S100. Fifteen homogeneous groups of 10 juvenile gilthead seabream with initial body weight (BW) 28.0 ± 0.13 g (triplicated groups for each treatment) were used. Fish were fed according to water temperature and BW. Mean water temperature was 21.1 ± 0.2 oC. When the fish reached commercial size, the trial ended (total duration: 8 �� months), weight and length were measured and blood samples, as well as liver, intestine and fillet tissues were taken. The growth parameters that were investigated include: fish weight (W) and its coefficient of variation (CVw), specific growth rate (SGR), feed conversion ratio (FCR), thermal growth coefficient (TGC) and organosomatic indices. Additionally, the physiological condition of the fish was evaluated by analyzing blood samples (cholesterol, glucose, albumin, triglycerides, total protein, total antioxidant capacity, catalase). Sample analysis is still ongoing, so presented below are the results that have been gathered up until now.
Results
Fish weight was significantly lower in the Control treatment. The K100 diet resulted in higher final weight, followed by K50. The S50 and S100 diets showed no significant differences from the Control diet. Similar results were obtained for SGR and TGC. CVw and FCR did not show significant differences among treatments. Serum biochemical parameters weren't significantly different from control except for cholesterol, which was higher in fish that consumed K100. Organosomatic indices (Hepatosomatic index, spleenosomatic index, viscerosomatic index, perivisceral fat somatic index) didn't show significant differences among treatments.
Discussion
Present results suggest that the genetically improved gilthead seabream that are currently used in intensive aquaculture have higher PL needs. Also, there is an indication that krill oil is a more efficient source of PLs than soybean oil. At the same time, the physiological state of the fish is not affected, as indicated by the analysis of the samples. To our knowledge, there is a limited number of studies regarding PL needs in Mediterranean aquaculture fish species, while there is little to no research on adult fish. Despite this, Kokou et al. (2021) reported similar results for gilthead seabream rearing from 12 to 50 g. Conclusions regarding growth, in addition to the physiological condition of the fish, will be clearer after further analysis of the samples. Nonetheless, there seems to be potential in increasing the phospholipid level of gilthead seabream growout diets.
Table 1: Results of selected parameters analyzed for the experiment.
Control
K50
K100
S50
S100
P
Weight (final), g
326.1±9.3a
350.5±2.0b
371.4±2.7c
343.6±3.9ab
337.8±5.8ab
**
CVw, %
10.2±1.8
9.7±2.6
9.3±1.3
7.8±3.7
9.6±1.3 ns
SGR
0.943±0.01a
0.972±0.0005b
0.995±0.003c
0.964±0.004b
0.967±0.008ab
**
FCR
1.35±0.01
1.34±0.02
1.31±0.01
1.37±0.02
1.36±0.01 ns
TGC
0.705±0.011a
0.736±0.0005b
0.761±0.003c
0.727±0.004b
0.720±0.009ab
**
FR, %
1.069±0.003
1.072±0.015
1.061±0.010
1.091±0.013
1.083±0.002 ns
Cholesterol, mg/dl
336.2±39.0ab
372.9±2.1abc
422.8±20.9c
328.7±13.1a
403.9±9.9bc
*
P: Significance Level; TAC: Total Antioxidant Capacity; HSI: Hepatosomatic Index ns: not significant, P>0.05; * P<0.05; ** P<0.01
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.
Kokou, F., Vasilaki, A., Nikoloudaki, C., Sari, A.B., Karalazos, V., Fountoulaki, E. 2021. Growth performance and fatty acid tissue profile in gilthead seabream juveniles fed with different phospholipid sources supplemented in low-fish meal diets. Aquaculture, 544.
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.