Introduction
Feeding seabream (Sparus aurata) on a sustainable fish feed implies to find alternatives to the use of fish oil and fishmeal in the diet. It means that attention must be paid to substitute long chain poly-unsaturated fatty acids (LC-PUFA) and essential amino acids from fish oil and fishmeal, with other sources, in order to maintain growth performances. Seabream is a predator for shellfish and is able to crush the shells, once the size is suitable. The area of Charente Maritime (France), is known to produce oysters and mussels. A part of the production, out of calibration, is discarded whereas it could be a suitable source of nutrient for gilthead seabream. Thus, to propose a sustainable diet, based on local resources, a formulated feed based on vegetal resources, supplemented with fresh mussels (Mytilus edulis) out of calibration was tested. In order to test the interest of this approach, two trials were carried out in the facilities of the Lycée de la Mer at Bourcefranc-le-Chapus (France): (i) An indoor experiment was conducted on small size gilthead seabreams in order to check the relevance of the experimental diet on rapid growth rate individuals without any other potential natural sources of nutrient; (ii) An outside second experiment was performed in which the experimental feed was delivered to larger seabreams in fishpond.
Materials and methods
Indoor experiment
The experiment lasted for seven weeks. The gilthead seabream were bought in a commercial hatchery at a mean weight of 6.7 g (± 1.3 g). At the beginning of the experiment, they were stocked at a density of 1.5 kg/m3 in 3 tanks of 1.6 m3 (342 to 345 fish per tank). The three tanks were associated in a single recirculating aquaculture system (RAS). The water circulated by air-lift and was maintained to a constant temperature of 21±1°C. Water was renewed at a rate of 2 m3water per kg of feed distributed. In each rearing tank, a different diet was delivered 6 days a week: (i) the “control” diet (C), corresponding to a commercial feed (Neo Start, Le Gouessant), specific to pre-growing marine fish (including marine ingredients); (ii) the “vegetable” diet (V), corresponding to an experimental formulated feed composed only of raw materials from plants. This experimental feed was formulated based on: (1) absence of marine ingredients; (2) use of raw materials only from European production; (3) without synthetic amino acids; (4) limitation in the use of refined products; (5) a protein/fat rate similar to the commercial feed; (6) and use of an attractive compounds to foster feed intake (Actipal HP1-S1. Dianaaqua); (iii) the “mussel” diet (M) corresponding to the formulated feed V (five days a week), supplemented with fresh flesh (shells were removed) from discarded mussels, on the basis of gross energy content one day a week. The mussels were purchased from a close aquaculture farm.
Outdoor Experiment
The outdoor trial was performed twice in 2020 and 2021, from early June to early October. The gilthead seabream were bought in a commercial farm, from an indoor controlled environment. At the beginning of the experiments, the fish had a mean weight of 200 g and were stocked at a density of 3 individuals/m² in a single earthen pond of 410 m². The fish feed was formulated on the same basis than in the previous experiment and adapted to the requirement of 200g seabreams. The formulated feed was delivered five days a week, supplemented with discarded whole fresh mussels on the basis of gross energy content one day a week.
For indoor and outdoor experiments, fish were weighed at the beginning and the end of the experiment, and at intermediate dates. During the experiments, the quantities of feed distributed were recorded. In each experiment, proximate composition of diets, mussels and whole body were analyzed and their fatty acid profile determined, at the beginning and at the end of the experiment.
Results
Indoor experiment
At the end of the experiment, the mean weight of fish from the C (23.1 ± 4.3 g) and M (23.1 ± 5.2 g) treatments were not significantly different, whereas they were significantly lower in the V treatment (15.5 ± 3.6 g) (p<0.05). The survival rate was slightly higher in C and M (99.7% and 99.4% respectively) than in V (98.0%). Feed Conversion Ratio (FCR) obtained for C, V and M were 1.00, 1.84 and 0.99, respectively. For M, The FCR, including the mussel, calculated on the basis of the formulated feed at 97.3% dry matter (DM).
A total of 5 182 g DM of feed were delivered in C, 5 057 g DM in V and 5 240 g DM in M (composed of 4 708.5 g DM of feed V and 531.5 g DM of mussel flesh). The quantities of total protein delivered in each treatment were 2 891 g for C, 2 781 g for V and 2 906 g for M, and the quantities of total energy delivered were 110 MJ for C, 122 MJ for V and 116 MJ for M. During the experiment, the lipid content of the fish increased, with a higher level in V and M (11.87% and 12.15% respectively) than in C (9.38%), at the end of the experiment. But, the proportion of EPA (Eicosapentaenoic acid) and DHA (Docosahexaenoic acid) in the body lipids was lower in V (1.31% and 2.10%) and M (1.82% and 2.21%) than in C (9.20% and 10.78%, respectively), at the end of the experiment. The retention rates in lipids and protein were similar between C (69% and 30% respectively) and M (70% and 29% respectively) but lower for V (45% and 16% respectively).
Outdoor Experiment
Similar growth performances were observed between the trials carried out in 2020 and in 2021 with a final mean weight of 352.4 g (± 59.4 g) and 363.8 g (± 72.1 g) respectively. The survival rate was similar too with 94% and 95% respectively. The FCR obtained were 1.90 and 1.76 respectively, calculated on the basis of mussel with the same dry matter than the formulated feed. During the experiment, the mean lipid content of the fish increased, from 13.8% to 17.5%, but, the mean proportion of EPA and DHA in the body lipids decreased from 3.9% to 1.8% and from 7.1% to 3.5% respectively. The retention rates in lipids and protein were 16% and 18% respectively.
Discussion-Conclusion
For both types of experiment, fish fed with formulated feed supplemented with fresh mussel exhibited growth performances similar to that from fish fed with commercial feed. Survival rates observed were over 90%.
If the protein content in fish remained stable during the experiments, the lipid content increased. The balances between saturated and polyunsaturated fatty acid, and n-3/n-6 FA ratio decreased with the use of the vegetable diet, and the supplementation with mussel did not allow to maintain the proportions of PUFA.
The use of the proposed feeding strategy has to be confirmed on longer periods and/or included in a global feeding strategy along the fish rearing cycle, including phases of feeds of different PUFA levels. This study shows that it is possible to propose new feeding strategies, based on the use of available nutritive sources in territories.