In aquaculture, loss of nutrients in the surrounding environment is an important concern. The benefits of associating aquatic organisms to use nutrients released from fish farming is well reported in the literature, but in general the documented applications at a commercial scale remain scarse. In the same time, Fish oil and fishmeal included in feed are also pointed out to contribute to the depletion of natural resources and to be in competition with direct human consumption. Hence, to maximize the use efficiency of nutrients, the system presented was designed to associate three different organisms of commercial interest and differing from seabream (Sparus aurata) for their trophic level, while using only local vegetal raw materials in the formulated fish feed. In addition, fresh discarded mussels, were directly supplied to the fish to balance their micronutrient and fatty acids needs. Thus, the aim of this study was to assess the performances of this system, by monitoring the water quality and the growth performances of the different organisms reared.
Materials and methods
The system was tested from 9th June to 7th October 2020. It was composed of 4 ponds, connected in cascade, to circulate the water by gravity, according to the following order (Figure 1): (i) in pond 7, seabreams were stocked at a mean weight of 210 g; (ii) in ponds 6 and 5, oysters (Crassostrea gigas) and shrimps (Penaeus japonicus) were stocked at a mean weight of 47.5 g and 0.52 g, respectively; (iii) in pond 4, clams (Ruditapes decussatus) and shrimps were stocked at a mean weight of 3.4 g and 0.52 g, respectively. From the pond 4, water was pumped back to the pond 7. Since salinity remained at an appropriate level in the system during the experiment, water was added from the inlet conduct (connected to the open sea) only to compensate evapotranspiration, avoiding discharge water toward the surrounding environment. During the experiment, the fish, shrimp and bivalves were weighed to estimate the growth. The water quality was weekly recorded to control temperature, oxygen, pH, turbidity and salinity. Once a month, water was sampled in each pond for concentrations in phytoplankton, nitrogen compounds and phosphorus compounds.
The mean concentration in oxygen for the entire period of experiment was significantly (p<0.05) lower in the pond 7 (6.5 ± 1.8 mg/l, n=13) than in the ponds 5 (9.5 ± 2.6 mg/l, n=13) and 4 (9.8 ± 2.8 mg/l, n=13), and in the inlet water conduct (8.7 ± 1.4 mg/l, n=9). An increase of the concentration in total chlorophyll was observed in the system during the cycle of production (from 2.5 µg/l in June to 60.6 µg/l in October) contrary to that in the inlet water conduct, in which the concentration remained quite constant (between 5.1 µg/l in June and 11.0 µg/l in October). No significant differences were observed (p<0.05) between compartments, in the mean concentrations in N (N total, NH4, NO2 and NO3) and P (P total and PO4). Nevertheless, the level of the N total in the ponds of the system was higher compared to the inlet water channel, in which the N total remained almost stable along the production cycle time. The mean values of P-PO4, were significantly lower (p<0.05) in all the ponds of the system than that in the inlet water conduct. Despite a period of adaptation due to changes in feed and environmental conditions, seabreams grew well according to the expected performances. Thus, 1 169 seabream were harvested at a mean weight of 352.4 g, with a survival rate of 86.6%, and a corrected FCR of 1.9. At the harvest, the mean weight of the shrimps from the pond 4 was significantly (p<0.01) higher (35.2 ± 10.0 g, n=120) than that from the pond 5. Unfortunately, the week before the harvest, a storm event depleted the oxygen in the water and all shrimps in the pond 6 died. The rearing performances of oyster were higher than expected. Despite a summer mortality syndrome observed in the area, the survival rate of oysters observed was 91% for the pond 4 and 87% for the pond 5. At the harvest, the mean weight of the oysters was significantly (p<0.01) better in the pond 5 (88.4 ± 20.6 g, n=89) than in the pond 6 (71.2 ± 10.4 g, n=89), as well as the mean filling ratio (21.5 ± 3.3 %, n= 50 and 18.8 ± 2.5 %, n=50, respectively). The number of clams harvested was estimated to 8 102, representing a survival rate of 69%, for a mean weight of 11.4 ± 3.3 g (n=100). Harvested clams had a filling ration of 20.0 ± 3.6% (n=50) which was significantly (p<0.01) higher than that measured in June (16.1 ± 3.0%, n=20).
The water concentration in oxygen was a key parameter of the system. In the fishpond, oxygen was at low levels in the morning despite the air adduction, so a special attention was paid to that in postponing the feeding time to the afternoon. The different concentrations in N and P, and physical indicators in water, like oxygen concentration, observed in the pond 4, reflected the ability of the system to improve the quality of the water released from the fishpond. Moreover, no water was discharged from the system along the rearing period, resulting in the save of water and in the preservation of the surrounding environment. In the system tested, good results were obtained for fish growth and feed conversion ratio, despite the lack of fishmeal and fish oil in the feed. Without additional specific source of nutrients in the system, except than the feed supplied to the sea bream, the body growth of the others organisms reared were similar and even higher (filling ratio and survival rate of the molluscs) compared to their usual monoculture in ponds (based on expert experience). Variations in the performances of the different species among the ponds give interesting perspectives in the improvement of this IMTA system, especially on optimization of animal densities and in adding a compartment dedicated to seaweed cultivation.