Introduction
Since Aquaculture is asked to produce sustainable animal proteins for the future growing world population, the large use of commercial feeds may represent a relevant bottleneck. Grains such as soybean, corn, wheat, barley, and even more precious raw materials such as fish oil and fishmeal, are commonly produced far away from the consumption sites. Moreover, their intensive production represents itself a severe hazard in terms of environmental pollution, soil and sea overexploitation, and biodiversity reduction. Yet, the overseas transportation of large quantities of raw materials and final products is characterized by a very high Carbon Footprint. In this context, an environmentally sustainable alternative to intensive aquaculture might be the Integrated Multi-Trophic Aquaculture approach. Stemming from IMTA, the Self-sufficient Integrated MultiTrophic AquaPonic system (SIMTAP , H2020 PRIMA-Programme ) encompasses four consecutive trophic levels , starting from microalgae, deposit- and filter-feeder organisms (DFFO), fish and plants, reared in a recirculating system using saltwater.
In the SIMTAP system, micro- and macro-algae are raised for feeding DFFO, which in turn are harvested for feeding (or partially sold) fish; finally, solid fish wastes “feed-back” DFFO, while soluble wastes such as nitrogen and phosphorous are absorbed by hydroponically grown halophytes, salt-tolerant glicophytes and macro-algae , w ith that bio-remediat ing the recirculating water.
The DFFO are heterotrophic species such as polychaetes, bivalves, and echinoderms, which may represent a sustainable, nutritionally valuable, alternative to fish oil and fish meal in fish nutrition.
To evaluate how the use of these organisms may affect the growth performances of carnivorous fish species, two different trials have been carried out so far. The first consisted in the diet inclu sion of increasing rates of mussels , until totally replacing the commercial feed . In the second trial, the commercial feed was fully replaced by a mixture of mussels and clams.
Material and Methods
All the experimental procedures were approved by the Organism for Animal Welfare of the University of Pisa and the Italian Ministry of Health (authorization code: B290E.N.AHZ) . The trials were carried out using six cylindric 420 L tanks of the SIMTAP system located at the Department of Agricultural Food and Environment of the University of Pisa (Pisa, Italy).
J uveniles of Gilthead Sea Bream (Sparus aurata ) were used and daily fed at 3% ( on dry matter base ) of their live body weight (BW) . The diet ingredients used were the following: INVE© O. range P15 as commercial feed; frozen mussels (Mytilus platensis) and clams (Chamelea gallina) as feed replacers. T he fish biomass of each tank was weighed every week in order to adjust the amount of diet to be supplied ; then, Feed Conversion Rate (FCR), Condition factor (Kf) and Specific Growth Rate (SGR) were calculated per each tank and dietary treatment . On day 0 and at the end of the experiment, fish were weighed and measured individually (total length, TL). At the end of the experiments , 25 fish from each tank were also euthanized with an overdose of tricaine methanesulfonate (MS222©), dissected for abdominal viscera and liver weight determination, and calculating Viscera-Somatic (VSI) and Hepato-Somatic (HSI) indexes.
Experiment 1: 1, 243 fish (mean weight 4.95±1.120 g; mean total lengths 7.39±0,600 cm) were distributed in the 6 dietary treatments: F100M0 (100% feed), F80M20 (80% feed, 20% mussels), F60M40 (60% feed and 40% mussels), F40M60 (40% feed, 60% mussels), F20M80 (20% feed, 20% mussels) and F0M100 (100% mussels). After tawing, mussels were minced, and diets supplied 4 time per day.
Water temperature, salinity and pH approximately maintained at 24 °C, 32 g L_1, 7.6, respectively, and DO above 6 mg L-1.
Experiment 2: 1,255 fish (mean weight 6,78± 1,41 g; mean total lengths 8,06±0,65 cm ) were used. Again, f ish were divided into 2 dietary treatments and 3 replicates : F100 (100% feed) and M100, this latter consisting of a mixture of 50% and 50% tawed and minced mussels and clams , respectively. W ater temperature, salinity and pH were kept at 22 °C, 25 g L-1, 7.5, respectively , and DO above 6 mg L-1.
Statistical analysis: One-Way ANOVA followed by Tukey-Kramer HSD (Honestly Significant Difference) for the Experiment 1 and Student’s test for the Experiment 2 were used for statistical analysis of growth performance parameters. Differences were considered significant at P<0.05.
Results
Experiment 1: t hawed minced mussels showed significantly higher palatability than dry commercial feed, with fish intensively competing for catching mussel particles . On day 0 , groups F100M0, F0M100 showed significantly lower (P<0.05) initial BW (4.76±1.074 and 4.76±1.042 g, respectively) than groups F80M20, F60M40 and F40M60 (4.86±1.076, 5.10±1.216 and 5.17±1.078, respectively) . T hese differences were not anymore significant on day 28 and 35 while on day 42 the group fed 100% mussels (F0M100) showed the lowest BW (13.02±2.608) , significantly different (P<0.05) from the group fed 100% commercial feed (14.69±3.498) . Moreover, this latter group showed a significantly lower (P<0.05) BW than the group fed 60% commercial feed and 40% mussels (15.57±3.327). T he group F0M100 showed the highest cumulative FCR (1.11), followed by the group F80M20 (1.06), F20M80 (1.04), F40M60 (1.02), F100M0 (1.01), and F60M40 (0.98).
Experiment 2: as observed in Experiment 1, fish fed on thawed minced mussels and clams showed higher competiti veness for the diet than those fed on commercial feed. Regarding initial BW and TL no significant differences were observed among treatments: 6.82±1.446 g and 8.08±0.638 cm for F100 and 6.75±1.378 g and 8.04±0.657 cm for M100. On day 48 of the experimental period, treatment F100 showed significantly higher BW and TL than M100: 23.62±4.471 g and 11.66±0.837 cm for F100 , 20.83±3.943 and 11.40±0.704 cm for M100. Mean weight gain was significantly higher for treatment F100 than for M100: 3,522.12±90.015 g and 2,884.16±150.177 g, respectively. Also, FCR w as significantly different among treatments : 0.88±0.050 for F100 and 1.05±0.062 for M100. Statistical analysis of TL, SGR, Kf, VSI and HSI data of both experiments is in progress.
Discussion and Conclusion
In general, results suggest better Gilthead Sea Bream growth performances when mixed diets are used. In fact, the use of mussels as only diet ingredient reduced fish growth b y 11.3% and 16.4% in comparison to commercial feed and to a diet consisting of a mixture 60% commercial feed and 40% mussels , respectively. Similar results were observed also when a diet consisting of 50% mussels and 50% clams rather than mussels only was used. Probably, the enrichment of the wet diet mixture with the introduction of additional ingredients such as polychaetes and/or echinoderms , may improve its nutritional value and fish growth performances enhanced.
Acknowledgments
The Authors thanks Blue Resolution® association (Monsummano Terme – Italy) for kindly supplying frozen mussels and clams. SIMTAP is part of the PRIMA Programme supported by the European Union.