Introduction
In a context of rapid development of aquaculture, ecosystem responsible practices are a deep challenge. Aquaculture feeds have been traditionally based on fish meal (FM) and fish oil (FO) derived from fisheries of small pelagic species. Efficient alternatives to FM and FO are crucial to preserve wild fish populations. An other concern of aquaculture is the release of high amounts of wastewater into the environment . The excess of particulate and dissolved nutrients , i.e., uneaten feed and metabolic wastes of fish, induce the eutrophication of surrounding ecosystem. The generic concept of Integrated MultiTrophic Aquaculture (IMTA) covers a large set of practices but is based on the production of fed species (e.g., finfish fed sustainable commercial diets) with extractive species, which utilize the inorganic (e.g., seaweeds) and organic (e.g., suspension-feeders) excess nutrients from fed aquaculture for their growth (Chopin, 2013) . IMTA have the principal advantages to decrease the dependence on external inputs, increase the system efficiency , diversify farm-products and decrease waste effluents. An important challenge, to increase the development of IMTA practices, is the understanding of trophic relationship between species co-habiting in the system. Carbon (δ13C) and nitrogen (δ15N) natural stable isotopes have proven to be a useful tool to characterize trophic interactions in aquatic ecosystem ( Fry and Sherr, 1984). The use of stable isotopes is based on the assumption that isotope signatures of consumers reflect those of assimilated dietary sources. The δ13C signatures of consumer tissues are usually close to those of their diets, making possible an elucidation of the origin of food sources. In contrast, δ15 N signatures are enriched by 3.5 ‰ between a consumer and its prey and thus are typically used to estimate the trophic position of a consumer.
In this study, seabream (Sparus aurata ) was raised in one pond and feed with commercial pellets exclusively containing terrestrial plant ingredients. U nmarked batches of mussels were given to seabream as supplementary food to underbalanced deleterious effects of the total replacement of FM and FO in their diet. In three other ponds, f ilter-feeders (oyster and clam) and detritivores crustacean (shrimp) have been raised . The four ponds were only connected by water flow. T he aim of our study was to determine the food sources used by seabream , shrimp, clam, and oyster raised in this IMTA system . The proportion of commercial diet and mussels used by seabream were quantified. We also evaluated the ability of shrimp, clam, and oyster to feed on fish detritus. The present study is part of SIMTAP ( Self-sufficient Integrated Multi-Trophic AquaPonic systems ) a PRIMA project aiming to improving food production sustainability and brackish water use and recycling.
Material and methods
From June 2020 to September 2020, e xperiment has been conducted in 4 ponds on the Atlantic F rench C oast at the “Lycée de la Mer et du Littoral , Bourcefranc le Chapus” (fig.1.) . Seabream (Sparus aurata ) were fed 5 a week with commercial pellets and once a week with mussels at a rate of 3.7 ± 1.3% of live weight day -1 and adjusted according to water temperature . Commercial feed and mussels were distributed to seabream with a ratio of 0.86:0.14. The oyster ( Crassostrea gigas), shrimp (Penaeus japonicus) and clam (Ruditapes philippinarum ) were not artificially fed. At the end of the experiment, 10 individuals from each species were collected in each ponds. Sediment organic matter (SOM) and suspended organic matter (SPOM) were also sampled in each pond . Sampled were dried, weighed and packed into pressed tin capsule for simultaneous δ13C and δ15N analysis, using an isotope ratio mass spectrometer interfaced with an elemental analyzer . The contr ibution of food sources to diet were estimated using Bayesian stable isotope mixing models (simmr package of R software, Parnell et al. 2013).
Results and discussion
Seabream, oyster, clam, and shrimp had significantly different δ13C and δ15N values (p < 0.05). Seabream were enriched by 5.5‰ and 5.49 ‰ in 13 C and 15N respectively compared to commercial feed. Mixing model indicated that commercial feed contributes for 72.7% while mussels contribute for 27.3 % to seabream diet. P roportion of mussel used by seabream was higher than those distributed (14%). Such difference is probably due to a better appetence and/or digestibility of mussel than plant-based diet. Oysters were enriched by 0.35 ‰ and 3.03 ‰ in 15 N and 13 C compared to SPOM that is consistent with a consumption of SPOM. Oyster did not directly ingest fish aquaculture waste. However, oyster helped to regulate the level of SPOM (i.e., bloom of phytoplankton and zooplankton) that is boosted by the release of nitrogen and phosphorus by fish. Mixing model indicated that clam consumed 67.3% of SPOM and 32.7% of SOM. Clams were in competition with oyster for SPOM but helped to reduce the level of organic matter in sediment. Shrimp had the higher δ13C and δ15 N values suggesting that they not directly fed on organic matter from sediment but had a carnivorous diet feeding on naturally present organisms in sediment. The results of this study confirm that seabream, oyster, clam , and shrimp have complementary diets and can be raised together in IMTA ponds.
References
Chopin T (2013) Aquaculture, integrated multi-trophic (IMTA). In: Christou P, Savin R, Costa-Pierce B, Misztal I, Whitelaw B (eds) Sustainable food production. Springer, New York, NY, p 184−205
F ry, B., Sherr , E. B. (1984) δ13 C measurements as indicators of carbon flow in m arine and freshwater ecosystems. Contribution in mar ine Science 27: 1347
Parnell, A., Phillips, D ., Bearhop, S., Semmens, B., Ward, E., Moore, J ., Jackson, A., Grey, J., Kelly, D., Inger, R. (2013). Bayesian Stable Isotope Mixing Models. Environmetrics . 24.