Food web in Mediterranean coastal integrated multi-trophic aquaculture ponds: Learnings from fatty acids and stable isotope tracers

Abstract

Ecosystem responsible aquaculture practices are today imperative to feed the world increasing population. The culture of extractive species such as oyster with fed species such as fish is a promising solution to recycle waste streams, which would be otherwise lost and discharged into the surrounding environment. In Mediterranean earthen ponds, meagre (Argyrosomus regius), white seabream (Diplodus sargus) and grey mullet (Mugil cephalus) are three fish species of different trophic levels that are good candidates to be raised in integrated multi-trophic aquaculture (IMTA) pond with oysters (Magallana gigas) and macroalgae (Ulva flexuosa). Trophic links between species raised in IMTA ponds were inferred using the analysis of body fatty acid composition as well as in carbon and nitrogen stable isotopes. A combination of 3 treatments was tested: fish, oysters, phytoplankton and macroalgae (T1); fish, oysters and phytoplankton (T2); fish, phytoplankton and macroalgae (T3). Our results highlighted that the three fish species were in competition for food sources as they mainly fed on commercial feed whatever the treatment. However, grey mullet also consumed polychaetes that naturally grown in ponds. Macroalgae were not used by fish. In T1 and T2, oysters actively consumed the excess of phytoplankton. In T2, the absence of macroalgae reduce nutrient competition for phytoplankton growth and oysters have more available food. In earthen ponds, the production of fish with macroalgae or fish with oyster should be privileged compared to usual semi-intensive fish polyculture as the presence of extractive species reduced fish wastes.