Response of the benthic ecosystem to fish farming waste in a tropical environment: observational and modeling approaches.

Introduction: One of the main challenge for the sustainable development of marine aquaculture is to determine the appropriate level of production that respects the ecological carrying capacity of the receptor ecosystems. Most environmental thresholds associated to aquaculture waste have been defined for temperate ecosystems but very few studies have quantified impact thresholds in tropical environments. 

Materials and Methods: In this study, we combined observational and modeling approaches to quantify the environmental impact of fish farming waste. A sampling campaign was carried out at a coastal marine fish farm located, in a shallow semi-enclosed embayment of Le Robert (Martinique). Particle traps were deployed to quantify the sedimentation rate (total and particulate organic matter) at different distances from fish cages (0, 25, 50 and 150 m) along two transects. Grab samples were taken to analyze the structure of macrofaunal benthic communities, supplemented by sediment cores to analyze sediment physico-chemical characteristics (% organic matter, sediment grain size). Isotopic signatures of each compartment (feed pellets, particulate organic matter in particle traps and sediment) were analyzed to trace the extent of fish farming waste dispersion. Current flow measurements and zootechnical data were also collected to predict solid farming waste with a farm waste dispersion model (MOCAA’pp).

Results: The environmental impact of farming waste seems to be limited to the close to fish cages (0-25 m). Significantly higher sedimentation rates of particulate organic matter were measured at 0 m sites on each transect compared to other stations (> 1.2 kg m^-2 yr ^-1), due to low current speed (3 cm s^-1 in average) and shallow depth under cages. Stable isotopes analysis in particle traps and sediment (d15N/d13C) also showed a low dispersion of farming waste, with an isotopic signature at stations 0 m closer to that of the fish farm. In the sediment, a significant reduction in species richness, abundance and diversity was observed between 0-25 m. From 25m upwards, the presence of sensitive species was observed leading to an improvement of the ecological status (M-AMBI index).The environmental variables that significantly explained the structure of benthic communities were the sedimentation rate of particulate organic matter and the sediment grain size. Ongoing analyses are in progress to confront field observations with modeling outputs using the farm waste dispersion model (MOCAA’pp).