Laboratoire d’Océanographie Physique et Biogéochimique

“Modelling aquaculture waste dispersion and benthic degradation in Ligurian coastal waters”

     The aquaculture is the foodstuffs production activity with the most rapid growth in the world and in particular marine fish farm in intensive system. Several studies have been highlighted that the particulate wastes from the cages increase the organic load on the benthic environment and might determine changes in the community structure and in the biodiversity of the benthic assemblages. Therefore we are in need for tools able to predict environmental impact due to fish farm waste and the numerical models can represent the right tools both to perform environmental impact predictions and to test different scenarios.

     For this aim, we improve the advection-dispersion model POM-LAMP3D coupling with a new numerical benthic degradative module FOAM. This module uses the outputs of the other units of the modelling framework to compute the sediment stress level and organic carbon concentration remaining on the seabed after the degradation. The natural capability of the seafloor in absorbing part of the organic load is modeled by the different minalization rates. These rates represent a key parameters of the module and the lack in literature of these ones specifically targeting for the Medititerranean condition drives us to designed a field campaign to measure the benthic metabolism in a typical Mediterranean fish farm. With the results of this campaign, several simulations were carried out switching suitable parameters such as the waste typologies (uneaten feed or faeces), the release conditions (continuous or twice a day) and the particle chemical-physical characteristics (settling velocity). The uneaten feed are confirmed to be the primary cause of ecological impact on the sediment, while the faecal waste are almost completely degraded. Furthermore the maximum impacts, in terms of both stress level and organic carbon concentration, are due to the quickly settling feed, released in periodical mode and during slow current periods.

     These results support the use of self-feeders that are already proposed by several authors in order to reduce the uneaten feed losses with no effect on growth rates. Moreover the benthic metabolism has been found dependent on water temperature, contrary to what was found on the Atlantic Salmon rearing. These results show how the new and more complete modelling framework here presented is able to improve the objectivity in decision making processes and it may be successfully used for planning and monitoring purposes.