Ecasa Toolbox

Shell-fish farming: Assessing a site's potential

'Shellfish' here refers to filter feeding molluscs, especially mussels and oysters. Assessing the potential of a site to grow these bivalves involves the following environmental considerations. Farmed animals use oxygen and release ammonia. Where water movements are weak, the build-up of ammonia and the decrease in dissolved oxygen at a farm will slow bivalve growth. And, of course, the farmed animals need a good supply of food, in the form of phytoplankton and particulate organic matter. If the supply is restricted, the bivalves will grow poorly.

Models can be used to estimate the maximum bivalve production that is likely to be supported at a given site. Three types of model are relevant:

    • A hydrodynamical model calculates water motions; it can be replaced by measurements of currents.
    • An environmental model is needed to predict the effects of bivalves on the water and the supply of phytoplankton and particulate organic matter.
    • An organism model predicts the growth of bivalves as a function of food availability, water temperature, etc.

An organism model can be used alone if food and temperature data are available. An environment model can provide the information about food supply, but in turn needs information about water flows and about conditions in the region surrounding that which is modelled. An organism model is independent of scale, but when you run an organism model for conditions local to the farm, you assume that water quality and food content remains unaffected in the water body with which the water at the site exchanges. This may not be true: a single farm can be so big as to change conditions throughout a water body, or the combined effects of several farms may do this. This explains why it might be necessary to combine an environment model with an organism model, in order to investigate effects of one or more shellfish farms on water quality and food supply as well as the effects of food supply on bivalve growth. See the page dealing with: scales.

Below we list relevant environment and organism models studied during ECASA. In general, the organism models must be provided with information about water temperature and the availability of food (as phytoplankton and/or particulate organic matter) throughout the year. Environment models may be able to provide this information, but must in turn be given information about boundary conditions such as water temperature and chlorophyll concentration outside the region that is the subject of the model, and they also need information about water movements. See also: choosing a model.

Model (alphabetical order) Model category: scale and relevance Environment category Bivalve type for which applicable Tested at ECASA sites:
DEB Organism model any pacific oyster Baie des Veys, Loch Creran
DDP Organism/population model microtidal regions of restricted exchange oysters and mussels not tested in ECASA
EcoWin B (water-body), environment model RREs and semi-enclosed coast waters (any - depending on organism model) Creran
FARM organism model, A (farm-) scale environment model including ASSETS eutrophication assessment all mussels, oysters Creran, Ria Formosa
Longlines organism model plus A (farm-scale) transport model RREs and semi-enclosed coastal waters mussels, scallops not tested in ECASA
MG-IBM organism model anys (Mediterranean) mussels Chioggia
ShellSIM can be coupled to EcoWin Organism model any clams, mussels, oysters, scallops Creran, Chioggia
TapesIBM Organism model any clam not tested in ECASA

 

Although ECASA documented some hydrodynamical models, it did not study them in detail. A number of well-documented models of this sort are already in use. Their employment, however, requires detailed information on the bathymetry of the water body concerned. See Hydrodynamical models page for some more advice.

Combining a hydrodynamical model, an environment model and an organism model, will allow the estimation of the carrying capacity of the water body for bivalve farming, as exemplified by the SMILE project in northern Ireland or the CANO project in Norway.