Mussels (Mytilus edulis), are important aquaculture suspension feeder species,
which can have a significant impact on coastal marine systems due to the removal of considerable amounts of suspended food particles from the water column.
Understanding the production carrying capacity (stocking density of bivalves at
which yields are maximised) is necessary for sustainable yields to be maintained.
The goal of the present study was to gain an understanding of the feedbacks and
interactions between the various components that determine production carrying
capacity; Mussel feeding behaviour, grazing rate and growth were investigated
together with the seston concentration and prevailing hydrodynamics using an in situ approach which has been lacking in previous modelling.
Mussel feeding behaviour in the Menai Strait was directly coupled to the
hydrodynamics and food concentration of the water column. Mussel feeding
behaviour in the subtidal area was only synchronized with the advected chlorophyll a (chl a) concentrations and not with other environmental factors measured (e.g. predators, suspended particulate matter, current velocity ... ). Mussel shell growth was also coupled with chl a concentrations. The development of the internal shell microgrowth methodology proved better than the use of traditional morphometric measurements to quantify mussel growth depending on the temporal scale used. The in situ defecation methodology, using chl a concentration as a proxy, was developed to estimate the clearance rate of M.edulis and then compared to other in situ methodologies (biodeposition and adapted suction) applied in the same natural conditions. This method was reliable and useful, if applied under certain constraints.
All these in situ methodologies (feeding behaviour, clearance rate, microgrowth)
developed during this PhD project were combined in one study over an intertidal
mussel bed of the Menai Strait. The same results were found using different
instruments and methodologies following the dual approach of "ask the water, ask the animals": the amount of food from the water distributed to the mussels was equal to the amount of food filtered by the mussels. The results from this study showed the robustness of the methodologies employed but also the importance of the mussel bed patterning (presence of bare patches within a mussel bed) as a facilitator for food supply.
Most of the results in this study contribute to the growing evidence that the use of n situ approaches is necessary to avoid under or over-estimations of production carrying capacity due to incorrect modelling. The interactions of the three components that determine production carrying capacity were demonstrated in this project. These findings open new perspectives for improving production carrying capacity modelling for future applications in the Menai Strait with new re-laying strategy and in other systems.