Population and ecophysiological modelling of the cultured mussel Perna perna : towards the development of a carrying capacity model

The overall aim of this thesis was to integrate the ecophysiology and population dynamics of the mussel Perna perna in Southern Brazil into a model that can ultimately be used for carrying capacity analysis in a tropical environment. The first chapter quantified and modelled the filter-feeding behavior of mussels feeding on natural seston. Models were generated that described each step of the feeding process and produced a predictive model of rates of food uptake. Feeding experiments using the biodeposition approach were conducted with mussels ranging in shell height from 3.94 to 9.22 cm of three sites, including turbid and clear water environments. Among the feeding steps characterized and modelled were filtration rate, rejection rate, organic selection efficiency, the organic content of ingested matter, absorption efficiency, and absorption rate. The coupling of the equations that described filter-feeding processes produced a robust model with relatively low complexity and specificity. The model can predict the P. perna feeding behavior in turbid or clear water and can be used with different species if the correct coefficients are used. In the second chapter, growth and mortality rates using size frequency distributions of P. perna in suspended culture in two locations were studied. Growth rates were used as forcing functions in a model to predict size class distributions on one meter mussel ropes. Settlement of new individuals was included in the model using the unique settlement rate observed in each location. Mortality rates were estimated at 0.06 year-1 in the populations at both locations. The integration of growth and mortality data in a predictive model resulted in good predictions of size frequency distributions at two locations in Brazil. The third chapter coupled the feeding model developed earlier with a model of energy balance and scope for growth. This model was divided in four sectors to facilitate description and explanation of the functions controlling feeding and metabolic responses to varying food availability and seawater temperature. The seston sector included characteristics of seston likely to influence food and energy acquisition in mussels. It also included relationships that estimated the energy content of phytoplankton and detritus, the main components of mussel diet. The feeding sector described suspensionfeeding behavior using natural seston as described in the first chapter. In the energy allocation sector, absorbed matter was transformed to absorbed energy using estimates of energy content of food provided in the seston sector. After accounting for the maintenance requirements of the mussels (heat loss, excretion, and mucus production), the scope for growth was directed to the growth sector. The growth sector included byssus, organic shell, and soft tissue production based on energy pa,rtitioning estimated from monthly measurements of tissue (somatic and reproductive) and shell growth. The model successfully predicted mussel shell length and dry tissue weight during the simulation period and estimated the response of mussels to food acquisition, energy expenditure, and allocation to growth. In the fourth and last chapter, mussel population dynamics and ecophysiology were coupled to model feedbacks from the bivalve population to the environment. These feedbacks included population level estimates of the rates of filtration, excretion, and biodeposition. Seston (TPM, POM and CHL a) dynamics were investigated in three temporal scales: biweekly for four years, weekly for eight months, and tidally (neap and spring tides). Characterization of the study area enabled an estimation of bivalve standing sock, total surface area, and volume of the area. Measurements of water level across tides allowed estimates of tidal water renewal inside the area. Some aspects of interest for carrying capacity analysis presented in the last chapter were the water mass residence time for the area, bivalve clearance time (the time needed for the total bivalve biomass to filter a volume of water equivalent to the system volume), and phytoplankton production time (the time it takes for the primary production within the system to replace the phytoplankton biomass within the system). Among the important aspects for carrying capacity studies not included in this model are the spatial resolution of hydrodynamics_ processes and physical/biological processes related to seston dynamics like sedimentation, resuspension, and mineralization of organic compounds, interacting together with the population ecophysiology to predict the exploitation carrying capacity for this system. Although there was good agreement of the model prediction with the observed mussel growth data, the model needs to be tested with an independent set of data before it is can be used as a management tool. Therefore, it appears that the integration of whole animal models with population models can be used in carrying capacity analysis for shellfish culture areas in Brazil.