A growth model for giant clams 'Tridacna crocea' and 'T. derasa'

Abstract

In this paper, a model is presented for the average growth of an individual giant clam. The model describes the clam’s basic metabolic processes of photosynthesis, filter-feeding, respiration and 'surplus' energy demand for unaccounted metabolic processes such as reproduction, and incorporates the effects of environmental variables including temperature, solar radiation, depth, visibility and nutrients in the seawater. A detailed mathematical description of the model is provided, and its operation is investigated by fitting it to growth data collected for 'Tridacna crocea' and 'T. derasa' from smallholder-mariculture trials in Solomon Islands. Model calibration is described and the sensitivity of the simulation results to parameter values is considered. Growth predictions for both species are found to closely fit the observed data and to be very sensitive to small errors in the value of those parameters that regulate photosynthesis, respiration, surplus energy intake and how respiration responds to temperature. These results lend support for scientific research to better understand the relationships between these parameters and clam growth. The effect of environmental conditions on predicted growth is also investigated. Results indicate that growth has a substantial positive correlation with both visibility and nutrients. These environmental factors are likely to be negatively correlated with each other, hence nutrient-rich environments are likely to have low visibility and the benefits from nutrients may be outweighed by the costs of turbidity and clams may grow poorly. Although the mathematical model presented here could be applied to wild-stock clams, parameterisation with data from mariculture trials restricts its use to predicting growth over only a few years.