Studies on the nutritional ecology and ecological energetics of oysters from Galveston Bay

Abstract

This study examines the relationship between what is available for the oyster to eat, its condition or state, and physical factors of its environment. Available food was evaluated by a series of measurements on the particulate matter of seston. The measurements made included chlorophyll a, phaeo-pigment, lipid, carbohydrate, and protein concentrations, as well as the dry weight, ash-free dry weight and inorganic weight of the seston. Proximate analysis, a dry-weight condition index, macroscopic appearance, gonadal index, and level of parasitism by Perkinsus marinus were used to evaluate oyster condition or state. Measured environmental parameters included water temperature, salinity and depth, oxygen concentration, turbidity, and current velocity. Changes in the proximate composition of oysters are associated with changes in the annual cycle of fattening, storage, and reproduction. The fattening phase is characterized by high dry-weight condition indices and elevated carbohydrate (glycogen) concentrations. A "storage cycle," the transition from stored glycogen to the lipid reserves in developing eggs, is evident in Crassostrea virginica. The gonadal index and percent lipid composition of the oyster were positively correlated. Oysters that have spawned have low lipid and carbohydrate concentrations, low condition and gonadal indices as well as high percent water and percent protein concentrations. Available food for the oyster was measured as a food index. The food index was defined as the percentage food (food = lipid + carbohydrate + protein) in the total seston. The food index was higher in the spring and summer and was correlated with the gonadal index of oysters. Apparently, the amount of food was greatest at the time of greatest energy demand; that is, during gametogenesis. Two Fortran models of oyster energy budgets were constructed with information gleaned from the literature. The results of the models are in fair agreement with published literature. This suggests the possibility that the food index is a useful measure of available food, that the simplifications made in the models are reasonable ones, and that enough particulate food is present in Galveston Bay to sustain oysters.