Mortality and production in natural, artificially-seeded and experimental populations of Anadara granosa (Bivalvia: Arcidae)

Declines in density of one natural, three artificially-seeded and eight experimental populations of Anadara granosa were monitored over periods ranging from seven to twelve months. For three of four large populations the decline in density has been adequately described by a model of the form N t=N 0·e zt where N t is density at time t, N o is initial density and z is the instantaneous mortality coefficient. The mean value of the latter was found to be-1.88·yr-1 with 95% confidence limits-1.54 to-2.25. The data obtained from the experimental populations demonstrated that mortality is unaffected by initial densities upto 2,500·m-2, but increases as shore elevation decreases due at least partly to greater access by predators at the lower levels.The data on mortality rates have been combined with previously-published information on growth rates in order to arrive at estimates of production. At one of the artificially-seeded sites the 95% confidence limits of estimates of mean production are 24 and 62 g dry tissue·m-2·yr-1 with an average value of 42 g·m-2·yr-1. Examination of the effects of shore elevation on production revealed that the latter is greatest at the uppermost shore level studied because down-shore increases in growth rate are more than offset by increases in mortality. Production per individual has been shown to decrease with increasing density. This latter fact has been used to estimate the maximum possible production which, for one site 250 cm above chart datum, lies between 49 and 72 g·m-2·yr-1 (95% confidence intervals). It has been demonstrated that for one of the culture sites variation about an estimate of mean mortality rate does not contribute as much to the variation of the final estimate of production as does the variation about the estimates of the mean values of the constants in a growth equation.As far as possible production of Anadara granosa has been compared with values reported for other marine and estuarine bivalve molluscs and it is concluded that A. granosa may be considered moderately to highly productive.