Population Studies on the Toheroa, Amphidesma ventricosum Gray (Eulamellibranchiata)

Abstract

The ecology, distribution, and commercial importance of the toheroa, Amphidesma ventricosum Gray, are discussed. The habitat favoured by that portion of the population at present known is a beach fully exposed to heavy surf, with a fine sand and sufficient residual moisture to prevent desiccation at low tide. Frequent dense blooms of phytoplankton are necessary to provide an adequate food supply. Estimates of numerical abundance for various beaches are given, showing that violent fluctuations in numbers may occur, sometimes, but not always, with epidemic mortalities. The sampling technique for estimat- ing numerical abundance is described and discussed. The growth rate has been estimated for several different beaches, both by interpreting the annual growth-rings of the shell, and by analysis of length frequency data. The South Island toheroa have a significantly lower growth rate than those in the North Island. It is shown that the distribution of age classes on the beach is inconsistent with the assumption that the species is confined to the littoral region. In particular, the younger toheroa are almost invariably fewer in numbers than would be expected in a self-contained popula- tion. From this, and from the extreme fluctuations in numbers which occur from time to time, it is concluded that some part of the population is located in the sublittoral. The spatial distribution of the beach toheroa shows a high degree of over-dispersion (or aggregation) which is mainly attributable to variations in environmental conditions. Vertical and horizontal zonation of size groups may sometimes occur, but the contributing factors are obscure. Statistical aspects are discussed in appendices.