The ecology of bivalves on Centre Bank, Tauranga Harbour

Abstract

An investigation into the ecology of bivalve molluscs on Centre Bank, the flood tidal delta of Tauranga Harbour was undertaken over the period December 1994 - March 1996. The primary purpose of which was to gather quantitative data on bivalve diversity, spatial patterns of distribution, abundance, and population size structure of bivalves present on Centre Bank. Experiments were conducted to assess the burrowing behaviour of the bivalve Paphies australis. It was intended that information obtained be such that it could be used as a base for future investigations into changes in the ecology of Centre Bank. Two broad scale benthic surveys of 27 sites on Centre Bank, conducted 6 months apart resulted in a total of 31 bivalve taxa being identified. Temporal and spatial variation in abundance was prominent for all species. Three species Paphies australis, Tawera spissa and Ruditapes largillierti occurred in far greater abundances than any others. Distribution of bivalves in both surveys was patchy with some species showing strong associations with particular sites. Small scale abundance patterns of P. australis was examined in detail by systematic sampling of a 50 m x 15 m grid. The grid was monitored at 2- monthly intervals from March 1995 to November 1995. Abundances at sites in the northern half of the grid were found to be very dense (up to 1400 m², mean shell length=55-65 mm), while densities at the southern end were noticeably lower. Through time densities across the grid appeared to decrease evenly in a constant north-west direction. The smothering effect of mobile sand ridges was speculated to be the cause of these density decreases. Experiments examining the burrowing behaviour of the infaunal bivalve Paphies australis were conducted both in the field and in the laboratory. The ability of P. australis to bury into the sediment if left exposed on the sediment surface was examined in the field. It appeared that P. australis were able to bury relatively quickly with the majority of experimental animals reburying within 40 minutes. Laboratory experiments showed P. australis to be capable of burrowing to the sediment surface after being inundated with sediment to a depth of 10 cm.