Ecology of the invertebrate predatory fauna across Centre Bank, Tauranga Harbour

Abstract

The distribution, abundance, population size structure and behaviour of three common invertebrate predators were studied on Centre Bank, the flood-tidal delta of Tauranga Harbour. The study assessed the role of the predators in controlling the distribution and abundance of bivalve shellfish and provided information concerning the biology of Centre Bank for Port of Tauranga Ltd. Two broad-scale surveys were conducted to describe the broad-scale spatial patterns of Patiriella regularis, Coscinasterias calamaria and Cominella adspersa. Results found P. regularis was the numerically dominant species across Centre Bank and it maintained a widespread distribution over the sampling period. C. calamaria was of low abundance and typically exhibited a patchy distribution. C. adspersa was also found to be of low abundance and similarly distributed as C. calamaria. Size structures of all 3 species comprised mostly adults in both surveys. Temporal monitoring at finer scales was undertaken, two-monthly, to describe small-scale spatial patterns. P. regularis was found at similar densities to that of the large-scale surveys and it was the most prevalent species across the grid; C. calamaria was present at higher abundances within a small patch of the grid and appeared stable over time. C. adspersa was initially absent from the grid, but became more abundant and widespead over the 9 month sampling period. Investigations included monitoring the natural movement behaviour of C. adspersa in relation to tidal currents and the body condition. Results showed the directions moved and distances displaced by C. adspersa may be influenced by the rate and direction of current movement and therefore potentially influencing the dispersal ability of C. adspersa, in relation to available prey. An experiment to determine the body condition of C. adspersa in the field suggested that food deprivation may be less detrimental than first realised, or that C. adspersa is able to feed on a greater variety of food types than known of. Little evidence was found that indicated predation was an important mechanism controlling bivalve populations across Centre Bank. However, predators may be potentially important as scavengers of decaying or dead organisms. Their predatory activities may also provide shell substrate for macroalgae, and this, in combination with dense aggregations of C. calamaria, appeared to provide habitat for small fishes.