Abstract
The Pacific oyster, Crassostrea gigas (Thunberg), native to Japan, China, Korea, Southeast Russia and Taiwan, was first discovered in Northland, New Zealand in 1971 and then in the Marlborough Sounds in 1977. The origins of both populations are unknown.
In Northland, the oyster spread quickly through high levels of natural reproduction and through incidental transfer on oyster sticks used for the farming of the native oyster, Saccostrea glomerata. In the inner Marlborough Sounds, including Kenepuru Sound, the population increased slowly over fifteen years, although large scale spawning and recruitment occurred in Tasman Bay. The low natural colonisation and agreements to limit the aquaculture of the species permitted an opportunity to study the dynamics of colonisation of Crassostrea gigas in the Kenepuru Sound.
The field work took place over two summers, 1992-93 and 1993-94, and quantitatively examined the abundance of the larvae of C. gigas in Nopera, Long and Mills Bays of the Kenepuru Sound and compared this abundance to the settlement on collectors on the beach. Both observations became tools for the measure of larval supply to the intertidal beach substrates. Recruitment on the beach was assessed by the annual survey of 500m long intertidal transects at a shore height of 1.3m chart datum in the three bays. Other data recorded included oyster condition in Nopera Bay, predators on the transects, and the base beach substrate types upon which the colonisation was building.
The data suggest abundance of larvae was significantly correlated to the density of settlement on collectors from bay to bay. Settlement on collectors was not significantly correlated , however, with the recruitment on collectors or in specific beach sections on natural substrate but general trends were apparent from bay to bay. Recruitment success was observed to be a function of substrate suitability and the survival of the spat. High larval supply did not assure high recruitment or did low supply limit beach sections to low recruitment. Predation was high on all the transects and was most often by Lepsiella spp., common intertidal whelks. There was no evidence that beach base substrate type limited or enhanced the recruitment or subsequent colonisation of C. gigas. Results of experimental manipulation showed freshwater appeared to enhance survival of settlers rather than induce larval settlement.
The aggressive reproductive characteristics of C. gigas, i.e. high fecundity and planktonic larval stage, will most likely result in sporadically large increases in the abundance of the species in the Marlborough Sounds, followed by periods of high mortality and population decline. In the short term, the species’ pattern of recruitment in the distinct patches should continue to be observed on the intertidal beaches of the Kenepuru Sound. In the longer term, a more general distribution is likely as documented in other C. gigas colonised localities of the world.