Aspects of the Biology of the New Zealand Scallop, Pecten Novaezelandiae Reeve 1853, in the Marlborough Sounds

Abstract

The ecology, reproduction, early life history, growth swimming behaviour and mortality of the scallop Pecten novaezelandiae Reeve were studied in Pelorus Sound and nearby areas during 1973 and 1974 as a contribution towards better management of the fishery for this species. Field observations and results from previous studies indicate the habitat of P. novaezelandiae, and the reproductive system and spawning cycle are described from histological sections and records of seasonal variations in external features, spat settlement spawning and gonadial weights. P. novaezelandiae occurs on a wide variety of substrates in semi-estuarine and coastal waters from low tide level to at least 90 m and is not associated exclusively with any single bottom community. It is a functional hermaphrodite with no distinct tendency to either protandry or protogyny. In Pelorus Sound partial spawning of some scallops occurs from at least August to March, and in 1973 and 1974 spawning peaked in November or December. Most juveniles have well developed gonads by the time they are one year old but contribute little to the spawning pool until the end of their second year. The early stages in the life cycle of P, novaezelandiae are described from observations of D-stage veligers reared in the laboratory, from umboned larvae collected from the plankton and from spat obtained from artificial and natural settlement surfaces. The larvae, which are similar in shape to those of the Tasmanian scallop (P. meridionalis), metamorphose soon after reaching a shell height of 200 2m and attach themselves with byssus threads to filamentous materials such as Zostera debris, algae and hydroids on the seabed. Spat detach themselves from their settlement sites on reaching about 5 mm in shell height and are thereafter free-living on the bottom. Studies of size frequencies and measurements of confined and free-living tagged scallops indicate that scallops in the outer part of Pelorus Sound reach the minimum takeable size (100 mm in length) about 2½ years after settlement and that the commercial catch in this area consists mainly of scallops between 2½ and 5 years old. The likely causes of slight reductions in growth rate which occurred in the late winter and spring of 1973 and in the winter of 1974 ate discussed. Most scallops formed 'growth' rings during these periods but the frequent and irregular formation of rings in other months precludes the use of ring counts as accurate indicators of age and may explain the low growth rate estimates given in previous studies. The swimming behaviour of Pecten novaezelandiae is described and evidence for and against the existence of scallop migrations is presented. Underwater observations, the recovery of tagged scallops and the persistence of characteristic differences between adjacent subpopulations indicate that adult scallops in Pelorus sound normally move very little. The likely causes of mortality among scallops in Pelorus Sound are discussed and annual mortality rates of adult scallops on two beds in the outer Sound are estimated from a study of the duration of resilia on dead shells and the relative abundance in dredge samples of live scallops and empty shells with resilia attached. Mortality on these two beds, estimated at 23% and 39% per annum respectively, is ascribed mainly to old age and predation by fish and starfish but there is evidence of high levels of indirect fishing mortality in some areas. Size frequency distributions recorded in this and previous studies indicate that annual recruitment of spat has been quite reliable during the last 20 years, but because the catch at any one time consists of only three or four year classes, it is anticipated that failure of a single year class could lead to a temporary collapse of the fishery. The prospects of expanding the fishing area to reduce the chances of such an occurrence are discussed and a number of tentative suggestions for the management of the fishery are given. It is considered that raft culture of scallops to marketable size would be uneconomic in New Zealand at present but partial culture and subsequent seeding of new scallop beds might be worthwhile especially if natural spat fall should fail.