A Population Study of the Tasmanian "Commercial" Scallop, Notovola meridionalis (Tate) (Lamellibranchiata, Pectinidae)

Abstract

The "commercial" scallop supports a dredge fishery in the D'Entrecasteaux Channel, Tasmania. There is probably a genetically homogeneous population in this area, and the juveniles possibly move from bed to bed, but the adult scallops form discrete, stable communities.

The statistical history of the fishery reveals a marked decline in the availability of scallops since some time between 1941 and 1945. A more exact slatement cannot be made because fishing efforts in war time were not comparable in efficiency with those made before and since. Furthermore, before 1941 a second species of scallop was also fished, but not distinguished in the catch records.

The rings in the shell are formed annually and can thus be used for age determination. This can be demonstrated directly in young scallops by comparing the growth at the edge of the shell at successive seasons of the year. In older scallops the growth increments are too small to compare in the same way; the evidence of the annual character of the rings is here mainly indirect, although in its totality convincing. There is distinct variation in growth rate between different beds, and apparent variation between different age-groups from the same bed.

The age and size composition of the stocks were studied from samples taken from several representative beds from 1944 to 1949. The age composition studies showed that the year-classes produced in the period extending approximately from 1939 to 1941 predominated, and that very little recruitment had occurred since that period; the results of the size composition studies confirmed this. Scallop fishermen agreed that snlall scallops had been unusually scarce in the Channel for many years.

It is concluded that the scallop population is suffering primarily from a severe long-term shortage of recruits, the cause of which, although apparently natural, is unknown; and secondarily from overfishing, as the fishermen exhaust, one after another, the aging, isolated subpopulations of adults on the different beds. Unless ample numbers of recruits soon reappear, the fishery may come to an end.

(Since Mr. Fairbridge wrote this paper some beds of juvenile scallops have been located. Although the beds are not large, their presence indicates that the Channel population is still reproducing and that there could be a return to years of good recruit broods.)