Underwater Studies on the Tasmanian Commercial Scallop, Notovola meridionalis (Tate) (Lamellibranchiata: Pectinidae)

Abstract

Population densities of scallops in the D'Entrecasteaux Channel (Tas.) fishing areas have been assessed by diving. In shelving positions the scallops occur in greatest densities between 4 to 8 fm, and on flat areas within these limits they may be found everywhere, but in varying concentrations. The so-called "strips" or "beds" of commercial fishermen are merely higher density aggregations. The most extensive bed, 1 sq. mile in area and averaging six scallops/sq. yd, contained about 18 million scallops.

Two scallops/sq. yd is considered the minimum economic limit for operation provided the meat yield (roe included) per 1000 scallops is not less than 33 lb.

The successful spatfall and subsequent development of a strong year class is very sporadic and, as yet, unpredictable. In 1954, the 1948 year class scallops entered the fishery, which until then had been almost entirely dependent on the scallops spawned about 1940.

Water movements, particularly tidal currents, have a marked influence on the distribution of the spat. In areas of strong tides the long axes of areas of high concentrations run lengthwise to the flow whereas in areas of slow movement the distribution is even, whilst eddy systems appear to concentrate the spat.

Concentrations of 30-36 scallops/sq. yd were reduced in 4 years by starfish to five/sq. yd, i.e. a 75-80 per cent. mortality occurred.

During the course of this underwater investigation the individuals of a selected bed were observed to make random movements, yet the known location of a bed did not change.