The New South Wales Tiger Flathead, Neoplatycephalus macrodon (Ogilby). I. Biology and Age Determination

Abstract

In a previous paper the catch statistics of the east Australian trawl fishery (Fairbridge 1948) were discussed, and in the present work, the biology of the most important species, the tiger flathead, is examined in greater detail. An age determination technique is developed for this species, and the age composition of the catch of recent years is analysed.

The sampling technique carried out at the Sydney Fishmarkets is described and tested. The results are given of the first experimental trawling in Australian waters using a fine-meshed cover on the codend.

The biology of the species is studied, chiefly from the point of view of establishing the age determination technique and the final analysis of the catch. Spawning is protracted, lasting throughout the six warmer months, and takes place throughout the geographical range of the species. The males spawn first in their second year, the females in their third or fourth. The "condition" (K) of the gutted fish is lowest in February, after spawning. It is probable that there is a single, genetically uniform stock, with a single centre of maximum occurrence. The younger age-groups probably do not undertake extensive coastwise movement; as a result, local areas are liable to become relatively depopulated by heavy fishing.

The method of collecting and reading the flathead otolith is described. The otolith is found to grow isauxetically above a fish-length of 10 cm., and the technique of back-calculation is therefore used as an aid in age determina- tion. The otolith in this species is certainly not as clear to interpret as that of the North Sea plaice, but is probably comparable with that of the cod or hake.