Sexual dimorphism in the red morwong, Cheilodactylus fuscus

Abstract

The red morwong, Cheilodactylus fuscus Castelnau (Pisces : Cheilodactylidae), is a conspicuous fish on the rocky reefs of the eastern Australian coast. Visual determination of the sex of individuals in the field is necessary to remove sex bias in home-range and associated bioaccumulation studies. A sample of 61 fish (30-50 cm fork length) was collected from spear-fishing competitions in the Sydney area, and from each fish 16 morphometric measurements were taken. A multivariate analysis revealed that, in comparison with females, males are generally larger, with a significantly larger orbit-tubercle - or horn - situated in front of each eye, a smaller eye diameter and a smaller head length. A discriminant function analysis derived from these four features correctly identified 87% of an independent sample of 40 fish. Visual appraisal of fish on the basis of the proportional length of the horns misidentified <10 % of fish >30 cm and <30% of fish 20-30 cm fork length. The orbit-tubercle in males is between 2% and 4% of the fork length and is composed of bone. According to this simplified visual technique, red morwong aggregations in the Sydney region have consistently biased sex ratios.