Biological and fishery characteristics of rubberlip morwong Nemadactylus douglasii (Hector, 1875) in eastern Australia

Abstract

Rubberlip morwong Nemadactylus douglasii are important members of temperate reef communities off south-eastern Australia where they are targeted by offshore commercial and recreational fisheries. Macroscopic gonad staging and gonadosomatic indices demonstrated that N. douglasii have a distinct autumn/winter spawning period. Males and females attained sexual maturity at approximately 24 cm fork length, corresponding to an age of approximately 3 years old. During the spawning period male gonadosomatic index values were of an order of magnitude smaller than those for females, suggesting that N. douglasii may belong to the group of cheilodactylids that are not group spawners. The method of counting opaque zones in sectioned otoliths to estimate age was validated using marginal increment analysis. The results showed that opaque zones in the otoliths of N. douglasii were similar to other cheilodactylids in this region and formed once each year during spring/early summer. Males grew at a similar rate to females until approximately 2 years of age, after which males grew faster and attained larger sizes. The longest and oldest fish observed during the study were 52.4 cm fork length and 22 years, respectively. N. douglasii were fully recruited to the commercial fishery by 4 years of age and the fishery was dominated (>93%) by fish between 3 and 15 years of age. The age composition in landings showed evidence of variable recruitment through time. The total instantaneous mortality rate was estimated using catch curve analysis and ranged between 0.27 and 0.37 yr −1. Natural mortality was estimated as 0.14 yr −1 based on the maximum observed age. Steady declines during the past three decades in the quantities and average sizes of N. douglasii in landings suggest that the stock has been substantially depleted.