Growth, Age Validation, Mortality, and other Population Characteristics of the Red Emperor Snapper, Lutjanus sebae (Cuvier, 1828), off the Kimberley Coast of North-Western Australia

Red emperor, Lutjanus sebae, were examined from commercial catches in the Northern Demersal Scalefish Fishery (NDSF) of north-western Australia from 1997 to 1999. Specimens ranged from 183 to 728mm fork length (FL); males had a mean FL of 509mm, and were significantly larger than females that had a mean FL of 451mm. Ages were estimated from thin sections of sagittal otoliths. Marginal increment analysis of sagittal otoliths showed a single annual minimum during September and October and indicated that one annulus is formed each year. Male L. sebae (n=977; 211–728mm FL) ranged from age 2 to 30 years and females (n=1384; 183–584mm FL) ranged from age 1 to 34 years. Sagittal otolith weight and height were significantly correlated with age for each sex. There was significant differential growth between sexes. The relationship of observed fork length at age was described by the von Bertalanffy growth equation for males, Lt=627·8 {1−exp [−0·151 (t+0·595)]} and females, Lt=482·6 {1−exp [−0·271 (t−0·065)]}. The slow growth, long life span and large size and age at maturity of L. sebae indicate that this species has a low production potential and hence spatial area closures are vulnerable to over-exploitation. The instantaneous rate of natural mortality (M) ranged from 0·104 to 0·122. The optimum rate of fishing mortality was estimated to be 0·052–0·061. The instantaneous rate of total mortality (Z) estimated from catch at age data for fully recruited ages, was 0·374 in 1997/98 and 0·242 in 1998/99. Hence, the NDSF population of L. sebae is exploited above optimum levels. Given their low production potential, populations of L. sebae in north-western Australia and elsewhere in the Indo-Pacific region require prudent management. Furthermore, fishery managers need to consider as part of any harvest strategy for these fish to preserve significant levels of the spawning stock.