Age-based demography and relative fisheries productivity of Spanish mackerel, Scomberomorus commerson (Lacepede) in Western Australia

Spanish mackerel, Scomberomorus commerson, is a species of economic and artisanal importance throughout the Indo-West Pacific region. In this study a total of 2973 S. commerson were examined in a comprehensive study of otolith structure and demographic characteristics across multiple management regions along the Western Australian (WA) coast. Marginal increment analysis of sagittal otoliths showed a single annual peak in opaque zone formation indicating that one opaque and translucent band (annuli) is formed each year. Juvenile S. commerson of undifferentiated sex (n = 67; 58–709 mm FL) ranged in age from 23 days to 350 days. Males (n = 1239; 301–1381 mm FL) ranged in age from 80 days to 22 yrs, and females (n = 1273; 396–1720 mm FL) ranged in age from 122 days to 18 yrs. Significant differential growth was evident between sexes, with females attaining a larger size-at-age than males. Initial growth of S. commerson is rapid with both sexes reaching maturity within 1.5 yrs of age. Growth parameters also differed among regions. The instantaneous rate of natural mortality (M) was estimated to be in the range of 0.16–0.20 yr−1. The instantaneous rate of total mortality (Z) estimated from catch at age data for fully recruited age classes, was different between sexes and among regions. Significant differences in demography, age structure and otolith readability among management regions support earlier studies that have documented meta-population structure in S. commerson and underpin the regional management regime for this species in Western Australia. Populations of S. commerson have a high production potential given their rapid initial rate of growth and low age at maturity. Despite this high production potential, productivity of this species in WA is relatively low compared to related fisheries throughout the Indo-Pacific region.