Variation in growth, mortality, length and age compositions of harvested populations of the herbivorous fish Girella tricuspidata
Commercial gillnet and beach-seine catches of Girella tricuspidata from seven estuaries in eastern Australia were examined for differences in fork length (LF), sex and age composition, and populations were assessed for growth and mortality. Fish 220–350 mm LF dominated landings across all estuaries sampled, regardless of gear type. Few fish >10 years of age were observed in the catches, with fish aged 3–5 years, and 4–7 years, being most abundant in the catches in the four most northern estuaries and three southern estuaries, respectively. There was considerable variation in the LF of G. tricuspidata at any given age and the oldest male and female were 21 and 24 years, respectively. There were no consistent differences between sexes or latitudinal regions in the growth and mean LF at age of fish in each individual age class between 3 and 8 years. Growth of females was greater than males in the northern region, but not elsewhere. Estimates of the instantaneous rate of total mortality (Z) were dependent on estuary and year, ranging from 0·30 to 1·01, whereas the corresponding estimates of fishing mortality (F) ranged from 0·12 to 0·90. Populations of G. tricuspidata appear to have been heavily exploited, primarily relying on young fish recruiting to the fishery. The ecosystem-wide effects of harvesting this dominant mobile teleost herbivore need to be assessed further.