Do recent age structures and historical catches of mulloway, Argyrosomus japonicus (Sciaenidae), reflect freshwater inflows in the remnant estuary of the Murray River, South Australia?

Patterns of annual freshwater flows in the Murray River and recruitment of mulloway, Argyrosomus japonicus (Scianidae) were reviewed in terms of recent age structures and historical catches and CPUE. Age distributions from the nearshore marine fishery were dominated by the 1993 age class which comprised 35% and 41% of 2001 and 2002 catches, respectively. In 1993 annual freshwater inflow was 2.4 times the 25 year average. Freshwater inflow explained 28% and 35% of the variability in year class strength in the nearshore marine fishery in 2001 and 2002, respectively. Over 80% of the current South Australian commercial catch of mulloway comprises juveniles taken from the remnant estuary of the Murray River. Our results suggest that recent low levels of recruitment in South Australia's fisheries for A. japonicus may reflect low fresh water inflows since 1993. Since 2000, southern Australia has experienced the worst drought in recorded history and management strategies for this fishery must take into account effects of both environmental factors and fishing mortality on this vulnerable sciaenid population. We suggest that the population of A. japonicus located about the Murray River system is estuarine dependent, that the estuary provides important refuge for juveniles, and that strong year classes, or their absence, may be related to freshwater inflow to this environment. We also suggest that age distributions of this apex predator may provide an indicator of environmental health for the Murray River estuary. Le modèle annuel des apports d'eau douce du fleuve Murray et le recrutement de Argyrosomus japonicus (Sciaenidae) sont étudiés en termes de récentes structures en âge, de captures historiques et de prises par unité d'effort (CPUE). La distribution en âge des captures, de la pêcherie côtière maritime, est dominée par la classe d'âge 1993, qui représente respectivement, 35 % et 41 % des captures 2001 et 2002. En 1993, l'apport annuel d'eau douce a été 2,4 fois celui de la moyenne de 25 années. L'apport d'eau douce explique 28 % et 35 % de la variabilité de la force d'une classe d'âge de la pêcherie côtière en 2001 et 2002, respectivement. Plus de 80 % des captures commerciales habituelles de l'Australie-Méridionale consiste en juvéniles, pris dans l'estuaire résiduel du Murray. Nos résultats suggèrent que les récents faibles niveaux de recrutement de A. japonicus dans les pêches de l'Australie-Méridionale pourraient refléter les faibles apports d'eau douce d