A synopsis of biological, fisheries and aquaculture-related information on mulloway Argyrosomus japonicus (Pisces: Sciaenidae), with particular reference to Australia.
For some time there has been concern over the population of mulloway (Argyrosomus japonicus) in NSW. This concern has been driven by continuing declines over the past two decades in reported commercial catches, recreational fishers reporting reduced catches and fewer large fish, and large numbers of juveniles being discarded as by-catch from estuarine and coastal prawn trawlers.
Mulloway are distributed in estuarine and nearshore Pacific and Indian Ocean waters surrounding Australia, Africa, India, Pakistan, China, Korea and Japan. They are commercially and recreationally fished throughout their distribution and form the basis of a growing aquaculture industry in Australia. A review of the published scientific literature on mulloway indicated a dearth of information concerning their biology and fisheries, except for southern Africa where the biology of mulloway is relatively well studied. In South Africa, mulloway is a fast-growing fish that can live to a relatively old age (42+ years) and large size (> 175 cm TL). In South Africa, mulloway is considered recruitment overfished.
This current study in NSW identified that, like in South Africa, mulloway grow fast, reaching, on average, nearly 40 cm TL in 1 year and 95 cm TL in 5 years. Mulloway in NSW reach sexual maturity at a size of approximately 68 and 51 cm TL for females and males respectively and at an age of 2+ to 3+ years. These lengths and ages are significantly smaller and younger than for mulloway in South Africa, highlighting the need for local data to fully understand the biological characteristics of a species for management considerations. Mulloway appear to predominantly spawn in ocean waters between November and March in NSW.
In NSW, mulloway are primarily caught using mesh (gill) nets in estuaries and by line in ocean waters. Analyses of the statistics of commercial fisheries of mulloway in NSW showed that reported catches have been declining in both the oceanic and estuarine sectors. However, reported fishing effort has also declined which may explain observed declines in total landings. Sampling of catches for length and age composition showed that most (approximately 80%) mulloway were within 15 cm of the minimum legal length (MLL = 45 cm TL) and that very few large (> 70 cm TL) fish contributed to commercial catches. Furthermore, commercial catches were dominated (> 70%) by fish aged 2 years. These data are of concern given that these fish can potentially grow to > 175 cm TL and reach ages of 40+ years. Estimates of total mortality based on catch-curve-analyses were relatively high (0.45 – 0.7) and yield-per-recruit analyses identified that the minimum legal length of mulloway should be increased greatly (to at least 70 cm TL) for optimal harvesting of the species. The data presented are indicative of a species that is growth-overfished.
Changes in the management arrangements for mulloway in NSW are required for their effective conservation and sustainable harvesting. Greater protection to the spawning population and to juveniles in estuaries from capture in prawn trawl fisheries is also required. Implementation of by-catch reduction devices in the estuarine prawn trawl fleet should help with the latter, but a significant increase in minimum legal length and possible seasonal and spatial closures to fishing may be required to protect the spawning population.