Sea mullet monitoring
Fisheries Queensland monitors the biology of key commercial and recreational species to help assess the sustainability of our fisheries.
Sea mullet (Mugil cephalus) has been monitored since 1999 as it is an important component of the commercial fin fish catch in Queensland.
In Queensland, sea mullet inhabit freshwater, estuaries and waters adjacent to ocean beaches along the entire east coast; however, the majority of fish are harvested from southern waters. Sea mullet are caught by commercial net fishers in estuaries year-round using mesh and tunnel nets (Non-Ocean Beach sector), and along ocean beaches from April to August on their annual spawning migration using seine/haul nets (Ocean Beach sector). They are processed and consumed locally, and some products such as roe are exported with World Trade Organisation approval.
The annual harvest of sea mullet in Queensland has been consistent over a long period of time. Between 2007 and 2015, 1,500 to 2,200 tonnes of fish have been caught annually (Graph A), with an annual average of 1,800 tonnes caught over this period.
Monitoring the fishery
Biological information from sea mullet taken commercially south of Baffle Creek contributes to assessing the status of the stock. Consistent landings and favourable signals in the biological information including age structure have resulted in sea mullet being classified as sustainably fished1.
1.Fisheries Queensland (2016) Summary: Stock Status of Queensland’s fisheries resources 2016. Department of Agriculture and Fisheries, Brisbane.
The age of a sea mullet can be estimated by examining a thin section of its otolith (ear bone) under a microscope. Annual growth rings are formed due to periods of differential growth. Approximately 1500 sea mullet are aged by Fisheries Queensland scientists each year. This information helps determine the age structure of the commercial harvest.
Most fish harvested commercially are between three and five years old, but fish as old as 16 have been sampled. Sea mullet grow at different rates due to a number of factors including their habitat, food availability and sex. On average, a sea mullet is three years old by the time it reaches the minimum legal length of 30 cm total length.
The age structure of the commercial catch assists in determining the status of fish stocks. If age structure is consistent over a long period of time, and old fish are being caught regularly, this indicates that fishing pressure is not excessive. Results from the monitoring of sea mullet show that age structure in southern Queensland has been consistent for a number of years, and old fish are present in the population (e.g. fish older than six years are caught regularly – see age structure graph).
Support and assistance
Thank you to commercial fishers and wholesalers who have generously assisted with the monitoring of sea mullet by allowing Fisheries Queensland scientific staff to sample their catches.