Summary

Sea mullet landings are highest in quantity and total value of all finfish species managed by NSW fisheries. Information regarding the biology and fishery of sea mullet in Australia has not been comprehensively reviewed since Thomson (1963). This report reviews biological information relevant to the NSW sea mullet fishery and the operations of the fishery, making it a useful reference document for managers, scientists and stakeholders.

In recent decades, there have been significant advances in understanding of sea mullet biology, particularly the development aging of techniques. Larval and juvenile rearing techniques have also been advanced, although our understanding of the early life history of wild fish is still poorly understood. The tag and release of Australian sea mullet over several decades has resulted in a partial understanding of movement during spawning migrations but spawning locations, post-spawning movement and larval dispersal/recruitment processes remain unclear. The dearth of information regarding reproduction and early life history is surprising, given the numerous important fisheries that target pre-spawning aggregations of sea mullet.

In NSW, sea mullet are caught on ocean beaches and in estuaries. Most landings occur in central or northern regions between March and June. In this period, fish aggregate and undertake a migration prior to winter spawning. Fish in commercial landings are typically 30-40 cm in length. Maximum observed length in NSW is 60 cm. Ocean and estuary landings are dominated by 4-6 and 2-3 y old fish, respectively. Maximum observed age in NSW is 12 y. Females typically attain greater maximum sizes and ages than males. Maturity is reached at approximately 30-34 cm in length and 3-4 y in age. Females are targeted by commercial fishers for their roe, which is a valuable export product. Fecundity has not been examined in the NSW population, but estimates from other regions range between 1 and 4.8 million eggs, depending on female body size.

Monitoring of the length and age of NSW commercial sea mullet landings has occurred annually since 1995, and intermittently in earlier years. Age and length data are used to infer rates of recruitment, growth and mortality. Stock size and catch rate are functions of these population parameters. In this report, preliminary population parameters are estimated for the NSW stock. The age/length composition of sea mullet landings is variable among years, among regions and between sexes. Such variability must be considered in the sampling design of monitoring programmes. Stock assessment of sea mullet requires spatially representative, annual monitoring of age/length composition of landings.

Prior to 1985, the NSW sea mullet fishery experienced a 30 year period of relatively stable catch levels, suggesting that these 'historical' catch levels were sustainable. Over this period, annual landings consisted of approximately 2000 t from estuaries and approximately 500 t from ocean waters. After 1985, quantities of oceanic landings increased very significantly and exceeded estuarine landings for a six year period during the mid-1990's. Unprecedented quantities of spawning fish were removed from the stock by the ocean fishery during this period and it is possible that the spawning component of this stock was depleted. Significant declines in catch occurred after this period. In 1999/00, ocean catch levels were approaching (but still above) historical levels. Significantly, historically stable catch levels may not currently be sustainable because of the depleted spawning biomass.

The cause of the decline in catch between 1998 and 2000 is unclear. Available data suggest a reduced availability of mature sea mullet in recent years, which may have resulted from recruitment variability and/or fishing pressure. Trends in catch and age structure are difficult to interpret in the absence of a reliable index of sea mullet abundance. If the quality of catch and effort data reported by commercial fishers could be significantly improved, the information may provide an index of abundance.

Under current market conditions, ocean-caught fish are more highly valued than estuary-caught fish and so is it likely that ocean fish will continue to be highly targeted. The impact of this harvest strategy on the sea mullet stock is uncertain, given the available data.