Spotted mackerel (Scomberomorus munroi, Figure 1) are only found in coastal waters of Southern Papua New Guinea and northern Australia. Multiple biological stocks occur throughout the species range across Australia, from Rottnest Island on the west coast to Wollongong on the east coast. Spotted mackerel are primarily harvested along Australia’s east coast, between Cooktown and central New South Wales. Fish in this area are known to belong to a single biological stock. This stock undertakes seasonal migrations with schools traversing between spawning grounds in northern waters in winter/spring and feeding grounds in southern waters in summer.
Spotted mackerel are primarily caught by line and show a harvest split of approximately 45% recreational and 55% commercial. The recreational harvest was most recently estimated in 2013-2014 at 69 tonnes . The annual commercial harvest has been relatively stable over the past four years and is below the 140 tonne quota. Commercially caught fish are processed and consumed in both local and interstate markets.
Routine biological monitoring of spotted mackerel by Fisheries Queensland scientists began in mid-2004. The monitoring program collects information on the length, sex and age of fish caught by both commercial and recreational fishers. The program addresses objectives such as understanding the long-term trends in the length and age of fish being harvested and tracks key population characteristics such as the rate of total mortality.
Fisheries Queensland scientists regularly review the times series of biological information obtained from routine monitoring. It is considered together with information from other sources including commercial fishing logbooks, recreational catch estimates and stock assessments, to evaluate the sustainability of the stock. The available information sources show favourable signals and indicate the east coast stock of spotted mackerel is sustainably fished .
More than 50% of the spotted mackerel harvested are between 65 and 80 cm total length (TL; Figure 2). Few fish longer than 95 cm TL are observed in the harvest (Figure 2). The largest fish sampled in 2015-16 was 109 cm TL. This fish was caught by a recreational angler in the Brisbane offshore area.
There is a slight difference in the size frequency of spotted mackerel caught by commercial and recreational fishers. Small fish (60-70 cm TL) typically make up a higher percentage of the commercial harvest than the recreational harvest (Figure 2).
Scientists often estimate a fish’s age from annual growth bands on its ear bones which are formed during periods of differential growth (Figure 1). Each year, Fisheries Queensland scientists age approximately 600 spotted mackerel. This information helps determine the age structure of the harvest, which assists in determining the status of fish stocks.
Spotted mackerel are fast growing and relatively short-lived; they reach maturity between 1-2 years of age; they are between 1-2 years of age at the minimum legal size of 60 cm TL; and can live as long as eight to ten years. Recent monitoring results show one to five year-olds continue to dominate the harvested part of the population (Figure 3). In 2015-2016, for the first time during the monitoring program, a sampled fish was determined to be 10 years of age (104 cm TL fish; Figure 3)
Figure 3: Age structure of spotted mackerel showing the relative abundance (percentage) of spotted mackerel within each age group for the recreational and commercial fishery sectors between 2013-2014 and 2015-2016. A recent pulse of young fish entering the fishery can be seen (1 year olds in 2013-2014; 2 year olds in 2014-2015; and 3 year olds in 2015-2016).
The sex of spotted mackerel can be determined by examining their gonads (reproductive organs). These organs are located towards the top of the gut cavity. Mature female fish will generally have orange coloured ovaries and male fish will have white coloured testes (Figure 4).
How old is your fish?
Figure 5: Estimate the age of spotted mackerel from a fish’s total length using this graph. For example, a 70 cm male would be on average three years of age with a 90% likelihood of being between two and four years. A female of the same size would be on average one year of age with a 90% likelihood of being between one and three.
Support and assistance
Thank you to all the fishers, fish wholesalers and seafood retailers who have generously assisted with the monitoring of spotted mackerel by donating frames and providing length measurements, or allowed scientific staff to measure their fish. Biological monitoring provides important information for assessing the status of the stocks in Queensland into the future. This ongoing assistance helps to ensure that the collection of biological data is representative of the commercial and recreational harvest each year.