OF the various different species of mackerels available to northern anglers, the broadbarred or grey mackerel (Scomberomorus semifasciatus) is one of the most enigmatic.
Grey mackerel can be encountered from Shark Bay in WA, throughout the Top End and down the east coast of Queensland, and seasonally even as far south as northern New South Wales. This adaptable species can be found from shallow tropical estuaries out to the edge of the continental shelf, with juveniles often being found inhabiting near shore areas such as river mouths and estuaries, with adults tending to be found further offshore.
As their common name suggests, in life S.semifasciatus usually have several indistinct, broad vertical bars on their sides, which either fade or form spots as fish grow larger. These, together with a large black area on the front of the dorsal fin, separate this species from the Spanish mackerel, which lacks the black front dorsal fin and tends to have more numerous narrow bars along the flanks. Upon their death, the broad bars on the flanks of S. semifasciatus tend to fade into a uniform grey colour, hence the name grey mackerel.
Like other mackerels, grey mackerel are speedsters which feed voraciously on baitfish such as herring, sardines and anchovies, allowing them to grow rapidly. Both male and female fish reach maturity at around two years old, at which time they are around 67 cm to 81 cm fork length, respectively. Maximum size for grey mackerel is around 11 to 12 kg and around 130 cm long, however, fish that size are exceptional, and most fish encountered by anglers tend to be less than half that weight.
Female grey mackerel produce more than 250,000 eggs per spawning and off the northern Queensland coast, spawning occurs several times during the spring/summer season between October and February. The larvae are subsequently found in coastal bays and the lagoon area between the coast and the Great Barrier Reef, with juveniles being particularly common near estuaries.
Commercial gill netting of grey mackerel has been controversial in some parts of the country with allegations that this activity has resulted in local stock depletions. These observations prompted more detailed research into the genetics of grey mackerel as fisheries managers attempted to determine whether grey mackerel populations were highly migratory or site attached. Recent stock discrimination work using parasites and genetic markers has found evidence for at least four different stocks of grey mackerel: one in Western Australia, one off the north-west coast of the Northern Territory, one in the Gulf of Carpentaria and the other on the eastern coast of Queensland.
Genetic analyses showed that mixing of the east coast stock with the Gulf of Carpentaria stock through movement of grey mackerel through Torres Strait was likely to be frequent. In contrast, the genetic data suggested that movements of grey mackerel from WA into the Northern Territory were rare. Commercial and recreational fishers reported the existence of annual spawning aggregations of Grey mackerel along certain parts of the east coast of Queensland. The evidence of genetic structure within different parts of the country highlights a need for careful management of this species, especially in relation to the question of fishing grey mackerel during their spawning aggregations.