Jack mackerel

Scientific name: Trachurus declivis, T. murphyi

Family: Carangidae

Other names: Cowanyoung, greenback horse mackerel, scaly mackerel, scad, common jack mackerel, Peruvian jack mackerel, Chilean jack mackerel

Description: Jack mackerel have elongate bodies and a forked caudal fin. They are dark blue-green above and silver to grey below, with a prominent black blotch on the rear of the operculum (the hard bony flap covering the gills). Jack mackerel have two dorsal fins. The first dorsal fin is ‘shot-based’ with eight spines in common jack mackerel (T. declivis) and nine spines in Peruvian jack mackerel (T. murphyi). The second dorsal fin is ‘long-based’ with the terminal ray being enlarged and slightly separated from the rest of the fin.

Size (length and weight): Up to about 65 cm in length and 1.6 kg. Commonly found at 25-40 cm in length and 0.2‑0.6 kg. Peruvian jack mackerel are larger than common jack mackerel.

Life span: Up to 17 years for common jack mackerel and 30 years for Peruvian jack mackerel.

Habitat: Jack mackerel are a pelagic schooling species found around the southeast and southern coasts of Australia over the continental shelf and outer shelf margin. T. murphyi is more common in oceanic waters off the edge of the continental shelf. They are commonly found at depths of 20-300 metres. Jack mackerel school by size. Juveniles tend to be found in shallower waters than adults. Feeding is thought to occur both during the day (T. declivis) and at night (T. murphyi).

Prey: Krill and other planktonic crustaceans, light fish and lantern fish.

Predators: Large fish such as tuna, barracouta and gemfish.

Reproduction: Jack mackerel reach reproductive maturity at 3-4 years of age. Spawning occurs during late spring to early summer. Spawning begins off the southeast coast of Australia and moves progressively southwards over the summer. Jack mackerel are serial spawners. Females produce about 34 000 eggs per spawning event.

The Commonwealth catch of jack mackerel is managed by quota. This means the catch of this fish by commercial fishers is restricted by weight. AFMA also restricts the type of gear that can be used to fish for Jack mackerel.

Commercial fishermen are required to fill in records of their catches, during each fishing trip and when they land their catch in a port. This helps us keep records of how much is being caught.

AFMA decide on the amount that can be caught each year from expert advice and recommendations from fisheries managers, industry members, scientist and researchers.

Jack mackerel are found in coastal waters of southern Australia from Wide Bay, Queensland to Shark Bay, Western Australia. They are generally located between 20-300m of water and form schools over the continental shelf and outer shelf margin.

Fishing for jack mackerel in the Small Pelagic Fishery has historically focused on the area of waters off South East NSW, Eastern Tasmania and South Australia.

Fishers use trawl nets and purse seine nets to catch jack mackerel.

Midwater trawl gear has minimal impact on the environment primarily because it does not come into contact with the seabed.

Sometimes, midwater trawling can catch unwanted species of fish (not the type of fish the net was supposed to catch). This is known as bycatch and it is monitored by on-board fishery observers who assess the environmental impact of the trawling.

AFMA’s management of commercial trawl fisheries aims to ensure trawl fishing has the least impact possible on the environment.

The purse seine method of fishing is very selective as it usually targets only one species at a time. This means that there is very little impact from purse seine fishing on other marine species. Purse seine nets are set near the ocean surface and do not touch the seafloor, so their impact on the marine environment is also very small.

AFMA carries out ecological risk assessments for all of its major fisheries which assess the impact of fishing on bycatch species and habitats. AFMA reduces any impacts through a number of management arrangements and strategies detailed in its ecological risk management strategy. These assessments indicate that fishing for jack mackerel is highly selective and has low rates of bycatch. However there are measures in place to minimise impacts on threatened endangered and protected species including compulsory seal excluder devices for midwater trawl nets and vessel management plans that include measures specific to the operations of individual boats to minimise interactions with seabirds and threatened, endangered and protected species, such as seals and dolphins.

Gear

• Midwater trawl
• Purse seine