Jack Mackerel

Interesting Info

• The Jack Mackerel is a schooling fish also known as the Greenback Horse Mackerel in Australia.
• It grows to around 60 cm (24 in) long and up to 1.6 kg (3.5 lb), though most caught are closer to 30–45 cm.
• In Australia, it ranges from southern Queensland down through New South Wales, Victoria, Tasmania, South Australia, and southern Western Australia.
• Globally, this species is mainly confined to southern Australian and New Zealand waters, making it a temperate regional species.
• It forms huge schools in coastal waters, sometimes stretching hundreds of metres and containing thousands of fish.
• Juveniles live in bays, harbours, and estuaries, while adults move offshore into deeper coastal waters.
• Jack Mackerel are fast swimmers with streamlined, silver-green bodies and a dark back for camouflage.
• Their diet includes plankton, krill, small fish, and squid, which they pick off in mid-water.
• They’re an important prey species for dolphins, seals, tuna, and seabirds.
• Spawning takes place offshore in spring and summer, with eggs and larvae drifting in the plankton before settling inshore as juveniles.
• They grow quickly, maturing at around 2–3 years old, and live up to 17 years—longer than many people expect for a schooling fish!
• Divers often see them as shimmering “bait balls” that pulse and twist in unison when predators approach.
• They’re called “horse mackerel” because of a hard scute (bony ridge) along the side line that feels rough like a horse’s mane.
• They are targeted in commercial fisheries in both Australia and New Zealand and have been the focus of sustainability debates.
• Jack Mackerel are known for diel migrations—coming closer to the surface at night and moving deeper by day.
• Schools can sometimes be so dense they show up as dark clouds on sonar.