Jack Mackerel – Trachurus novaezelandiae
Also known as horse mackerel and yellowtail
The average size of fully grown adult jack mackerel is between 30 and 50cm. Jack mackerel are relatively slow growing reaching maturity at 6 to 7 years of age. Big specimens can be as much as 30 years old.
Distribution is from Australasia to Japan. This is an important commercial species not so much because of its high value but because of its large size. The commercial catch for Trachurus novaezelandiae, together with the similar Trachurus declivis, and T. murphyi, for 2013 is 24, 281 tonnes. The commercial catch is taken mostly by licensed foreign trawlers.
Jack mackerel is easily identified by its peculiar armour in the form of overlapping bony plates or scutes which cover its lateral line running down each side. These plates are hard to the touch and proud of the flanks. The lateral line has a distinct bend midway along the side of the body which makes this species easily distinguished from the similar looking koheru.
They are particularly broad near the tail where they form a spiky keel. The tail can be quite yellow in colour on its trailing edge.
The lower jaw is slightly longer than the upper. The colour is a darkish green with a faint blue tinge. Below the bony side plates the colour is paler fading to creamy white underneath. Overall the jack mackerel is streamlined in appearance and built for fast swimming.
Jack mackerel are found in vast schools throughout New Zealand in particular off the north-eastern coast. They are a pelagic species found mostly in mid-water or near the bottom in shore out to at least 300m. Smaller fish tend to be golden brown coloured rather than green and are mostly found closer to land. Jack mackerel are common from Tasman Bay to Cape Egmont. They are also common in the Bay of Plenty.
The flesh is quite dark in colour but lightens when cooked. It has a high fat content. It is almost always available in New Zealand surpermarkets sold either whole or hot smoked fillets. It is generally it is regarded as a poorer quality fish at the lower end of the price scale.
Jack mackerel makes excellent surfcasting bait being firm and oily and stays on the hook well. This species deteriorates quickly after capture and must be iced immediately. For fish intended for the table, the best practice is to head and gut jack mackerel straight away. This fish reportedly dies very quickly when lifted from the sea.
I have regularly caught juvenile jack mackerel in the Marlborough Sounds. Though not fully grown they are capable of strong darting runs when taken on light line.
Jack mackerel caught in deeper water were found to be feeding on squid, anchovy, octopus, sprats, pilchards, flatfish, swimming crabs, krill (whalefeed) and shrimps. Jack mackerel are often found with throat parasites.
Though unlikely to be caught by surfcasters, at least not in the South Island, boat anglers report that jack mackerel will take almost any bait or lure put in front of them. Jack mackerel sometimes enter bays and harbours as far south as Otago. In deep water, they live near the bottom at night and move up near the surface during the day.
The species spawns in summer over the central continental shelf. Females lay as many as 400,000 eggs.