Oceanic fish are those species that are found some distance from coastlines or islands. This is commonly defined as the area beyond the continental shelf (around 200 metres deep) where the continental slope begins to fall away.
Many of these fish are seen only by deep-sea fishermen, or occasionally on shore if a specimen is washed up.
Around the world, sailors refer to ‘blue-water sailing’. The water in the open ocean is dark blue because it is very deep, and there is little land-derived sediment (which causes discoloration) suspended in the water column. Blue waters are the domain of open-ocean fish – which may nonetheless also swim near shore.
The pelagic zone is the water column – all of the sea apart from that near the coast or the sea floor. All fish that live in the water column are known as pelagic species. They do not feed from the sea floor, relying on the open water for their feeding grounds.
The pelagic zone can be subdivided. The epipelagic zone is the depth to which sunlight penetrates, typically down to 200 metres. Most oceanic fish live in this upper layer. The middle layer, the mesopelagic, is between 200 and 1,000 metres deep.
Species that live in the upper water column hundreds of kilometres offshore are known as oceanic pelagics. (Those living closer in are coastal pelagics.) Migrating to specific destinations, or swept by the currents, they are the most widely distributed fish in the world.
Many oceanic pelagic fish seasonally swim into New Zealand’s waters. Living out in the open is hazardous, and some species travel in schools for protection. Others are loners, using their speed and agility to avoid predators.
Many pelagic fish live in the upper 200 metres of the ocean, usually near the surface. Some eat plankton, while others survive as mid-water predators and scavengers (at depths below 200 metres). Many large fish of the open ocean are predatory wanderers that reach New Zealand waters in summer from subtropical or tropical waters. They include species of tuna (skipjack, albacore, and yellowfin) and billfish (marlin, swordfish).
Many of these species feed along ocean-current boundaries, at convergences (where cold and warm water masses meet), over the edge of the continental shelf, or sometimes closer inshore.
Sunfish, moonfish and saury are found in most of the world’s oceans, or at least in temperate waters. Some tropical bigscale pomfret, whale sharks and manta rays and other oddities occasionally show up in New Zealand’s northern waters.
Tuna are large fish (0.5–1.5 metres) which, because of their wandering habits, are said to have no native country. They form large schools and are generally more common in northern New Zealand, arriving from the north in summer.
The eyeballs are flush with the head, and the body is teardrop shaped. This bullet of muscle is driven forward by a stiff crescent-shaped tail. They need this speed to catch their prey – mackerel, herring, mullet, krill and squid.
Tuna are sought-after as big game and commercial fish. Top-quality specimens can fetch high prices at Tsukiji, the huge fish market in Tokyo.
The main species found in New Zealand waters are the albacore (Thunnus alalunga), southern bluefin (Thunnus maccoyii) and skipjack tuna (Katsuwonus pelamis). Other less common tuna found off New Zealand’s shores include the yellowfin (Thunnus albacares), bigeye (Thunnus obesus), slender (Allothunnus fallai), butterfly (Gasterochisma melampus) and frigate (Auxis thazard). The Australian bonito (Sarda australis) is rarely seen.
Tuna are superbly streamlined, and can retract their gills at high speed to reduce drag. The southern bluefin can reach speeds of 80 kilometres per hour. It does this in part by heating up its blood. As heated muscle is more efficient than cool muscle, the fish can achieve greater power.
Albacore found in New Zealand waters average 50–70 centimetres in length. An 80-centimetre fish weighs about 10 kilograms. Albacore spawn in the tropical Pacific, and are plentiful along the boundaries between currents.
Those that reach New Zealand waters are probably between two and five years old. Domestic vessels catch albacore by trolling (trailing baited lines) in coastal waters west of the country. Between 1991 and 2000, annual catches varied from 1,437 to 5,180 tonnes.
The southern bluefin lives only in the southern hemisphere, where it makes long migrations between tropical and temperate waters. In New Zealand waters a typical fish is about 1 metre long and weighs about 20 kilograms. The species is widely distributed between latitudes 10º and 50º south, and is the most southerly-ranging common tuna.
Internationally, stocks are thought to be severely depleted by overfishing. During the 1970s about 100,000 fish were taken annually by Japanese vessels. In the 2000s the species formed a small but still very valuable New Zealand fishery.
Skipjack tuna occur in all the world’s oceans, with their southern limit in New Zealand waters. They usually arrive in December, staying until May and ranging as far south as Cape Farewell. They feed opportunistically on fish and crustaceans.
Purse seiners (fishing boats with seine nets that draw up like a purse) hunted skipjack during the 1970s, taking between 5,000 and 10,000 tonnes annually. Catches in the 2000s were smaller (around 3,000–4,000 tonnes), with most fish landed in the Bay of Plenty and along the north-east coast of the North Island.
The term billfish is used for marlin and their relatives. They are large migratory fish, with a distinctive upper jaw that forms a pointed spear or bill. Along with the swordfish, five species are known to frequent New Zealand waters seasonally.
Marlin are likely to feed on a broad range of pelagic fish and squid, and small tuna in open waters. Nearer the coast, they also eat bottom-dwelling fish. Marlin are caught commercially, and are sought by anglers fishing off the northern North Island.
In the past, marlin were rumoured to make unprovoked attacks on boats. It is more probable that the fish were lashing out with their bills as fishermen hauled them into the boat.
The striped marlin (Tetrapturus audax) occurs in cooler waters and is the most abundant marlin around New Zealand. It grows up to 3.5 metres long and weighs up to 180 kilograms, but averages around 130 kilograms.
Blue marlin (Makaira mazara) taken from New Zealand waters average around 100–200 kilograms. The heaviest caught in New Zealand was 412 kilograms. Like other marlin, they arrive in the peak of summer and are known from the Bay of Plenty northwards, most notably off Northland’s east coast.
The large black marlin (Makaira indica) typically weighs between 100 and 200 kilograms and is around 2–3 metres in length. The heaviest caught in New Zealand was 444 kilograms. These fish are widespread in the tropical and subtropical waters of the Indian and Pacific oceans.
The broadbill swordfish (Xiphias gladius) is widely distributed and moves from tropical to temperate waters. Although it swims near the surface, it is thought to descend to deeper waters. The bill, which is longer and flatter than the marlin’s, is used for defence and to herd, stun and slice prey such as squid and small fish.
In 1875 a swordfish over 3 metres long was stranded on Shelly Beach in Auckland. It was examined and described by the Auckland Museum curator, Thomas Cheeseman, who wrote:
It has been said to attack the whale with its sword, but this is extremely improbable ... Instances not unfrequently occur of ships’ bottoms being perforated by the sword, but there is no good reason to think that an intentional attack is ever made. 1
The average length is 2–3 metres. Specimens weighing 100–300 kilograms are taken by anglers off the New Zealand coast.
Commercial fishing of the species is now prohibited, so any swordfish catch is taken by boats fishing for other species such as tuna.
Other billfish occasionally seen in New Zealand waters include the shortbill spearfish (Tetrapturus angustirostris) and the sailfish (Istiophorus platypterus).
These vibrantly coloured fish are found in oceanic waters of warm and temperate regions. They are usually only seen in northern New Zealand when cast up on beaches. Often caught unintentionally by offshore boats fishing for bigeye tuna, they are sold commercially.
The main species in New Zealand waters is Lampris guttatus, a round, flat fish which can grow to 1.5 metres in diameter. The related opah (Lampris immaculatus) is also occasionally caught.
Flying fish have enlarged pectoral fins that help them to escape from predators. The fish breaks through the water surface at high speed and its fins act like wings, enabling it to glide over the water for up to 50 metres. They are often sighted by yacht crews, and sometimes land on deck.
About 120–380 millimetres long, they feed on plankton and crustaceans just below the surface. At least five species occur in offshore surface waters around northern New Zealand.
The huge sunfish (Mola mola), which can reach 3 metres long, feeds with its tiny mouth on soft-bodied plankton (salps and jellyfish). This bizarre creature resembles a squashed rugby ball with twin rudders – these are its fins, which swish in a figure-of-eight movement, stabilised by the large tail. Although it does not look agile, the sunfish can leap above the water. If one swims near the surface, its fin is sometimes mistaken for that of a shark.
This seasonal visitor moves into northern New Zealand waters from November to late June. Stragglers reach as far south as Otago Peninsula. Although they are often seen alone, they may also congregate.
Several species living in the mid-water zone (100–500 metres deep) frequent New Zealand waters.
Saury (Scomberesox saurus) are common in the temperate seas off northern New Zealand. They are about 350 millimetres long and eat small crustaceans and fish.
Dolphinfish are tropical fish found worldwide in the open ocean. They do not look much like dolphins, as they have a large blunt head.
The species known in New Zealand is the mahi mahi (Coryphaena hippurus), occasionally seen off northern shores. They are about 1–2 metres long and feed on large fish.
The warehou family has about 20 species, common in New Zealand waters. They are also known as raft fish, as they are often found with drifting debris. They often have short, blunt heads, and they feed on jellyfish and plankton.
The blue warehou (Seriolella brama) is commonly found around Cook Strait and the South Island, but is rare further north. The silver warehou (Seriolella punctata) is found around most of New Zealand, but is a deeper-water species. They are typically 40–60 centimetres long.
Pomfret live in tropical to temperate oceans. New Zealand has at least six species, although they are rarely seen. They are Ray’s bream (Brama brama), southern bream (Brama australis), bigscale pomfret (Taractichthys longipinnis), spiny pomfret (Pterycombus petersii), Taractes asper and wingfish (Pteraclis velifera).
Pomfret are about 600 millimetres long, and mainly eat shrimps and small fish. Ray’s bream is occasionally taken in commercial quantities.
The red bonnetmouth is about 200 to 400 millimetres long, with a deeply forked tail and jaws that open up like a car bonnet. The two species found in New Zealand are the rubyfish (Plagiogeneion rubiginosum) and redbait (Emmelicthys nitidus). They feed on plankton.
The bluenose (Hyperoglyphe antarctica) is widely distributed around New Zealand, usually near rough seafloor. They feed on fish, crustaceans and small squid. They are an important commercial species, and catches increased rapidly from around 1980. They grow to 30–45 centimetres in length.
Acknowledgements to Malcolm Francis.
Doak, Wade. Fishes of the New Zealand region. Auckland: Hodder & Stoughton, 1984.
Doak, Wade. A photographic guide to sea fishes of New Zealand. Auckland: New Holland Publishers, 2003.
Francis, Malcolm. Coastal fishes of New Zealand: an identification guide. 3rd ed. Auckland: Reed, 2001.
Paul, Larry. New Zealand fishes: identification, natural history and fisheries. Auckland: Reed, 2000.
Paul, Larry, and John Moreland. Handbook of New Zealand marine fishes. Auckland: Reed, 1993.
Paulin, Chris, and others. New Zealand fish: a complete guide. Wellington: Te Papa Press, 2001.