Pelagic fish have streamlined bodies and are therefore usually fast swimmers. They are usually coloured blue, green, or mottled above and silvery below – a colour scheme which camouflages them well in open waters. The sunfish usually drift along in the ocean currents and they may be considered as the larger members of the plankton (wanderers) rather than the nekton (active swimmers).

There are two main groups of pelagic fish – oceanic and inshore.

Oceanic fish undergo long migrations and consequently have wide distributions. Marlins and tunas are typical examples, usually found beyond the continental shelf in clear blue oceanic water. The marlins and tunas migrate southwards to New Zealand during the summer and autumn. At least four Indo-Pacific species of tuna visit our shores. Other warm-water oceanic species which occur seasonally are the southern mackerel, dolphin (not to be confused with the marine mammal of the same name), flying fish, and large sharks, such as the thresher, mako, and tiger sharks. The barracouta is an example of an oceanic species characteristic of cooler waters, occurring also in southern Australia, South Africa, and South America. It is distinct from the tropical aggressive barracuda, which is not found round New Zealand.

Inshore fish also migrate in response to seasonal water-temperature changes or to spawning stimuli, but they stay close to land and do not travel the vast distances of the oceanic species. Examples are kahawai, trevally, horse mackerel, and kingfish (yellowtail).

The thermocline generally acts as a barrier to the depth these pelagic fish can reach.

Marine fish are divided into two main groups, those with a cartilaginous skeleton (e.g., sharks and rays) and those with a bony skeleton (e.g., snapper). The cartilaginous species are a relatively small group in New Zealand, making up only about 11 per cent of the total of about 400 species, the bony fish contributing the other 89 per cent. Less than half of the species of cartilaginous fishes are common. The ghost shark and elephant fish (Chimaeroids), although not true sharks, are included in this group. The bony fishes are more diverse and total about 350 species, which are grouped into some 100 families. The majority of the families are, however, represented by very few species, and only a few contain more than six species. Examples of these larger families are the Carangidae (trevally and horse mackerel family), Pleuronectidae (flounder family), Cheilodactylidae (tarakihi family), Gadidae (red cod family) and Labridae (wrasse or parrot fish family). Almost a quarter of the species are native to New Zealand, while a slightly larger proportion are Australasian. The remainder are found mainly in the south-temperate zone of the Indo-Pacific area and some are even more cosmopolitan, occurring in all but the coldest seas.

Fish are cold-blooded animals and take on the temperature of the water mass surrounding them, and most species can survive only within narrow limits of temperature fluctuations. Water temperature and currents, therefore, play an important part in their distribution.

New Zealand is surrounded by a complex system of warm and cool currents. The trade wind drift and the Tasman current are responsible for the presence of the warm currents round our coasts, while the west wind drift brings cool sub-Antarctic water to the South Island. These different water masses meet to form “convergences”, which are areas of variable temperature and salt content, and can act as barriers to the movements of fish. The positions of the convergences move with the seasons, but the summer positions of the major ones are to the west of Stewart Island and to the east of Banks Peninsula.

The warm and cool water masses support their typical fish species. Characteristic of the warm coastal water are such species as snapper, kingfish, trevally, and kahawai. Besides these, several oceanic species which normally inhabit the Indian and Pacific Oceans come to our shores in the summer and autumn. Such fish as the tunas (at least four species), marlins (three species), and large sharks of the big-game type are examples of these seasonal visitors. Typical fish of the cooler sub-Antarctic water are ling, hake, blue cod, red cod, southern rock cod, trumpeter, and elephant fish. More species of the cooler waters can penetrate into the warmer regions than vice versa; the mixed temperatures in the convergences appear to act as a more effective barrier to the warm-water species than to the cool-water group. A third group, comprising about half the total number of fish species in New Zealand, can withstand a much wider range of temperature, and these are therefore much more widespread in their distribution. Tarakihi (with their centre of abundance near Cook Strait), groper (hapuku) and bass, moki, barracouta, and wrasses (parrot fish) are examples of this third group.

So far, only the various water masses have been discussed in reviewing the distribution of fish in New Zealand. The depth of the water and its effect on fish movements should also be considered. Associated with increasing depth are higher pressures, decreased light intensity, and a narrow band of water, between 100 and 500ft below the surface (varying seasonally), where there is a rapid drop in temperature (known as a thermocline). Each of these three physical properties may restrict the movements of fish. Water pressures increase rapidly with depth and, although fish are adapted to withstand high pressures, rapid changes in depth will usually kill them. Sunlight is essential for the growth of green plants, and the diatoms, which are the initial source of food in the complex oceanic food chain, are therefore found only in the upper layers of the sea. Small marine animals, including small fish and fish larvae, which feed on these diatoms, are also found only in the sunlit layer, and these in turn affect the distribution of larger fish feeding on them. The thin deep layer of rapid temperature changes acts as a distinct barrier to the downward movements of many surface fish.

Fish living in the various depths of the sea are classified as pelagic (surface), demersal (sea bottom), and deep-sea fish (abyssal or bottom dwellers, and bathypelagic or deep-swimming fish). Each group has its own features of shape, colour, and movements.

(Carassius carassius). Carp appear to have been introduced about 1867 and are present in many waters throughout New Zealand, but principally in the Rotorua and Waikato hydro lakes and Lake Taupo.

Other, but less successful introductions, have included tench (Tinca tinca), still present in sluggish rivers near Oamaru; and bullheads (Amerius catus) in Lake Mahinapua and the lower Waikato River and adjacent lakes; sockeye salmon (Onchorhynchus nerka), which are present as a land-locked stock in Lake Ohau; and the mosquito fish (Gambusia affinis) in Lake Ngatu, North Auckland.

Unsuccessful attempts to acclimatise many other species, such as whitefish, lake herring, gudgeon, dace, etc., have been made.

by Brian Turnbull Cunningham, B.SC., Senior Fishery Officer, Marine Department, Wellington.

• The Fishes of New Zealand, Vol. 1, Phillipps, W. J. (1940)
• Fresh Water Fishes of New Zealand, Stokell, G. (1955).

(Cristivomer namaycush). Mackinaw, or the Great Lakes Char, were introduced in 1906 and hatched at Christchurch. They are present in Lake Pearson, Canterbury, but other liberations appear to have been unsuccessful.

(Salvelinus fontinalis). In 1877 shipments of this char were received at Christchurch and Auckland, but earlier consignments could have been made to Christchurch. A native of the east coast of North America, these fish are also called brook trout. Fontinalis were liberated in many areas, but today only isolated populations are present, mainly in the Hinds River, Lake Emily, and the Hinemaiaia Dam, near Lake Taupo, where they were liberated recently.

(Perca fluvitalis). Perch were introduced in 1868 and have been released in many waters throughout New Zealand. They prefer still or sluggish water and are common in the Otago, Wellington, and Taranaki Districts.

(Salmo salar). Salmon eggs were first imported in 1868, and until about 1911 many shipments of eggs from Great Britain, the Rhine, and North America were received. Although liberations were made in many river systems, they became established only in the Te Anau – Manapouri – Waiau River watershed.

(Onchorhynchus tschawytcha). Between 1875 and 1880 unsuccessful attempts were made to establish quinnat, native to the Pacific coast of Canada and the northern United States, but large-scale releases into the Waitaki River system ex the Hakataramea hatchery from 1901 to 1942 were successful. The first adults returned from the sea in 1905. From the Waitaki, quinnat spread north as far as the Waiau by about 1920. It appears the Clutha River and Lake Okarito stocks resulted from single liberations made in 1917 and 1935 respectively.

(Salmo gairdnerii). It appears this fish was introduced from California, where it is indigenous, by the Auckland Acclimatisation Society in 1877. The society received further consignments of eggs in 1883. The eggs were taken from steelheads (sea-run fish) from the Russian River. The fish were liberated in the Auckland district initially, later in the Rotorua lakes, and in 1903 in Lake Taupo. Subsequently rainbow eggs from the McLeod and Shasta Rivers were hatched at Auckland and Christchurch. The progeny of the rainbow and steelheads were released throughout New Zealand.

(Salmo trutta). Browns from Great Britain were established in Tasmania in 1864. A few browns were hatched at Christchurch from a consignment of Tasmanian eggs received in 1867, but it is doubtful whether any were released. The first successful liberation is regarded as that made in the Water of Leith, Dunedin, in 1869, when trout hatched from Tasmanian eggs, received in 1867, were released. Further shipments of browns from Tasmania during the 1870s, and other varieties, such as Loch Leven and sea trout from Great Britain, were hatched and released throughout New Zealand.