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.