FISH, MARINE

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.

Pelagic

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.

Demersal

Demersal fish are dependent on the sea bottom for food and shelter and it is upon this group of fishes that the commercial fisheries are based, although interest is also shown in the exploitation of some pelagic species.

Species living on the continental shelf (depths to 600 ft or rather more) are extremely varied in shape and colour, but typically they are less streamlined and slower swimmers than the pelagic group and are usually red, brown, or grey – colours which act as good camouflage in their various environments. They do not migrate to any great extent. Their food consists of such items as shellfish, crabs, worms, and seaweed.

Sea-bottom topography also has an influence on the general distribution of fish. Rocky sea bottom attracts the greatest variety of species, but some are common over sandy or muddy bottom.

Most species living in rocky areas are weak swimmers, some being capable of only short, darting movements; many are solitary, especially the larger members of a species, while some swim in small schools just above the sea floor. Some are brilliantly coloured (e.g., the wrasses or false parrot fishes); some are eel-like (hagfish or blind eel, conger eel, moray eel); some have large heads and jaws (groper, John Dory); several species are spiny (gurnard, scorpionfish); while some do not resemble fish at all (seahorse, pipefish). The small fish of intertidal pools are adapted to surviving under continuous wave action, either by clinging to rocks by means of a ventral sucker (clingfish) or by sheltering under rocks or stones (blennies).

Fish characteristic of a sandy or muddy bottom tend to be flattened from above and are typified by flounders, rays, and stargazers. Their colours are usually grey or brown. Flounders are further protected from enemies by being capable of colour changes to match the sea floor. They also partially bury themselves in sand, a habit that is carried further by stargazers, sand eels, and lancelets.

Some coastal species, although classed as demersal, spend considerable periods in the middle depths. They resemble pelagic fish in their swimming ability and in their general colouring and shape, but they are mainly bottom feeders and they form small schools. A number of our commercial species are included in this class, such as snapper, tarakihi, mullet, moki, and trumpeter.

Deep Sea

Deep-sea representatives are not well known in New Zealand waters, but as the environment is uniform everywhere, they are likely to be very similar to those in other parts of the world. They live in waters of cold temperatures with no seasonal fluctuations, an absence of water currents, and no solar light.

Abyssal or bottom dwellers are not streamlined in shape; are usually dark in colour; and frequently have poorly developed eyes. They are all carnivorous, many having unusual methods of catching their prey, including the development of light-producing organs.

Bathypelagic or deep-swimming fish live beyond the continental shelf in the mid water below the sunlit zone. They are usually grey or silvery in colour and have large eyes, and often possess light organs. They characteristically undergo extensive vertical migrations, living in deep water by day and rising some hundreds of feet by night, being unaffected by changes in water pressure. Myctophids (lantern fishes) and some deep-water rat tails are typical of this region.

Life Histories

The sexes are separate and generally occur in about equal numbers.

Most sharks and rays bear their young alive in an advanced stage of development after an embryonic period of six to 12 months. The skates and elephant fish, however, lay relatively few large eggs in horny capsules, depositing them on the sea floor.

The vast majority of bony fish shed several thousand very small transparent eggs, which float in the upper layers of the sea. Some species produce fewer eggs, which are attached to seaweed or rocks, etc. Spawning usually occurs in spring or summer, sometimes during the autumn. Hatching takes place within a few days, the incubation period depending on the temperature. The tiny larvae in their first few days live on their store of yolk, but when this supply is depleted they begin feeding on small planktonic organisms. At this early stage they are entirely planktonic and may be widely dispersed by means of currents.

by Lawrence James Paul, B.SC., Fisheries Division, Marine Department, Wellington.

• New Zealand Sea Anglers' Guide, Doogue, R. B., Moreland, J. M., Heath, E. W. (1961)
• A Treasury of New Zealand Fishes, Graham, D. H. (1956)
• Sea Angler's Fishes of New Zealand, Parrott, A. W. (1957)
• The Queer and Rare Fishes of New Zealand, Parrott, A. W. (1960).