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.