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
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.