Open ocean
The open sea, far from land, near the surface. 42 fish on EveryFish so far.
The open ocean, far from land, is the largest habitat for fish near the surface. Its sunlit upper layer, down to about 200 metres, is home to fast, wide-ranging fish: tuna, swordfish, marlin, sailfish, mahi-mahi, flying fish and oceanic sharks. Food is patchy, so many of these fish swim great distances, following currents, fronts and schools of smaller prey across whole ocean basins.
Built for distance
Open-ocean fish are streamlined and countershaded, dark above and silver below. Tunas and some sharks keep their muscles warm and never stop swimming. Swordfish and marlin warm their eyes and brain to hunt in cold, deep water. Sailfish are often called the fastest fish, though measured speeds are lower than the popular claims. The ocean sunfish drifts and dives, eating jellyfish. Flying fish glide above the waves to escape predators.
Fishing the high seas
Tuna, swordfish and sharks of the open ocean are caught by longlines and purse seines, often far from any coast, and are managed by regional tuna commissions. Oceanic sharks, such as the oceanic whitetip, have declined steeply. A United Nations treaty on the high seas, adopted in 2023, came into force in January 2026 and allows marine protected areas to be set up in international waters, which cover nearly two thirds of the ocean.
Still to be written: Atlantic bluefin tuna, Skipjack tuna, Swordfish, Indo-Pacific sailfish, Mahi-mahi, Ocean sunfish, Oceanic whitetip shark, Blue shark.
Further reading
Photo credits on this page
The small pictures above come from Wikimedia Commons under free licences. Some are cropped to fit.
- Ribbonfish: Personnel of NOAA Ship DELAWARE II, Public domain, source