Only the freshwater salmoniform fishes can be studied in any detail by direct observation. Most of what is known about the deep-sea species is based on preserved specimens, and, for most species, behaviour and locomotion can only be surmised from an examination of the morphology and anatomy.

The generalized body form of trout and salmon is characteristic of active, swift-moving fishes. A trim fusiform body, powerful caudal (tail) muscles, and a well-developed tail combine to propel the fish against strong currents with a minimum of resistance. These features also give the trout or salmon the ability to leap barrier falls as high as 3 metres (10 feet) or more.

Predatory fishes that dart out to grasp their prey are exemplified by the pike, in which the dorsal fin is situated posteriorly on the body to act more as a rudder than a keel. The pikelike body form has been evolved independently many times among predatory fishes such as the barracuda (Sphyraena sphyraena, of the order Perciformes). Among the deep-sea protacanthopterygians, however, certain predatory species are sedentary and have only weak swimming ability. Such fish remain immobile until unsuspecting prey ventures close enough to be grasped. Some deep-sea fish dangle a luminous lure to attract their prey.

The behaviour of a fish toward other members of its species can be highly variable. Often, predator species are territorial and aggressive, whereas plankton-feeding species typically form schools and do not function normally unless they are close to other members of their species. Although behaviour patterns are largely innate and species-specific, striking differences occur between closely related species. On hatching, pink salmon fry seek each other and form schools prior to seaward migration. The young of the coho, or silver salmon (Oncorhynchus kisutch), however, establish territories and aggressively attack other young cohos that invade their territory. This difference in aggressive behaviour is associated with the longer period of freshwater life and limited food supply experienced by the coho salmon.

One fascinating aspect of the behaviour of trout and salmon is their homing instinct—that is, the ability to return to the stream of their birth after migrating thousands of kilometres in the ocean for one to three years. Homing to the site of birth for reproduction is apparently a universal trait among the Salmonidae. Trout, char, and whitefishes in lakes segregate into discrete populations during the spawning season, each at a specific site.

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It is now generally accepted that the sense of smell plays the major role in guiding an anadromous trout or salmon to its precise natal stream once it enters a river drainage from the ocean. How it finds the mouth of the river system leading to the natal stream from the open ocean is not yet understood; celestial navigation and detection of fields of gravity by some unknown means have been hypothesized. Several senses besides smell may be used to locate the natal stream. Cutthroat trout (O. clarkii) in Yellowstone Lake, Wyoming, have been found to be able to return to their spawning stream after experimental blocking of the senses of smell and sight.

Homing behaviour has allowed the development of discrete populations among anadromous species of salmon and trout. Different life-history characteristics can be maintained because different populations segregate for spawning, and individuals of a population spawn only with each other, perpetuating hereditary traits. In major river systems such as the Columbia and Fraser in North America, one species may include several distinct races, each having different life cycles; such a situation greatly complicates the management of a species.

Ecology

As with other aspects of the biology of protacanthopterygian fishes, the ecology of species of the family Salmonidae is best known. All species of salmonid fishes evolved in clear, cold water, and they thus require pure, well-oxygenated, cold water; for this reason salmonid fishes are the first species to suffer when water quality is degraded. The esociforms, although not quite so sensitive to water quality as the salmonid fishes, are also susceptible to the inimical effects of human-induced environmental degradation.

Most salmoniform fishes are predators, feeding on other fish and large invertebrates. The process of evolution, however, works to modify and adapt species for certain ecological specializations in order to exploit a variety of food resources. In the lakes of the Northern Hemisphere, several whitefish species (Coregonus) are comparable, ecologically, to the herrings in the ocean. Such whitefishes, which are often called freshwater herrings, cruise the open water of lakes, filtering out minute organisms by straining the water through a fine mesh of gill rakers—minute bony elements attached to the gill arches. The sheefish, or inconnu (Stenodus leucichthys)—a large predatory whitefish of the Arctic—demonstrates that evolution for ecological adaptation is occasionally reversible. Adult sheefish feed on other fish and have evolved a pikelike body shape and large, powerful jaws; the development of teeth take precedence over that of the gill rakers. Consequently, the sheefish is quite unlike the typical whitefish from which it has evolved.

There probably has been strong selection for freshwater protacanthopterygians. All have species that migrate to the ocean for feeding. This presents a problem of osmotic regulation in waters of different salinities. The physiology of most fishes is fixed for life in fresh water or in the sea, but most of the freshwater salmoniforms are able to live in the sea because they can excrete excess salts through cells in the gills. They also possess well-developed kidneys, which, in the freshwater environment, handle the excess of water that diffuses into their blood via the gills.

Little is known of the ecology of the wholly marine protacanthopterygians. They may be ecologically grouped by the depths that they inhabit and by their feeding preference. Those found in the twilight zone of the ocean (200–1,000 metres [650–3,300 feet]) consist of plankton feeders and predators. The plankton feeders typically are more active and have a more fully developed and functional swim bladder than is typical of the predatory forms.

Because virtually all primary food production in the oceans takes place in the upper, sunlit layer, the deep-sea fishes live in a food-poor environment. At first, it may seem contradictory that they are able to maintain such numerical abundance; certain features of the biology of the deep-sea protacanthopterygians, however, allow them to attain great numbers. The body of the typical oceanic protacanthopterygian is feebly developed, appearing to consist of little more than gelatinous material. The skeleton and muscles are reduced, so little energy is needed to maintain the body. Many of the deep-sea species make nightly migrations to the food-rich surface zone for feeding. The species inhabiting the deepest parts of the ocean must depend on a food supply that filters down from above. This food is concentrated in the ocean’s thin bottom layer (the benthic zone), with the result that the benthic fish species may attain a relatively high abundance.