The rainbow trout, Oncorhynchus mykiss, is a cold-water fish species renowned for its adaptability and varied feeding habits. These fish are categorized as opportunistic carnivores, meaning their diet is determined by the available food sources in their immediate environment rather than specific preference. Their life history includes both resident individuals that spend their entire lives in freshwater and anadromous forms, known as steelhead, which migrate to the ocean before returning to spawn. This ecological flexibility across diverse habitats, from high-mountain streams to large lakes, necessitates a highly generalized diet.
Primary Food Sources in the Wild
The foundational diet of wild rainbow trout consists primarily of invertebrate life, both aquatic and terrestrial, which provide a reliable source of protein and fat. Aquatic invertebrates constitute the largest portion of their intake, including the larval, pupal, and nymph stages of insects like caddisflies, stoneflies, and mayflies. Trout actively forage along the substrate, consuming these insects before they emerge into their adult flying forms.
A significant part of the wild trout’s diet comes from the surface drift, composed of terrestrial insects that fall into the water. This surface feeding includes ants, beetles, grasshoppers, and moths, with feeding activity often increasing during rainstorms when bank-side insects are washed into the current. The diet is further supplemented by bottom-dwelling organisms such as crustaceans like scuds and crayfish, as well as small mollusks and aquatic worms.
As rainbow trout grow, their predatory focus expands to include smaller vertebrates and eggs. They will consume the eggs of other fish species, or even their own, if encountered during spawning periods. Larger adult trout become increasingly piscivorous, actively preying on smaller forage fish such as minnows, sculpins, and the fry of other trout species.
How Diet Changes with Age and Size
The feeding strategy of a rainbow trout undergoes a distinct transformation as the fish matures from a newly hatched sac-fry to a large adult. Newly emerged fry, after absorbing their yolk sac, begin feeding almost exclusively on microscopic organisms found in the shallow, slower-moving water near the streambanks. These initial food items are primarily zooplankton and small waterfleas, which are the only prey small enough for their tiny mouths.
As the juvenile trout grows from a fry into a fingerling, its diet shifts to small aquatic invertebrates like midges and blackfly larvae. This stage is characterized by frequent feeding on small, easily digestible items to fuel rapid growth. The energy requirement of a trout increases exponentially with its size, driving a progression toward larger, more energetically rewarding prey items.
By the time the trout reaches adult size, it can afford to expend more energy to capture substantial meals, moving away from a diet dominated by insects. Adults begin to focus heavily on forage fish, which provide a much greater caloric return for the effort. Very large, opportunistic individuals have been known to consume small mammals, such as mice or voles, that fall into the water.
Influence of Aquatic Habitat on Feeding
The specific environment a rainbow trout inhabits fundamentally determines the composition of its diet, leading to noticeable differences between river and lake populations. In flowing water systems like rivers and streams, the fish rely heavily on the current to deliver food directly to them. Trout typically position themselves in strategic lies, such as behind large rocks or log jams, where they can intercept the continuous downstream drift of aquatic insects and terrestrial prey without expending excessive energy.
The stream diet is highly seasonal, dictated by the synchronized emergence of various insect species, known as “hatches.” During a major caddisfly or mayfly hatch, trout will become highly selective, focusing almost entirely on the specific stage of that insect, whether it is an emerging nymph or a floating adult. Surface feeding on flying insects is a defining behavior in streams, especially during the spring and summer months.
In contrast, the diet of a rainbow trout living in a large lake or reservoir is structured around a pelagic food web. These trout spend more time cruising the water column, hunting for schools of small forage fish like kokanee salmon or smelt, which are often the most significant source of calories in large, deep water bodies. Benthic organisms also play a role, with trout foraging on deep-water scuds and snails when they move closer to the lake bottom. The lake environment often allows trout to achieve larger sizes due to the abundance of high-energy forage and the reduced metabolic cost of not constantly fighting a current.
Feeding Practices in Hatcheries and Stocked Waters
Rainbow trout raised in commercial hatcheries or stocked into public waters are fed a diet that is radically different from their wild counterparts. The commercial feed is a manufactured pellet, scientifically formulated to provide the precise nutritional balance required for optimal growth and health. These pellets are high-protein, often containing 40 to 50 percent protein derived from ingredients like fish meal, animal byproducts, and plant proteins such as soy.
The pellets also contain fats and oils to ensure the fish receive adequate energy and essential fatty acids. The composition is carefully adjusted based on the trout’s size, with fry receiving a feed with a higher protein content to support their rapid early growth. An interesting component sometimes added to commercial feed is astaxanthin, a natural carotenoid pigment.
This pigment is incorporated to give the trout’s flesh the pink or reddish color that consumers often associate with salmon. Wild rainbow trout that consume crustaceans like scuds naturally acquire this color from the pigments in their prey. Hatchery-raised fish require the dietary supplement since their pellet diet would otherwise result in a paler, white flesh.