The question of whether salmon are omnivores depends heavily on the context of their life—whether they are wild or farmed. Salmon are anadromous fish, meaning they are born in fresh water, migrate to the ocean to mature, and return to fresh water to spawn. Their diet changes dramatically throughout this complex life cycle, making a fixed classification challenging. The confusion stems from the vast differences between their natural diet and the manufactured feed given to them in aquaculture.
Understanding the Classification of Omnivores
Biologically, animals are classified as herbivores, carnivores, or omnivores based on their primary food source. An omnivore is defined as an animal whose diet consists of both plant and animal matter, and whose digestive system is adapted to process both for energy and nutrients. A carnivore primarily consumes other animal tissue and has a digestive tract specialized for high-protein diets.
The traditional classification for salmon is as a carnivore, or more specifically, a piscivore (fish-eater). This definition is subject to debate because classification should also consider an animal’s ability to derive sustenance from diverse sources. Salmon are opportunistic predators, demonstrating a flexibility in their natural diet that suggests a move toward the omnivore label. The key scientific distinction lies in the physiological ability to digest and metabolize components from both plant and animal sources.
The Diet of Wild Salmon Across Life Stages
The diet of wild salmon is highly dynamic, shifting as the fish moves between freshwater and marine environments. In their early freshwater stages, known as fry, young salmon feed on small aquatic invertebrates, zooplankton, and insect larvae. As they grow into the parr stage, they become more predatory, consuming aquatic insects, microcrustaceans, and small bait fishes.
Once salmon migrate to the ocean as smolts, their feeding habits shift to a predominantly carnivorous marine diet. Adult salmon prey on smaller fish, such as herring and sand eels, along with crustaceans like krill and shrimp. They spend between one and five years at sea, depending on the species, growing to maturity on this high-fat, high-protein diet.
This oceanic diet introduces an indirect, yet significant, intake of plant-derived nutrients. The prey items consumed by salmon, such as zooplankton and small forage fish, have fed on algae and phytoplankton, transferring plant-based compounds up the food chain. This highlights that while wild salmon are predators, their nutritional intake is not exclusively animal-based.
How Aquaculture Changes Salmon Feeding Habits
The feeding habits of farmed salmon are dictated by human intervention, fundamentally altering their dietary classification. The commercial pellets fed to salmon in aquaculture are designed for rapid growth and sustainability, resulting in an explicitly omnivorous diet. These manufactured feeds represent the main reason consumers question the salmon’s traditional carnivorous status.
Modern salmon feed is composed of a complex mix of ingredients, often containing a significant amount of plant-based materials. Vegetable protein sources, such as soy, corn, wheat, and rapeseed, can make up 40 to 70 percent of the total ingredients in some formulations. These plant materials are used as an affordable substitute for traditional marine ingredients like fishmeal and fish oil.
While fishmeal and fish oil are still included to provide omega-3 fatty acids and proteins, the bulk of the protein and energy comes from these vegetable sources. This shift is driven by the need to reduce reliance on limited wild fish stocks and improve the economic viability of farming. By thriving on a largely plant-derived diet, farmed salmon effectively function as true omnivores within their controlled environment.