Salmon and trout often cause confusion due to their similar appearances and shared aquatic habitats. This article clarifies the biological relationship between these two popular fish groups.
A Shared Family Tree
Salmon and trout belong to the same scientific family, Salmonidae, which includes over 200 species like char, graylings, and freshwater whitefishes. Within Salmonidae, these fish are categorized into different genera. For example, Pacific salmon species like Chinook and Sockeye, along with Rainbow trout, are in the Oncorhynchus genus. Atlantic salmon and brown trout belong to the Salmo genus. This classification highlights their shared ancestry, despite common names suggesting otherwise.
Key Similarities
Salmon and trout share several characteristics. Both exhibit a streamlined, fusiform body shape, efficient for swimming in flowing water. A distinguishing feature common to salmonids is the adipose fin, a small, fleshy fin located between the dorsal fin and the tail. They generally prefer cold, clean freshwater habitats, thriving in environments with high oxygen levels. As predatory fish, both feed on aquatic insects and crustaceans when young, progressing to smaller fish as they mature.
Distinct Differences
Despite their shared family, salmon and trout exhibit several distinguishing characteristics. Many salmon species are anadromous, hatching in freshwater, migrating to the ocean to grow, and then returning to their natal freshwater streams to spawn, typically dying after spawning. Some trout species, like steelhead (sea-run rainbow trout), also display anadromous behavior, but many trout populations remain in freshwater for their entire lives. Steelhead, unlike most salmon, can survive spawning and return to the ocean to breed multiple times.
- Adult salmon generally grow larger than most trout, with Atlantic salmon being among the largest salmonids, averaging 20 pounds. Steelhead also tend to be larger and more streamlined than resident rainbow trout.
- Spotting patterns differ: trout often have numerous dark spots scattered across their body, including below the lateral line and on the gill covers. Salmon typically have fewer spots, concentrated mainly above the lateral line and on the front of their bodies.
- A trout’s upper jaw often extends past the eye when closed, whereas a salmon’s jaw usually does not.
- Salmon typically have a more slender tail wrist and a forked or notched tail, while trout often have a broader tail wrist and a squarer or less forked tail.
Why the Confusion?
Several factors contribute to the ongoing confusion between salmon and trout. The sheer number of species within the Salmonidae family means there is significant variation, with some “trout” and “salmon” species appearing very similar. For example, the common names “salmon” and “trout” do not always align with scientific classification, as Atlantic salmon are more closely related to brown trout than to Pacific salmon.
Regional naming conventions further complicate identification; for instance, sea-run rainbow trout are commonly known as “steelhead,” blurring the lines between typical trout and salmon life histories. Young fish, known as parr, of both salmon and trout species can look very similar, often having distinct vertical stripes on their sides for camouflage. These factors, combined with the presence of both anadromous and resident populations within the same species like rainbow trout, make visual identification challenging for the average person.