Salmon are members of the family Salmonidae, encompassing both Pacific and Atlantic species. They are well-known for their anadromous life cycle, migrating from freshwater to the ocean and back again to reproduce. Salmon hold commercial importance and play a substantial ecological role, particularly as a keystone species in North Pacific ecosystems.
General Physical Characteristics
The typical adult salmon in saltwater is characterized by a sleek, torpedo-like body shape built for swift movement in the open ocean. This streamlined form allows for efficient long-distance travel during feeding migrations. Their coloration in the “ocean phase” is a bright, reflective silver along the sides and belly, contrasting with a darker, bluish-green or gray back. Size varies dramatically by species, with the largest Chinook salmon occasionally exceeding 45 kilograms, while the smallest Pacific species, Pink salmon, typically average less than two kilograms. All salmon possess a small, fleshy adipose fin located on the back between the dorsal fin and the tail. This powerful structure and silvery appearance mark them as strong, cold-water predators.
Key Visual Differences Between Species
While the ocean phase makes identification challenging, specific physical markers allow for distinction between species. The most reliable features are the pattern of spots on the back and tail, and the color of the mouth and gum line.
Chinook salmon (King salmon) possess a black mouth with a black gum-line, and distinct spots cover both the upper and lower lobes of their tail fin. Coho salmon (Silver salmon) also have a dark mouth but exhibit a white gum-line, and their spots are generally confined to the upper lobe of the tail. Sockeye salmon lack distinct spots on their back or tail, and both their mouth and gum-line are white. Pink salmon are identified by large, oval spots on the back and both tail lobes, coupled with a white mouth and a black gum-line. Chum salmon generally have no spots and feature a white mouth with a white gum-line. Atlantic salmon, the sole species native to the North Atlantic, are more slender than Pacific relatives and have fewer spots, often appearing as distinct x or y-shaped markings.
Appearance During the Spawning Migration
As salmon transition from saltwater into freshwater for spawning, they undergo a rapid visual transformation known as developing nuptial colors. The silver ocean phase is replaced by vivid hues that vary significantly by species. Sockeye salmon, for instance, change to a bright red body contrasting sharply with a green head. Other species develop deep maroon, mottled brown, or multi-colored bands, such as the green and purple stripes on spawning Chum salmon.
Alongside the color change, male salmon develop pronounced morphological features for competitive spawning. The lower jaw elongates and hooks upwards, forming a structure called a “kype,” and they develop enlarged, canine-like teeth. Males of certain species, notably Pink and Sockeye salmon, also develop a substantial hump on their back, giving the Pink salmon its nickname, “Humpback.” These changes are temporary, driven by hormonal shifts, as the fish rely on stored energy reserves for the upstream migration.