Pacific Lamprey

The Pacific lamprey (Entosphenus tridentatus) is a jawless, anadromous fish native to the coastal Pacific Ocean and its tributary rivers, ranging from Baja California, Mexico, north along the coast of North America to Alaska, and across to Japan and Russia. As a member of the ancient lamprey lineage (Petromyzontiformes), it lacks jaws, scales, and paired fins, relying instead on a circular, tooth-lined oral disc. Adults spend an extended period at sea feeding parasitically on marine fish and marine mammals before returning to fresh water to spawn. The species holds significant ecological importance in Pacific coast river systems, both as a prey source for other wildlife and, historically, as a culturally significant species for Indigenous communities of the Pacific Northwest. Many populations have declined due to river damming and habitat degradation.

Pacific lampreys share the general jawless, eel-like body plan of the family but have distinguishing tooth and coloration details.

Key field marks:

• Circular oral sucking disc with sharp, well-developed teeth arranged in a distinctive pattern, including three large cusps on the supraoral tooth plate (source of the species name tridentatus)
• Seven round gill openings in a row on each side of the head
• Dark blue-grey, brown, or nearly black back, lightening to a pale silvery belly
• Long, scaleless, cylindrical body with two closely spaced dorsal fins and no paired fins
• Single central nostril atop the head
• Adults typically 50-70 cm long

It can be told apart from the sea lamprey largely by geography (Pacific versus Atlantic drainages) and from smaller freshwater lamprey species by its larger size and well-developed, sharply cusped dentition.

Pacific lampreys inhabit coastal marine waters of the North Pacific as parasitic-feeding adults, ranging from nearshore areas to deeper offshore waters where they attach to host fish and marine mammals. They are anadromous, migrating from the ocean into coastal rivers and streams from California to Alaska and the western Pacific rim to spawn in gravel-bottomed, flowing stream reaches with cool, well-oxygenated water. After spawning, hatched larvae (ammocoetes) burrow into fine silty or sandy substrate in low-velocity stream margins, where they remain buried as filter feeders for several years before transforming and migrating downstream to the ocean. Adults require unobstructed river passage, and dams pose a major barrier to their historic spawning migrations.

As adults, Pacific lampreys are parasitic, attaching to host fish or marine mammals with their sucking oral disc and rasping through tissue to feed on blood and fluids while at sea, sometimes leaving distinctive circular wound scars on hosts. They undertake long spawning migrations upriver, often navigating challenging obstacles including waterfalls and rapids, using their oral disc to grip surfaces and move upstream in bursts. Adults cease feeding once they enter fresh water and rely on stored energy reserves through their migration and spawning. Multiple individuals may build nests together in gravel substrate, and all adults die after spawning. Larvae live buried in soft sediment for several years as filter feeders, playing an important role in stream nutrient cycling, before metamorphosing into free-swimming, sea-migrating juveniles.