Pacific Lamprey: The Ancient Fish in Humboldt's Rivers

Pacific lamprey have been migrating up the rivers of what is now Humboldt County for approximately 450 million years — a tenure that predates the coast redwood tree by 350 million years, the Roosevelt elk by roughly 449 million years, and the first salmon by somewhere between 60 and 80 million years. Lady Humboldt notes this without editorial elaboration. The lamprey arrived first and did not appear to need assistance from any subsequent species to establish their arrangements.

Pacific lamprey (Entosphenus tridentatus) are jawless, eel-like fish in the ancient class Agnatha — a lineage so old it predates the evolution of jaws, paired fins, and most of what is now recognizable as a fish. Adults reach 30 to 75 centimeters in length, migrate from the Pacific Ocean into coastal rivers to spawn, and die after reproduction. They are native to every major watershed along the North American Pacific coast from Alaska to Baja California, with the Klamath, Trinity, and Eel rivers in Humboldt County among their most significant remaining California strongholds. (NOAA Fisheries, 2025)

Pacific lamprey are classified as a Species of Concern by NOAA Fisheries — a designation below Threatened or Endangered but signaling that population data and conservation attention are warranted. Surveys on the Klamath and Trinity systems show the species has declined significantly from its historical abundance, for reasons involving dams, water temperature, habitat simplification, and the decline of the large marine fish on which lamprey depend during their ocean phase. The county's rivers still support lamprey populations. They are, by many measures, in need of support in return.

The Pacific lamprey life cycle is among the most elaborate of any North American fish. It unfolds across three distinct phases occupying different habitats, involving complete metamorphosis, and spanning seven to twelve years in total.

The larval stage (ammocoetes). After hatching in river gravel, lamprey larvae — called ammocoetes — burrow into fine sediment in the calm margins and backwater areas of rivers and remain there for three to seven years. They are essentially blind, lack functional eyes and teeth, and feed by drawing river water through a specialized filtering structure that extracts organic matter and microorganisms. An ammocoete in the Trinity River sediment is functionally invisible: soft-bodied, no larger than a nightcrawler, producing no disturbance in the substrate. Lady Humboldt suspects there are more of them in the county's rivers than any reasonable observer would guess, for reasons best attributed to their successful concealment rather than to their rarity.

Metamorphosis. In late summer and early fall — typically August through November — ammocoetes transform over a period of three to five months into juvenile lamprey. Eyes develop. Teeth form in circular rows around a suction-disc mouth. The body changes color from dull tan to the silver-bronze of a seagoing fish. This transformation requires no food: it is powered entirely by stored energy accumulated over years in the river sediment. Transformed juveniles overwinter in the river and migrate downstream to the ocean the following spring, typically February through May.

The ocean phase. In salt water, adult Pacific lamprey are ectoparasites: they attach to larger fish — Pacific salmon, halibut, Pacific hake — using the suction disc and teeth, rasping through the host's skin to consume blood and body fluids. The ocean phase lasts one to three years. Hosts survive individual attachments, though multiple parasitic events reduce their condition. Lady Humboldt notes that this phase of the lamprey's career is typically the point at which the creature's public standing undergoes revision.

The return migration. Adult lamprey cease feeding before entering fresh water and remain without food for the duration of their upstream migration and spawning — a period of up to eighteen months. They migrate upstream from roughly October through June, with peak activity in fall and early spring. Males and females construct shallow redds in river gravel, spawn, and die, completing a cycle that began in the same watershed several years prior. (Pacific States Marine Fisheries Commission, 2026)

The following table summarizes the Pacific lamprey life cycle stages, their timing, habitat, and duration in Northern California river systems including the Klamath and Trinity.

Life Stage	Location	Approximate Timing	Duration	Key Feature
Egg (in redd)	River gravel, shallow riffles	March–June	2–3 weeks	Female produces 30,000–100,000 eggs
Ammocoete (larval)	River sediment, backwater margins	Year-round	3–7 years	Blind; filter-feeds on organic matter; essentially invisible
Metamorphosis	River sediment to open water	August–November	3–5 months	Eyes and teeth develop; no food consumed during transformation
Downstream migration (smolt)	River to ocean	February–May	Days to weeks	Juveniles travel at night; mostly passive, drifting downstream
Ocean parasitic phase	Pacific Ocean	Year-round	1–3 years	Attaches to salmon, halibut, and Pacific hake as ectoparasite
Upstream migration (adult)	River system	October–June (peak: fall and spring)	Weeks to months	Ceases feeding entirely; uses suction disc to cling to rocks
Spawning and senescence	River headwaters and tributaries	April–July	Days to weeks	Both adults die after spawning; carcasses enrich riparian system

Sources: NOAA Fisheries Pacific Lamprey Conservation Agreement, 2025; CDFW Inland Fisheries Program, 2024.

Pacific lamprey range along the Pacific Coast from the Aleutian Islands to southern Baja California, but their California populations have concentrated in the state's northernmost river systems — the Klamath, Trinity, Smith, and Mad rivers — as suitable habitat contracted over the twentieth century due to dam construction and watershed degradation.

The Klamath River, which enters the Pacific at Requa in Del Norte County and drains significant portions of south-central Oregon, has historically hosted one of the largest Pacific lamprey runs on the West Coast. ODFW and NOAA surveys in the lower Klamath documented consistent lamprey presence through the 1980s, with noticeable decline in upstream abundance beginning in the 1990s coinciding with the operational effects of the four PacifiCorp dams whose removal was completed in late 2024.

The Trinity River, a major Klamath tributary joining it at Weitchpec, supports a distinct lamprey population that encounters a different dam situation: Lewiston Dam and Trinity Dam on the upper Trinity remain in place, with their bypass structures designed primarily around salmon passage. Lamprey are generally poor ladder climbers compared to salmon — their suction disc permits them to ascend rough, wet rock faces, but standard fish ladders with deep pools and high velocity jets represent a significant passage barrier. Lamprey-specific passage improvements have been installed at a small number of facilities statewide, as is documented in the NOAA Pacific Lamprey Conservation Agreement framework. (NOAA Fisheries, 2025)

The Eel River and its South Fork also support lamprey, though populations there have been more difficult to survey given the river's flashy hydrology and heavy sediment loads — conditions that paradoxically provide abundant fine-sediment larval habitat but complicate snorkel and electrofishing surveys.

As detailed in Lady Humboldt's account of the Klamath dam removal, the reopening of 420 miles of river corridor in 2024 represented the most consequential change to Pacific lamprey habitat in Northern California's recorded history.

Pacific lamprey have been a significant food source for Yurok, Karuk, and Hupa peoples for thousands of years — a dietary and cultural relationship documented in oral traditions, ethnographic records, and contemporary tribal food sovereignty programs.

Lamprey are harvested during their upstream migration, primarily by hand or dip net at falls, cascades, and constrictions in the Klamath and Trinity rivers where migrating adults congregate. The harvest requires detailed local knowledge: specific falls where lamprey accumulate on particular tide and flow conditions, timing windows within the broader migration, and preparation methods that differ from salmon preservation. Adults are traditionally dried and smoked on wooden racks, producing a shelf-stable food with a fat content considerably higher than most dried fish — a characteristic that made lamprey an important caloric resource through winter months. (Yurok Tribe Environmental Program, 2024)

The decline of lamprey runs has had consequences that extend beyond the ecological. Tribal members who grew up harvesting lamprey with their families describe reaching sites where lamprey were reliably abundant and finding none — a discontinuity in the transmission of ecological knowledge and cultural practice that cannot be remedied by fisheries management programs alone. The Yurok Tribe and Karuk Tribe have both established formal lamprey restoration programs, documenting traditional ecological knowledge of harvest sites, conducting population surveys in collaboration with NOAA, and advocating for lamprey-specific passage modifications at dams and culverts that salmon-centric designs overlooked.

Lady Humboldt observes that the Pacific lamprey restoration effort occupies the same landscape as the prescribed fire revival documented in the account of traditional burning on the North Coast and the dam removal story covered in the Klamath restoration record: tribal ecological knowledge, long institutionally marginalized, now recognized as the most detailed and place-specific available account of how these systems functioned before their disruption. These facts may be related.

Pacific lamprey face a compound of pressures that, taken individually, each represent a partial barrier; taken together, they constitute a serious population constraint across much of the California range.

Dams and passage barriers. The four Klamath dams removed in 2024 are the largest single passage barrier eliminated in the species' California range — but they are far from the only ones. Small diversion dams, road culverts, and tide gates on tributaries throughout Humboldt County present partial or total barriers to lamprey passage. Because lamprey use their suction disc to ascend rough, wet rock rather than leaping like salmon, a fish ladder designed for salmon may require modification — typically a roughened substrate or a gentle, continuous slope — to function for lamprey. Many existing structures were designed without lamprey in mind. (Pacific Lamprey Conservation Agreement, NOAA, 2025)

Fine sediment loss. Ammocoetes require specific substrate conditions: fine, mixed organic sediment in calm backwater areas where they can burrow and remain stable for years. River channelization, gravel mining, and altered sediment dynamics downstream of dams reduce the availability of suitable larval habitat. On some regulated river reaches, the combination of reduced sediment input and altered flow timing has simplified the riverbed in ways that reduce carrying capacity for lamprey larvae specifically.

Ocean conditions. Pacific lamprey depend on a diverse assemblage of large marine fish as hosts during their ocean phase. The same warming ocean conditions that have stressed Pacific salmon runs on the North Coast have also affected the forage fish and host fish on which lamprey depend. Population surveys cannot distinguish between river-based and ocean-based mortality, which complicates management responses.

Reduced run diversity. Unlike salmon, lamprey spawning runs are not well-differentiated by population segment — they are managed as a single coastal stock, meaning a localized extirpation in a tributary receives little specific management response. This aggregation approach, which obscures local population trends, is recognized in the NOAA conservation framework as a gap in current monitoring.

The removal of Iron Gate, Copco 1, Copco 2, and J.C. Boyle dams on the Klamath River between 2023 and 2024 reopened approximately 420 miles of river corridor — the largest dam removal project in United States history by any measure of water, sediment, or ecological consequence. The primary public narrative around the project centered on Chinook salmon; lamprey received less attention, but their potential benefit from the project is substantial.

Prior to dam removal, Pacific lamprey access above Iron Gate Dam required passage through a fish ladder documented as partially ineffective for lamprey — adults were detected accumulating below the dam during migration windows, with reduced passage compared to salmon. The Shasta River, Scott River, and Salmon River tributaries above the former dam sites provide habitat conditions — cold, clean water, mixed sediment, reduced gradient — well-suited to lamprey larval development.

Yurok Tribe biologists deployed passive integrated transponder (PIT) tag detectors in the mainstem Klamath above the former Iron Gate site in 2025, documenting the first confirmed adult lamprey presence in that reach in at least three decades. The 2025 data represent a single year of observation and do not yet indicate recovery of the spawning population above former dam sites, but they establish presence in newly accessible habitat that, if conditions remain suitable, could support significant expansion of the spawning and larval population over the next decade. (Yurok Tribe Environmental Program, 2025)

Lamprey recovery, unlike salmon recovery, does not produce a commercially visible return — there is no Klamath lamprey fishery that generates economic data, and the species does not trigger the same management attention as commercially harvested or culturally high-profile salmon runs. Lady Humboldt considers this a circumstance worth noting, without venturing an opinion on what should follow from it.

Adult Pacific lamprey during their upstream migration are occasionally observable at river riffles and falls where they pause to rest, using the suction disc to cling to submerged rock surfaces before continuing upstream. The most likely viewing windows are October through April during periods of moderate river flow — low enough for visibility into the water column, high enough to have triggered the migration.

The Weitchpec confluence of the Trinity and Klamath rivers, accessible via State Route 96 in Trinity County (immediately adjacent to Humboldt County's eastern boundary), has historically been a lamprey migration corridor and is the type of location where patient observation from the river bank produces lamprey sightings during active migration periods. The confluence provides a constriction in the channel that concentrates migrating fish. Lady Humboldt notes this site is also one of the more productive fall salmon-watching locations in the region, as detailed in the salmon runs guide — the two audiences converge on the same geographic point, though they are typically not looking at the same animal.

Lamprey ammocoetes are not realistically observable by casual visitors — they are buried in river sediment and require electrofishing or benthic sampling to detect. Spawning adults, which congregate in river gravels from April through July at falls and constrictions, are visible with polarized glasses at appropriate sites during their narrow behavioral window.

The species' presence in the county is, in one sense, its own observation: every coastal river in Humboldt County that has retained native salmon runs also retains Pacific lamprey, moving through the same corridors on a schedule that has not required revision in 450 million years. Lady Humboldt's weekly field guide tracks seasonal natural events across each watershed, with notes from the same rivers where lamprey and salmon share the gravel. A subscription is here.