While spawning, females lay eggs in redds created by using body movements to dig into the sand and gravel. After spawning, the blind larva, called ammocoetes, move downstream to find fine silt deposits where they bury themselves in a U-shaped form. Larva filter feed from its mouth at the surface of one end. Larvae feed upon algae, diatoms and detritus. Their bodies process 30-40% of food taken in, the remaining particulate matter is flushed downstream or taken up by other filter feeding organisms. The larvae are also predated upon by coho fry while changing locations or by being dug out of the sand or silt. The larva remains in the burrows for four to six years, changing locations to deeper water and courser sand as they mature. Upon reaching the eyed-lamprey or eyed-juvenile stage, the lamprey migrates to the ocean usually during November through June.
Pacific lamprey live in the ocean for six to forty months in waters up to 70 meters deep. During this period the pacific lamprey begin its parasitic state, feeding upon blood and bodily fluids of fish and whales. In turn, marine mammals and large fish predate upon Pacific lamprey. When they return to streams to spawn, they have not reached sexual maturity. During this time pacific lamprey become sexually mature and do not feed. After reaching sexual maturity spawning takes place.
Lamprey are important as a food source for numerous animals. High in fat content, concentrations of adult and larval lamprey once made them an important and dependable food source for birds, fish and mammals, especially seals and sea lions. Adult pacific lampreys also function as a buffer to reduce predation on adult migrating salmon from seals and sea lions. Similarly, Pacific lamprey is found in the diets of other fish and gulls, that otherwise might prey more heavily on young salmon. Similar to salmon, lamprey transport important nutrients such as nitrogen to freshwater ecosystems.
Pacific lamprey have the widest distribution of all lamprey. They are found in the Pacific Rim from Japan to Korea to California and inland in the Columbia Basin to sections of the Snake River Basin. Within the United States their populations have drastically declined in the last forty years. At Winchester Dam in the North Umpqua River, the pacific lamprey run count dropped from 46,785 in 1966 to less than 50 annually since 1995. At Gold Ray Dam on the Rogue River the population counts have varied from 155 to 2,370 since 1993. These numbers are considered to be much lower than historical Pacific lamprey runs.
Population declines are attributed to flow regulation, channelization, poor water quality, and chemical treatments. Dams lower Pacific lamprey runs because of blocked passages. Dams also lower water levels in rivers and streams, which dewater necessary sand and silt habitat used by ammocoetes. Irrigation ditches along rivers and streams allow ammocoetes to move through screens into the irrigation canals, but do not allow them to return to the stream. The ammocoetes die in the irrigation ditches for lack of proper maturing habitat and food sources. Also, introduction of exotic predators, such as small mouth bass, can be attributed to population declines.
Before its decline the lamprey was a very important fish for many of the Tribal people of the Pacific coast and interior Columbia River basin. Tribal people harvested these fish for subsistence, ceremonial, and medicinal purposes. Tribal members of the Columbia River Basin have attributed three main negative impacts on their culture from the reduced lamprey runs. These negative impacts are as follows: 1) loss of culture; 2) reduction or loss of fishing in traditional areas; and 3) members must travel to lower Columbia River tributaries to find lamprey. The accumulative effects of the loss of lamprey within the culture are that the young do not know how to catch and prepare lamprey for drying and the loss of lamprey myths and legends.