As mentioned they were quite numerous but in recent decades their populations have crashed and headed to ESA listing across the region; likely for much the same reasons that our steelhead populations have fallen on hard times.
A bit of background on why lamprey haven't been listed yet and why we should care. Lamprey are and have been an important food source for native people from the Yukon to the Sacramento River. The Arctic lamprey is the common one in the north and the Pacific lamprey is the one found most often in our region. As Smalma stated they are spring spawners, entering the river in the previous summer and fall. They swim into the rivers, like salmon they do not feed on their spawning migration. After spawning they die within 4 days. When the eggs hatch the larval lamprey bury themselves in sediments, in back eddies and sloughs, from 4 to 6 years. When they emerge from the mud the juveniles have neither eyes or a mouth. At this stage they are referred to as transformers, as they migrate towards the sea they develop eyes and a mouth and are quickly ready to feed. They feed on fish primarily but also attach to marine mammals.
At this time the US Army Corp of Engineers and BPA are spending several million dollars annually attempting to improve lamprey populations in the Columbia Basin. Aside from the usual problems of habitat and fisheries the unique problem in the Columbia Basin is the fish passage facilities at the dams. They were designed for salmon, with gradients and flow suitable for salmon. Lamprey have difficulty swimming against the high flows so they use a sort of attach to the wall crawling technique to move through the passage facilities. Even the smallest obstruction, such as caulk in a seam can stop them. USACE is spending big money every year to develop passage facilities that lamprey can use. The USACE Anadromous Fish Evaluation Program meeting earlier this month had a full 1/2 day of talks about lamprey research and programs. An interesting fact from the speakers. When salmon and steelhead are collected for transport down stream juvenile lamprey are also collected. By the time they reach the release site almost all of the lamprey have attached themselves to a fish. Smolts that died during transport often have up to 6 lamprey attached. It seems that they like their meal already dead. Lamprey are found attached to salmon by researchers on Japanese high seas research vessels.
What has delayed lamprey from being listed? The primary reason that they haven't been listed is that an ESU hasn't been determined. Not enough genetic work has been done to say if populations in different watershed are uniquely identifiable. It is not known if lamprey have fidelity to a home stream or spawn where fate takes them. It is possible that all of the lamprey along the Northwest coast are one population but it seems more likely that different river systems would have different populations. A listing of lamprey could cause problems for all of us. In the Columbia and Snake the upstream passage problem would require an even greater investment. Downstream migration is also a concern. The bigger disruption in things as usual is a habitat issue arising from the unique life history of the lamprey. The 4 to 6 years they spend buried in the sediments is where the problem arises. Those sediments are where many of the toxic pesticides, fertilizers, and industrial pollutants accumulate. The only way to protect the lamprey from the pollutants is to stop pollutants from entering the system and to remove the toxic sediments. It may not be much of a problem in higher elevation headwater streams but it would be a major disruption in lower elevation agricultural and urban areas.