Fair Hooker -
Interesting question - while there probably is not a clear answer to the lamprey declines I think we can rule out several of the usual suspects.

In spite of Spawnout's early efforts I see or heard of little interest in a bonk season for them. By default we have Wild Lamprey Release (WLR) management. As many lamprey as want to or can are allowed to spawn so they aren't limited by MSY management. So the bios must have the fishery management thing right for them.

Lamprey are one of a handful of native anadromous species where there has not been a hatchery program or basin relocation program. Without a hatchery program hard to blame mis-guided hatchery practices for the decline.

While it would be tempting to blame predators the fact is that they evolved with them and man has not stepped in an attempted to use some of the lamprey's natural productivity for our own use so over the long haul hard too imagine predation being higher today than say 200 years ago.

That leaves us with the last 2 of the usual suspects as the probable cuase(s)-

Poor marine survival - there is some indication that their survival varies much the same as the various salmonid species. On the Oregon they took an accerlated nose-dive in abundance at the same time as the cutthroat coho, etc did.

The other of course is degraded freshwater habitats. They may be even more sensitive to freshwater habitat conditions then steelhead. They are spring/early summer spawners making shallow nests (redds ) in the gravel and have eggs that are not buried but rather fall into the spaces between the rocks. As such they are more vulnerable to high waters. Their young spend a number of years in the freshwater. In British Columbia the Pacific ammocoetes were found to be 5 or 6 year old when they left the river - spending twice as long in the freshwater than steelhead so they are exposed longer to any freshwater survival impacts. The ammocoetes spend their freshwater time buried in the muck found in slow areas so may be vulunerable to flooding and channel changes (both more common in our altered watersheds than historically).

In short their plight seems to indicate the problem is habitat and habitat. In fact as stated in other posts if we are serious about having abundant anadromous fish the focus needs to be on habitat. While hatcheries and harvest play roles the information at hand indicates in the majority of cases habitat issues over shadow them.

For you record seekers - while the average Pacific lamprey may be 18 inches or so long as pointed out by others they do get larger. The biggest one I could find a record of was 30 incher that weight a full pound - fish on!

Tight lines
Smalma