Life History: Pacific lampreys within the same river system may have more than one run (Anglin 1994), or individuals that do not migrate to sea. For example, two forms of Pacific lamprey exist in the Trinity River, one smaller and paler than the other, representing either separate runs or resident and anadromous individuals (T. Healey, CDFG, pers. comm. 1995). Migratory Pacific lampreys are micropredators (feeding on prey larger than themselves) during their oceanic existence, consuming the body fluids of a variety of fishes, including salmon and flatfishes (Beamish 1980) and marine mammals (Close et al. 2002). Beamish (1980) found that 14-45 percent of the salmon returning to British Columbia had scars from lamprey attacks. Similar data are not available for salmon in California. Lampreys themselves are prey for other fishes, including sharks, and are often found with parts of their tails missing. Sea lions, near the mouth of the Rogue River, Oregon, also have been observed eating large numbers of migrating lampreys (Jameson and Kenyon 1977). Because lamprey prey are most common in estuaries and nearshore coastal areas, it is unlikely that Pacific lampreys migrate far from the mouths of their natal streams. However, some individuals have been caught in waters up to 70 m deep (Beamish 1980) and as far as 100 km from shore (Close et al. 2002). The oceanic phase lasts approximately 3-4 years in British Columbia, but is likely of shorter duration in southern waters. Predaceous adults appear to have little effect on fish populations (Moyle 2002, Orr et al. 2004, Brown et al. in press). Pacific lampreys have given rise to landlocked forms, such as Klamath River lamprey and Goose Lake lamprey, but Beamish and Northcote (1989) suggest that landlocked populations are not easily or quickly established.
Adult (30-76 cm TL) spawning migrations usually take place between early March and late June, but initiation of migration has also been documented in January and February (ENTRIX 1996, Trihey and Associates 1996), as well as in July in northern streams. Spawning migrations also have been documented in August and September in the Trinity River (Moffett and Smith 1950). It is possible that lamprey in the Klamath and Eel rivers, as well as other large river systems, have a number of distinct runs, similar to salmon. One indication is that many adults migrate upstream and hide under logs and boulders for months until they mature (Beamish 1980, ENTRIX 1996). Two distinct runs may exist in the Klamath River: a spring run of adults that spawn immediately after upstream migration and a fall run of individuals that wait to spawn until the following spring (Anglin 1994). A large spring run and smaller fall run have been observed in the Russian River (Brown et al. in press). In the Russian River, the two runs were observed with the use of underwater video (at Mirable, 37 rkm), primarily from the beginning of August to the onset of heavy rains (November to December) and in the spring of 2000 to 2007 (S. Chase, Sonoma County Water Agency, unpubl. data). The general trend of these runs are for low numbers in October and November and higher numbers in the spring (207 in 2000; 570 in 2007).
Most upstream movement occurs in surges at night, with some individuals migrating upstream fairly continuously over the course of two to four months. In the Santa Clara River (Ventura County), migration is initiated after the sand bar blocking the lagoon at the mouth is breached by winter rains in January, February or March; adults first reach a fish ladder 16.8 km upstream within 6-14 days of the breach (ENTRIX 1996). In the Santa Clara River, lamprey will migrate mostly during high flows, but will migrate in flows ranging from 25 to1700 m3/min (ENTRIX 1996). Lampreys migrate considerable distances and are stopped only by major barriers, such as dams. Lampreys were seen spawning by one of the authors in Deer Creek (Tehama County), about 440 km from the ocean. Presumably migrations of more than 500 km were once common. In the Klamath River, Humboldt County, radio tagged lampreys migrated an average of 34 km over the course of 25 days at a travel rate of 1.97 km/day (McCovey et al. 2007). Adults do not feed during spawning migrations (Beamish 1980) but can survive extended periods (months to two years) without food, allowing them to migrate long distances (Whyte et al. 1993).
Once at a spawning site, both sexes build a nest, 35-60 cm in diameter, by removing larger stones from a gravelly area with swift water, depths of 30-150 cm, and temperatures between 12 and 18 ?C (Moyle 2002). However, mean depth of nests range from 30-82 cm (mean of 59 cm) in the American River, while ranging from 36-73 cm (mean of 50 cm) in Putah Creek. Nest construction has been observed in waters as deep as 1.5 m in Deer Creek. Water velocity at nest in the American River ranged from 24-84 cm/sec, while ranging from 17-45 cm/sec in Putah Creek. Although Pacific lampreys most commonly spawn in flowing water, spawning has been observed in lentic systems also (Russell et al. 1987). Lampreys attach to the downstream end of rocks and swing vigorously in reverse to remove them from the nest. More than one individual may pull at the same rock until the combination of pulling and pushing from the current dislodges the rock (Stone 2006). Adults may test several nest sites (false digs) before fully digging a nest (Stone 2006). Nests are shallow depressions with piles of stones either at the downstream (Moyle 2002) or upstream (Susac and Jacobs 1999) end of the nest. In order to mate, the female attaches to a rock on the upstream end of the nest, while the male attaches himself to the head of the female and wraps his body around hers. Occasionally, both will attach to rocks while staying side by side (Wang 1986). Eggs and milt are released when both vibrate rapidly. Fertilized eggs float downstream where most adhere to rocks at the downstream end of the nest. After spawning, the lampreys loosen sediment upstream of the nest to cover the eggs. Spawning is repeated in the same nest until the adults are spent. Males may mate with more than one female spawning in the same area (Wang 1986). As many as 48 individuals were observed using the same nest in the Smith River, Oregon (Gunckel et al. 2006). The average time spent in the spawning areas is less than seven days for both sexes (Brumo 2006). Both sexes usually die after spawning. However, some adults may live to spawn for one more year in Washington streams (Michael 1984). Repeat spawning may also occur in the Santa Clara River, as live adults have been caught by downstream migrant traps (ENTRIX 1996). The fecundity of females ranges from 20 000 to 238 000 eggs (Kan 1975). Egg predation by speckled dace, Rhinichthys osculus, was observed in the South Fork Coquille River, Oregon (Brumo 2006). Adults may defend their nests. Stone (2006) observed a male using his suctorial disc to remove a sculpin (Cottus spp.) from the nest in Cedar Creek, Washington.
At 15 ?C, embryos hatch in 19 days. Upon hatching, juveniles (ammocoetes) stay in the nest for a short period of time and then swim into the water column where they are washed downstream to areas of sand or mud. Ammocoetes burrow into soft stream sediments tail first, at which point they will filter feed by sucking organic matter and algae off the stream substrates. Survival to this stage may be related to stream discharge at time of spawning and density dependent effects (e.g. amount of rearing habitat and prey items) associated with ammocoete abundance (Brumo 2006). Ammocoetes will drift from soft sediments at night throughout their freshwater residency (White and Harvey 2003). Larger ammocoetes more commonly will drift in spring high flows, while smaller ammocoetes drift during the summer. Consequently, they can be trapped during much of the year (Moffett and Smith 1950, Long 1968). In the Trinity River, ammocoetes as small as 16 mm recolonized areas from which they had been removed during winter floods (Moffett and Smith 1950). Most movement occurs at night. The ammocoete stage probably lasts 5-7 years, at the end of which ammocoetes measure 14-16 cm TL and metamorphosis to predatory adult begins. Lampreys develop large eyes, a sucking disc, silver sides, and dark blue backs during metamorphosis. Their physiology and internal anatomy (McPhail and Lindsey 1970) also change dramatically. One important physiological change will allow lampreys to tolerate salt water, which is lethal to ammocoetes (Richards and Beamish 1981). Saltwater tolerance coincides with the opening of the foregut lumen (Richards and Beamish 1981). Downstream migration begins when metamorphosis is completed and is often associated with high flow events in the winter and spring, perhaps coincident with adult upstream migration. Most volitional movement occurs at night (Dauble et al. 2006).
An under-appreciated aspect of Pacific lampreys is their importance in stream ecosystems. Ammocoetes can be important in breaking down detritus and as prey items to other fishes (Brown et al. in press). Adult carcasses may be an important source of marine derived nutrients (e.g. nitrogen) to oligotrophic streams (Wipfli et al. 1998, Close et al. 2001, Brown et al. in press).