Weitkamp et al.: Seasonal abundance, size, and host selection of Lampetra ayresii and Entosphenus tridentatus 
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14.0 
12.0 
10.0 
8.0 
6.0 
4.0 
2.0 
-o— American shad 
-A - Chinook salmon 
X- - Pacific herring 
Shiner perch 
Time period 
Figure 4 
Mean percentage of fish observed with lamprey wounds by 2-week time pe- 
riods during 2008-2012 for American shad ( Alosa sapidissima), shiner perch 
( Cymatogaster aggregata), and Pacific herring ( Clupea pallasii) and during 
2006-2012 for Chinook salmon ( Oncorhynchus tshawytscha ) in the lower Co- 
lumbia River estuary. Note that the axis for shiner perch is on the right side. 
adult salmon, http://www.fpc.org/adultsalmon/adultque- 
ries/adult_table_submit.html, accessed August 2013), 
indicating that it may take 2-5 months for adults to 
migrate from the estuary to the dam. In British Co- 
lumbia, it appears that adult Pacific lamprey re-enter 
freshwater typically between April and September, later 
than they do in the Columbia River estuary (Beamish, 
1980; Beamish and Levings, 1991). In California, adult 
Pacific lamprey move upstream between early March 
and late June, although earlier (December and Janu- 
ary) and later (July-November) migration timing has 
also been reported (Chase, 2001; Moyle, 2002; Moyle et 
al., 2009). Although available data are geographically 
limited and are often based on 1 or 2 years of study 
(including our study), there appears to be considerable 
regional or annual variation in the timing of movement 
of adult Pacific lamprey from marine to freshwater 
habitats. 
In contrast to the variation among studies in tim- 
ing of adult migrations, our estimate of migration 
timing for juvenile Pacific lamprey during winter- 
spring (Fig. 2) is consistent with observations from 
other studies; timing of this downstream migration is 
thought to be driven by river discharge (Beamish and 
Levings, 1991; van de Wetering, 1998; Moyle, 2002). 
Kostow 1 reported outmigration of Pacific lamprey (of- 
ten mixtures of ammocoetes and juveniles) during 
fall, winter, and spring at locations throughout the 
Columbia River basin. Counts of “juvenile lamprey” 
(species were not identified) made at Bonneville Dam 
show an abrupt increase in mid- 
April, soon after sampling com- 
mences in the spring (FPC smolt 
data query page, http://www.fpc. 
org/smolt/SMP_queries.html, ac- 
cessed August 2013). Hayes et al. 
(2013) indicated that, in Puget 
Sound, downstream migration 
of Pacific lamprey juveniles oc- 
curs during fall or winter, on 
the basis of the absence of that 
life stage in salmon smolt traps, 
which typically only operate dur- 
ing spring and summer. More- 
over, in Tenmile Creek on the 
Oregon coast, van de Wetering 
(1998) observed peak downstream 
migration of juvenile Pacific lam- 
prey during November and De- 
cember, slightly earlier than the 
timing observed in our analysis 
for the Columbia River estuary 
but consistent with the patterns 
described by Kan (1975) for large 
versus small rivers. Similar win- 
ter-spring timing of ocean entry 
has been reported for juvenile 
Pacific lamprey in California 
(Moyle, 2002) and a slightly later 
timing (through mid-summer) in 
British Columbia (Beamish 1980; Beamish and Lev- 
ings, 1991). 
The presence of western river lamprey in the Co- 
lumbia River estuary from spring through fall (Fig. 2) 
is also similar to timing reported in other studies. Bond 
et al. (1983) reported western river lamprey in the Co- 
lumbia River estuary, in Yaquina Bay on the Oregon 
coast, and nearby in marine waters from May through 
September. Hayes et al. (2013) stated that western river 
lamprey juveniles were common in downstream salmon 
smolt traps in Puget Sound in all months of operation 
(February-August), and the highest abundances oc- 
curred during June-August. For western river lamprey 
populations in Canada, Beamish (1980) reported the 
highest abundances in the Fraser River estuary in late 
May and in the Fraser River plume from early May 
through early July. 
Upstream migration of adult western river lamprey 
is reported to occur from September through late win- 
ter in both the Fraser and Columbia rivers (Beamish, 
1980; Bond et al., 1983). Our observation of an abrupt 
absence of western river lamprey in the Columbia Riv- 
er estuary beginning in October (Fig. 2) is consistent 
with this timing. However, neither the locations where 
western river lamprey go to spawn once they leave the 
estuary nor the total abundance of this species in the 
Columbia River (or other coastal rivers) are known 
(Kostowl; Mesa and Copeland, 2009). This information 
is especially needed to avoid misidentification of early 
life stages of lampreys in the field (Mesa and Copeland, 
