218 
Fishery Bulletin 113(2) 
— O — CREDDP trawl 
— o — CREDDP purse seine 
— a — - EPS purse seine 
Month 
Figure 2 
Mean monthly densities (individuals/10,000 m 2 ) of 
Pacific lamprey ( Entosphenus tridentatus) (A) adults 
and (B) juveniles and (C) western river lamprey 
(. Lampetra ayresii ) by study and gear type in the 
lower Columbia River estuary. The Columbia River 
Estuary Data Development Program (CREDDP) was 
conducted during 1980-1981, and the Estuary Purse 
Seine (EPS) study sampled during 2001-2012. Error 
bars indicate 1 standard deviation. 
venile Pacific lamprey showed the highest densities in 
December and lower densities during January-March; 
a few individuals were captured in June. High densi- 
ties of juveniles documented in December 1980 were 
spread across 8 stations sampled over 5 days, indicat- 
ing a large continuous surge of juveniles in the estuary 
rather than a single, isolated concentration of individu- 
als. Lastly, western river lamprey were present in the 
estuary over 6 months (April-September) in data sets 
from both studies, but there was a slightly earlier den- 
sity peak in the EPS study (June) than in the CREDDP 
study (August or September; Fig. 2). A single western 
river lamprey was also caught by the CREDDP trawl 
in January, but no parallel catches were observed in 
the CREDDP purse seine. 
The spatial distribution of both western river and 
Pacific lampreys within the Columbia River estuary 
was evaluated with data from the many CREDDP sta- 
tions (Fig. 1). Abundances of Pacific lamprey juveniles 
and adults were centered at rkm 21.8 and 31.8, re- 
spectively, and the average location for western river 
lamprey was rkm 20.0. The difference in mean location 
between western river and adult Pacific lampreys was 
statistically significant (MW: H-7.9, P<0.05), but the 
difference between Pacific lamprey juveniles and the 
other groups (Pacific lamprey adults and western riv- 
er lamprey) was not (MW: H<1.1, P>0.1). All 3 groups 
were widely distributed throughout the estuary, but 
neither species was recorded at the most downsteam 
(rkm 3.8) or upstream (rkm 58.2) trawl and purse seine 
sites (Fig. 1). 
The mean lengths of lampreys caught in the Colum- 
bia River estuary across both studies were 132.8 mm 
TL (range: 102-157 mm TL) and 596.0 mm TL (range: 
401-745 mm TL) for juvenile and adult Pacific lam- 
prey, respectively, and 198.1 mm TL (range: 102-324 
mm TL) for western river lamprey. Length and weight 
data from the CREDDP showed the same patterns: Pa- 
cific lamprey juveniles, on average, were 132.8 mm TL 
and weighed 3.6 g; mean values for adults were 605.2 
mm TL and 460.2 g and for western river lamprey were 
211.7 mm TL and 19.2 g. 
The size of western river lamprey differed by study and 
gear type. Western river lamprey caught in the CREDDP 
(regardless of gear) were longer (adjusted mean: 206.3 
mm TL; ra=41) and increased over time at a steeper rate 
(0.84 mm/d) than those caught during the EPS study 
(adjusted mean: 189.5 mm TL; slope=0.27 mm/d; n- 42; 
ANCOVA: F>3,8, P<0.G5). Within the CREDDP, west- 
ern river lamprey caught with trawls (adjusted mean: 
189.7 mm TL; re=18) were significantly smaller than 
were those caught with purse seines (adjusted, mean: 
224.0; ANCOVA test of means: P--6.4, P<0.05). It is not 
clear whether these size differences between studies and 
gear types reflect differences in size selectivity of gears, 
size differences between individuals occupying different 
parts of the water column, or differences in the lamprey 
population over time. 
Fishes with lamprey wounds 
We observed lamprey wounds on 142 individual fishes 
representing 8 species (Table 2, Fig. 3). The highest ab- 
solute number of wounded fish by species occurred in 
non-native A m erican shad (71 wounded fish), followed 
by subyearling Chinook salmon (33), shiner perch (25), 
and Pacific herring (8). Other species with wounds in- 
cluded juvenile steelhead (2), juvenile sockeye salmon 
(1), surf smelt (1), and longfin smelt (1) (Table 2). The 
frequency of lamprey wounds by cruise was highest for 
shiner perch; wounded individuals were observed in 
41% of all cruises in which at least one shiner perch 
