216 
Fishery Bulletin 113(2) 
Table 1 
Sampling effort, mean depths of sampling, and percent frequency of occurrence (%) and 
total numbers (n) of Pacific lamprey ( Entosphenus tridentatus) adults and juveniles and 
western river lamprey ( Lampetra ayresii) caught in the lower Columbia River estuary by 
year and gear type. Sampling occurred during 1980-1981 as part of the Columbia River 
Estuary Data Development Program (CREDDP) and in 2001-2012 during the Estuary 
Purse Seine (EPS) study. The 2 gear types used were purse seine (P) and trawl (T). 
Frequency of occurrence 
Year and 
net type 
Number 
of hauls 
Depth (m) 
Pacific lamprey (n) 
Adults Juveniles 
Western river 
lamprey (n) 
CREDDP 
1980-T 
241 
11.9 
9.1 (1) 
18.2 (21) 
54.5 (11) 
1981 -T 
153 
12.2 
0.0 (0) 
66.7 (12) 
66.7 (7) 
1 980 -P 
171 
14.0 
27.3 (5) 
18.2 (2) 
54.5 (22) 
1981-P 
109 
14.0 
16.7 (1) 
0.0 (0) 
16.7 (2) 
EPS 
2001-P 
79 
5.1 
0.0 (0) 
0.0 (0) 
0.0 (0) 
2002-P 
78 
5.1 
0.0 (0) 
10.0 (1) 
0.0 (0) 
2003-P 
54 
7.5 
0.0 (0) 
0.0 (0) 
30.8 (7) 
20Q6-P 
13 
7.5 
0.0(0) 
0.0 (0) 
33.3 (5) 
2007-P 
91 
9.4 
0.0 (0) 
0.0 (0) 
42.9 (6) 
2008-P 
95 
8.7 
0.0 (0) 
0.0 (0) 
62.5 (11) 
2009-P 
102 
9.1 
20.0 (2) 
0.0 (0) 
10.0 (3) 
2010-P 
98 
8.9 
0.0(0) 
11.1(1) 
22.2 (6) 
2011-P 
125 
9.0 
0.0 (0) 
0.0 (0) 
9.1(1) 
2012-P 
85 
9.0 
0.0 (0) 
0.0 (0) 
33.3 (5) 
DP data only) of western river and Pacific lampreys 
caught in the Columbia River estuary, and 2) we docu- 
mented the size and seasonal abundance of fishes with 
lamprey wounds and their co-occurrence with lampreys 
(EPS data only). 
Lamprey abundance and size Investigation of the sizes 
of Pacific lamprey revealed 2 clearly separated groups, 
which we designated as juveniles (<160 mm TL) and 
adults (>400 mm TL), according to reported sizes for 
each age class in the literature (Beamish, 1980; Far- 
linger and Beamish, 1984; Kostow 1 ). Because the 2 
groups of Pacific lamprey did not overlap in size and 
we expected their use of the estuary to differ, we ana- 
lyzed smaller juveniles and larger adults separately as 
different age classes. In contrast, one continuous size 
distribution was observed for western river lamprey, 
which were treated as a single age class. This latter 
group likely included individuals that were moving 
downstream (juveniles) and upstream (adults), but 
there was no clear break in the size distribution to 
indicate where to differentiate between juveniles and 
adults. 
To determine variation in the presence of western 
river lamprey and juvenile and adult Pacific lamprey 
in the estuary, we first estimated frequency of occur- 
rence (FO) for each species and age class for each 
year as the total number of cruises in which at least 
one lamprey was caught, divided by the total number 
of cruises conducted each year with each gear type, 
expressed as a percentage. For CREDDP data, each 
month of sampling was treated as a cruise, and, for 
EPS data, as described previously, each sampling trip 
was considered a cruise. 
To document seasonal abundance, we calculated 
mean monthly density (individuals/ 10,000 m 2 ) as the 
number of lamprey of each species, and of age class 
for Pacific lamprey, caught each month by each study 
or gear type, divided by effort and averaged across all 
years. Effort was calculated as the number of sets con- 
ducted each month multiplied by the area swept during 
each set by gear type. Because we wanted to include 
lampreys caught during nonquantitative tows of the 
EPS study (/i=14) in our density estimates, we needed 
to estimate the area covered during tows. We did so by 
calculating the ratio of the number of all fish caught in 
tows to the number of all fish caught in quantitative 
round hauls made immediately before or after the tow. 
For tows in which at least one lamprey was caught, 
this ratio averaged 3.1; therefore, tows were estimated 
to encompass 5927 m 2 (3.1x1912 m 2 ), when using the 
longer EPS purse seine. Estimates of density should be 
interpreted with caution, however, because all density 
estimates were based on the assumption that lampreys 
of both species and age classes are equally vulnerable 
to all net types — an assumption that is unlikely to be 
