The frequency of occurrence (FO) of lampreys in the Columbia River estuary varied in both studies by species, age class, and year and in CREDDP, by gear type (Table 1).
The density of western river and Pacific lamprey juveniles and adults exhibited clear seasonal patterns that were consistent across 3 data sets: the data set from EPS (purse seine) study and 2 subsets of data from CREDDP (trawl and purse seine) study (Fig 2).
The spatial distribution of both western river and Pacific lampreys within the Columbia River estuary was evaluated with data from the many CREDDP stations (Fig.
Length and weight data from the CREDDP showed the same patterns: Pacific 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.
Western river lamprey caught in the CREDDP (regardless of gear) were longer (adjusted mean: 206.3 mm TL; w=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 [greater than or equal to] 3.8, P<0.05).
The catch of each lamprey group (western river lamprey and Pacific lamprey juveniles and adults) in the CREDDP study was different with gear type, and the variation between groups due to gear type was mirrored by purse seine catches in the EPS study.
This habitat preference may be responsible for the absence of lampreys during 2001-2002 in the EPS study, when depth at sampling sites was shallower than depths at sites during 2003-2012, and for low catches (4 individuals) in shallow fyke nets and beach seines during the CREDDP study.
In addition, in the CREDDP, lampreys and other fishes were sampled only during fall and winter of a single year; therefore we were unable to estimate interannual variability in the timing of winter migrations of Pacific lamprey.