Analyses for Effect of Survey Week and Reach in Mark-recapture Studies of Metolius River Kokanee Oncorhynchus nerka
Mark-recapture/resight procedures are used to estimate the annual spawning escapement of kokanee Oncorhynchus nerka migrating from Lake Billy Chinook into the Metolius River, Jefferson County, Oregon. A sample of pre-spawning adults are marked with one or two plastic anchor tags, and several weeks later the fish are (re)sighted during three successive spawning ground surveys of the Metolius River, spaced two weeks apart. The spawning area is subdivided into multiple reaches, and a survey consists of a single pass through each. Data for the number of fish observed, and the number that possessed one or two tags, are summed across survey reaches and week. Initially the summary data were entered into standard Petersen estimators to derive population abundance. Recently, a likelihood model (MRmix; Hyun et al. 2012) has been used. Both analytical approaches assume that recapture/resighting is a single event, involving a random sampling of the population. Summing of the data across reaches and survey weeks assumes that neither time of the survey (week) nor location (reach) has a significant effect on the proportion of resighted tagged fish among the fish observed. To test the validity of this assumption, two-way ANOVAs were performed on the data for 2007 to 2012. A significant effect of reach was never observed. A significant effect of survey week was detected in two of the six years, and an effect was nearly significant (p>0.05 but <0.10) in two additional years. Average percent of marked fish in these four years tended to decline from the first to third week, inferring that loss of tags continued over the period of the spawning surveys. Nonetheless, a resulting bias in the point estimate of population abundance is likely small and within the confidence limits for the estimate based on the pooled data. In our judgment, it remains reasonable to continue to sum the data across survey weeks and reaches for the purposes of estimating abundance of Metolius River kokanee.
Galbreath, P.F. and S-Y. Hyun. 2013. Analyses for effect of survey week and reach in mark-recapture studies of Metolius River kokanee Oncorhynchus nerka. Columbia River Inter-Tribal Fish Commission Technical Report 13-07. Portland, OR. 15p.