Population Estimation of Larval Lamprey from the Pacific Northwest Using Depletion and Mark-Recapture Estimates

Pacific lamprey Entosphenus tridentatus populations have declined across their entire range and the status of resident lampreys Lampetra spp. in the Pacific Northwest is unknown.  Much basic information on the biology, ecology, and distribution of these species is poorly understood and monitoring and evaluations are necessary.  Larval lampreys are cryptic and burrow into the sediment which further complicates understanding occupancy and abundance in tributaries.  Traditional techniques developed for stream fishes may not be adequate for larval lamprey. 

We used maximum-likelihood depletion estimators and mark-recapture (M-R) techniques to estimate larval lamprey population sizes.  Depletion estimates were made using multi-pass electrofishing techniques in stream net-pens and blocked stream reaches with known numbers of larval lamprey.  M-R estimates (modified Peterson-Lincoln) were also made in blocked stream reaches.  Our goal was to examine the assumptions and estimate accuracy of the models and compare the accuracy and utility of the techniques.

Depletion methods commonly underestimated actual population sizes.  Maximum-likelihood models failed when there was not a descending pattern of catch, there was complete catch in the first pass, or there were zero catches.  Model accuracy in varied in net pens (mean=0.85) and blocked reaches (mean=0.44).  In addition, the actual abundance was outside of the 95% confidence bounds for the depletion estimate in many trials (48% in net-pens, 100% in blocked reaches).  Capture efficiencies were underestimated (mean=83% in net-pens, 100% in blocked reaches) and were heterogeneous (an assumption violation).  Measured capture efficiency ranged 43-100%. 

Mark-recapture estimates in blocked stream reaches were consistently more accurate than the depletion methods in blocked reaches.  M-R model accuracy ranged 45-128%.  M-R models underestimated the actual number in all but two cases. 

Population estimates were relatively more accurate using mark-recapture methods compared to depletion methods, although accuracy was variable and generally poor.  A disadvantage of M-R estimates is the increased level of effort required.  Stream reaches must be visited multiple times, reaches blocked (or fish movement assumed negligible), and fish tagged with specialized techniques.  The utility of depletion estimates is rapid completion in one visit.  We are currently investigating Bayesian depletion models which allow heterogeneous cactchability and may provide more accurate estimates.  Alternative techniques using occupancy models have shown promise for inferring abundance of fishes.  Further studies of these techniques are warranted.  Our results indicate that mark-recapture techniques produce more accurate estimates of larval lamprey abundance and that depletion estimates do not, necessarily, produce accurate estimates.