Observer error structure in bull trout redd counts in Montana streams: Implications for inference on true redd numbers

Transactions of the American Fisheries Society
By: , and 



Despite the widespread use of redd counts to monitor trends in salmonid populations, few studies have evaluated the uncertainties in observed counts. We assessed the variability in redd counts for migratory bull trout Salvelinus confluentus among experienced observers in Lion and Goat creeks, which are tributaries to the Swan River, Montana. We documented substantially lower observer variability in bull trout redd counts than did previous studies. Observer counts ranged from 78% to 107% of our best estimates of true redd numbers in Lion Creek and from 90% to 130% of our best estimates in Goat Creek. Observers made both errors of omission and errors of false identification, and we modeled this combination by use of a binomial probability of detection and a Poisson count distribution of false identifications. Redd detection probabilities were high (mean = 83%) and exhibited no significant variation among observers (SD = 8%). We applied this error structure to annual redd counts in the Swan River basin (1982–2004) to correct for observer error and thus derived more accurate estimates of redd numbers and associated confidence intervals. Our results indicate that bias in redd counts can be reduced if experienced observers are used to conduct annual redd counts. Future studies should assess both sources of observer error to increase the validity of using redd counts for inferring true redd numbers in different basins. This information will help fisheries biologists to more precisely monitor population trends, identify recovery and extinction thresholds for conservation and recovery programs, ascertain and predict how management actions influence distribution and abundance, and examine effects of recovery and restoration activities.

Study Area

Publication type Article
Publication Subtype Journal Article
Title Observer error structure in bull trout redd counts in Montana streams: Implications for inference on true redd numbers
Series title Transactions of the American Fisheries Society
DOI 10.1577/T05-129.1
Volume 135
Issue 3
Year Published 2006
Language English
Publisher Taylor and Francis
Contributing office(s) Northern Rocky Mountain Science Center
Description 12 p.
First page 643
Last page 654
Country United States
State Montana
Other Geospatial Goat Creek, Lion Creek, Swan River
Online Only (Y/N) N
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
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