Abundance from Mark-Recapture Data

Ugly River Brown Trout

Use single census mark-recapture techniques to estimate abundance of Brown Trout in a stretch of the Ugly River (New Zealand).

Derek H. Ogle


Mar 8, 2019


Feb 13, 2023

Young and Hayes (2001) described a study where Brown Trout (Salmo trutta) in several rivers were captured by experienced fly fishers, tagged at the base of the dorsal fin with a colored dart tag, and then observed by divers drifting through the sample area two days later. In the Ugly River, 43 trout were marked, 123 fish were observed by the divers, and 16 fish observed by the divers were tagged.

  1. Assign one of these symbols (\(N\), \(M\), \(n\), \(m\)) to each of the numerical results from this study.1
  2. Construct a population estimate, with 95% confidence interval, for the Brown Trout in this section of the Ugly River. Carefully interpret the results.
  3. Which method did you use to construct the confidence interval? Explain why you chose that method.

1 Symbols are as used in Ogle (2016).

Solution Code:

Available upon request to students not in a class. Contact fishR maintainers.


Ogle, D. H. 2016. Introductory Fisheries Analyses with R. CRC Press, Boca Raton, FL.
Young, R. G., and J. W. Hayes. 2001. Assessing the accuracy of drift‐dive estimates of brown trout ( Salmo trutta ) abundance in two New Zealand rivers: A mark‐resighting study. New Zealand Journal of Marine and Freshwater Research 35(2):269–275.