Abundance from Depletion Data

European Bullhead in England

Estimate abundance of European Bullhead in a headwater stream in England.

Derek H. Ogle


Dec 23, 2022


Feb 13, 2023

Pritchard et al. (2021) used three methods to determine the abundance of benthic European Bullhead (Cottus gobio) in a stony headwater stream in Northern Yorkshire, England. In one part of their study they set block nets at the up- and down-stream ends of a site called “Footbridge.” The researchers then used a backpack electrofishing unit deployed from the lower to the upper limit of the site to capture and remove all Bullheads from the site (the removed fish were placed downstream below the lower block net). After a short break, this process was repeated two more times. A total of 138, 55, and 48 Bullheads were removed on the successive electrofishing passes. Use this information to answer the questions below.

  1. What is the best estimate and 95% confidence interval for the population size? How does your estimate compare to the estimate published in Pritchard et al. (2021)?
  2. What is the best estimate and 95% confidence interval for the probability of capture during each electrofishing pass.
  3. Pritchard et al. (2021) estimated a “capture efficiency.” Use your results to show and then describe what they mean by this.
  4. Is there evidence that the probability of capture differed after the first pass?


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Pritchard, E. G., D. D. A. Chadwick, M. A. Chadwick, P. Bradley, C. D. Sayer, and J. C. Axmacher. 2021. Assessing methods to improve benthic fish sampling in a stony headwater stream. Ecological Solutions and Evidence 2(4):e12111.