How many fish are in the river?

Thu, 05 Feb 2026 00:00:00 +0000

On February 28th, 2025 Joe Thorley, presented on “How many trout are in the river?” Rethinking the logic of riverine population abundance assessments at the Alberta Native Trout Science Workshop & Gathering II

The abstract was as follows

“Obtaining a reliable answer to the question of ‘how many trout are in the river?’ plays an important role in monitoring threats to fish populations. Although approaches to answering this question are often formulated in terms of statistical procedures, we present the underlying logic and rethink some misconceptions. If a method was 100% efficient at capturing fish and was applied to the entire river, the population abundance would simply be the total number of fish caught. However, typically, the capture efficiency is less than 100% and can be highly variable which necessitates methods to estimate the efficiency. Currently, depletion-removal methods estimate the number of fish based on the rate of decline of the catch under the often-erroneous assumption that the capture efficiency does not change between passes. Mark-recapture makes the more reasonable assumption that marked and unmarked fish have the same probability of recapture. As a method can rarely be applied to the entire river it is also necessary to estimate the number of fish at the unsampled sites. Index sites with high fish densities do not allow the number of fish at the other sites to be estimated and may even be decoupled from changes in the population abundance. At the other extreme totally random site selection introduces unnecessary uncertainty. A stratified approach requires more thought but produces more reliable estimates. Fish densities are often calculated in terms of the number of trout by wetted area but this assumes that doubling the stream width is equivalent to doubling the stream length and means that trout densities change with discharge. Lineal fish densities, in contrast, facilitate comparisons within and among systems while also allowing the effect of stream width to be estimated. We provide examples from the literature and via thought experiments of both logically valid and invalid estimates.”

The presentation is available here.

Study Design | Poisson Consulting

How many fish are in the river?