Estimating Salmon Spawning Escapement Using Capture–Recapture Methods

Abstract

We describe a method of estimating the spawning escapement of coho salmon (Oncorhynchus kisutch) from capture–recapture data. Traditional capture–recapture analyses do not directly provide estimates of escapements; however, we show how simple modifications to the Jolly–Seber method can estimate the total number of fish returning to a river including those that enter and die between sampling occasions. Spawning runs of Pacific salmon were simulated and their escapements estimated using capture–recapture. The performance of the maximum likelihood estimators (MLEs), the censored MLEs, the constrained MLEs, and less-biased estimators in estimating the run sizes and providing estimates of precision were evaluated. Simulation results indicated that constrained MLEs provided the most appropriate estimates of escapement and that standard errors be computed using the large-sample variance formulae evaluated at these estimates. These methods were used to estimate the escapements of coho salmon to a small river on Vancouver Island in 1989 and 1990.

Use of the superpopulation approach to estimate breeding population size: an example in asynchronously breeding birds

“…This parameter includes all animals that enter the population at some time during the entire study period and either survive until the next survey occasion or emigrate or die before they are available to be sampled (Schwarz et al 1993, Schwarz andArnason 1996). The model's calculated gross superpopulation size was originally defined as the total number of organisms that were part of the population of the study location, in the sense that they were present at some time during the period between the first and last sampling occasions (Cooch and White 2007).…”

“…Using the counts from each survey date and the estimated encounter and survival probabilities from each survey date or interval (adjusted for time elapsed between surveys), the number of new entries (''births'') into the population between each consecutive set of surveys can be estimated. This value can then be used to estimate gross entries, which includes new animals that are never available to be detected at a sampling occasion (Schwarz et al 1993, Schwarz andArnason 1996). The gross superpopulation size is estimated in MARK by summing these gross entries between each consecutive set of survey dates, and adding the sum to the estimated number of nests present during the first survey (after Schwarz et al 1993, Schwarz andArnason 1996):…”

“…whereN i is the estimated total number of individuals (or active nests) in the population at sampling occasion i; n i is the number of individuals seen at occasion i; p i is the encounter probability at occasion i;B i is the estimated number of individuals entering the population between sampling occasions i and i þ 1 and available for detection at occasion i þ 1; / i is the survival probability per sampling interval (expressed as a proportion of a week) between occasions i and i þ 1; t i is the time between surveys i and i þ 1 (expressed as a proportion of a week); B Ã i is the estimated gross number of individuals entering the population between i and i þ 1 (including nests that entered and departed between i and i þ 1, estimated assuming a constant recruitment rate over time during the interval, after Schwarz et al [1993] and Crosbie and Manly [1985]);N* is the estimated gross superpopulation size; and k is the total number of surveys.…”

“…This latter issue can arise when individuals are not individually identifiable, and they join and leave surveyed populations at an unknown frequency. This problem has been recognized in colonially breeding birds (Frederick et al 2006), ungulates (Gould et al 2005), and spawning fishes (Schwarz et al 1993), among other taxa, as well as being a common circumstance at migration stopover sites (Williams et al 2002).…”

No evidence for effects of infection with the amphibian chytrid fungus on populations of yellow-bellied toads

“…Bayesian 95% credible intervals in Table 1) modification of the Jolly-Seber model (Schwarz et al 1993, Schwarz & Arnason 1996, which primarily aims at estimating abundances (Pollock et al 1990). One disadvantage is that population sizes are only estimated for the capture occasions (i.e.…”