Combining Field Data With Computer Simulations to Determine a Representative Reach for Brook Trout Assessment

Abstract

Fisheries biologists often use backpack electrofishing to sample stream fish. A common goal of sampling is to estimate density and/or biomass to make inferences about the status and trends of fish populations. One challenge when estimating population size is determining an appropriate site or reach length to sample. In this study, we empirically determined the required length of stream that needs to be sampled, assuming the study design is one site per stream, in order to achieve a desired level of accuracy for brook trout density and biomass estimates in Pennsylvania headwater streams. Long sample reaches (600 m) were chosen on seven first to third order streams and these sites were broken into twelve 50-m subreaches. Each subreach was sampled by removal electrofishing techniques until either five electrofishing passes were completed or no brook trout were captured. The total density and biomass of brook trout over all 50-m subreaches was considered the “true” density and biomass for the entire reach. We then performed computer simulations in which various numbers of 50-m subreaches were randomly selected and catches from each subreach were summed within the first three electrofishing passes to simulate removal sampling of site lengths ranging from 50 to 550 m. Population estimates were made using a removal estimator and density and biomass were calculated using various stratification schemes based on fish age and size. Estimates of density and biomass were then compared to the true values to assess the possible range in bias of estimates for a given reach length. Results from our simulations suggest a 200- to 250-m-long or a 400- to 450-m-long stream reach or site is needed to estimate brook trout density and biomass within 50% and 25%, respectively, of the true density and biomass. This information and our methodology will be valuable to fisheries managers in developing standardized protocols for assessing trout populations in small streams.

Spatiotemporal Stability Patterns of Brook Trout Abundance and Implications for Stream Research and Monitoring

“…Our results showed that estimating the population or biomass with increasing precision required an extensive amount of sampling, regardless of how sections were chosen. In nearby Pennsylvania streams, 450 m of stream was required to effectively estimate Brook Trout populations to within 25% of the census size (Sweka et al 2012). Our results also agree with those of Bohlin et al (1989), suggesting that extensive survey is required to estimate trout abundance with enhanced precision.…”

“…The patchy distributions of stream fishes and their habitats may result in reaches that insufficiently characterize a population (Ensign et al 1997). Increasing the total survey area reduces extrapolation errors (Sweka et al 2012) but at the expense of additional cost. Using randomization to select reaches has become an established approach (Klauda et al 1998, and others).…”

Influence of Seasonal Extreme Flows on Brook Trout Recruitment

“…Fish sampling.-On each study stream, a 400-m study reach was established and multiple-pass backpack electrofishing was conducted to sample fish in the summer (July and August) of each year from 2010 to 2019. We chose a 400-m study reach because Sweka et al (2012) found this reach length to be representative in estimating Brook Trout density and biomass within 25% of the true density and biomass. Electrofishing proceeded in an upstream direction with a two-person crew; one person operated the backpack electrofisher equipped with a handheld anode and rat-tail style cathode and handled a dip net, while the other person handled a dip net and a bucket to hold captured fish.…”

“…2006), but sampling of such a large reach as we did (400 m) ensured that large numbers of Brook Trout were collected and that this bias was minimized (Sweka et al. 2012).…”

“…Population estimates were converted to density estimates by dividing the population estimates by the area sampled (400-m stream length × mean wetted width) and were expressed as number per 100 m 2 . Removal estimators of abundance are known to be negatively biased, especially when small numbers of individuals are collected (Riley and Fausch 1992;Sweka et al 2006), but sampling of such a large reach as we did (400 m) ensured that large numbers of Brook Trout were collected and that this bias was minimized (Sweka et al 2012).…”