Digital Hydrography Underestimates Stream Length and Leads to Underestimates of Trout Population Size

Abstract

Stream length is measured for many fisheries management applications. Characteristics of populations and habitats measured at field sites are commonly generalized to unsampled areas using estimates of stream length or stream network length. There are many ways to measure stream length, but map-based stream length measurements are commonly used in fisheries applications even though they are known to be biased. We evaluated how length of headwater streams in Arizona may be underestimated by the National Hydrography Dataset and how that bias influences streamwide abundance estimates for adult Apache Trout Oncorhynchus apache. As expected, stream lengths measured using National Hydrography Dataset flowlines underestimated true length revealed by National Agricultural Imagery Program imagery on average 11.1% (SD = 4.1%), and this bias was higher in meadow versus forested habitats. The observed bias led to streamwide estimates of adult Apache Trout abundance that were only 88% on average (SD = 5%) of those made with more realistic imagery-based stream measurements. As we have shown, high-resolution imagery, now widely available, can be used to assess and quantify stream length bias, and we conclude that it is important to assess whether this bias has the potential to negatively impact important fishery management decisions.

UPRLIMET: UPstream Regional LiDAR Model for Extent of Trout in stream networks

“…In addition, the usefulness of fish or habitat observations, regardless of source, depends on an adequate characterization of the stream network that provides a spatial context for the observations. For example, underestimating stream length has been shown to lead to underestimates in population sizes of an endangered trout (Apache Trout O. apache) 20 .…”

Resiliency of Apache Trout habitats in a warmer and drier climate future

“…Adaptative potential should be considered in concert with the reality that many populations reside in small habitat patches (Hemstrom et al 2022). Small patch size can constrain long‐term viability and is one of the trade‐offs that comes with isolation management (Fausch et al 2009), but our identification of climate‐resilient habitats incorporated the patch size of suitable habitat as a driver of long‐term persistence (patch sizes are also known to be biased low; Dauwalter et al 2022). Habitats with high resiliency could also be the focus of active habitat management, such as riparian vegetation management and habitat restoration (Williams et al 2015; Baker and Bonar 2019), to improve or ensure their climate resiliency into the 2080s and beyond (Angel et al 2018; Overpeck and Bonar 2021).…”

Applying High-Resolution Satellite and UAS Imagery for Detecting Coldwater Inputs in Temperate Streams of the Iowa Driftless Region

“…This study utilized the IA DNR's stream polygon layer for Iowa. Sometimes these layers are not developed using the highest resolution base imagery available and they are therefore prone to underestimating the actual stream length, especially in headwater areas where the stream width is narrow and significant canopy cover exists [54]. In some cases, small coldwater spring runs or seeps may not even be designated as a stream segment.…”