Utility of SAR and Forward-Looking Infrared (FLIR) Imagery for Identification of Salmon Spawning Habitat During Winter in a Large, Glacial River
By Lisa Wirth, Alaska Satellite Facility
The large, glacial rivers that characterize the Alaskan interior are extraordinarily dynamic: glacial flow regimes, combined with heavy sedimentation, create a complex habitat template for the variety of fish species that use these systems. The remoteness and size of interior rivers present difficulties for characterizing the availability, dynamics, and use of critical habitats. Traditional methods of mapping and quantifying riverine habitats are typically limited to a small spatial and temporal extent (e.g., foot-based surveys conducted on reach scales with a singlesite visit), and fail to capture important multi-scale ecological and physical processes that contribute to fish productivity (Geist and Dauble, 1998; Baxter and Hauer, 2000; Fausch, et al., 2002). However, by integrating traditional methods of habitat data collection with the use of remote-sensing technologies, it is possible to characterize fish habitat at intermediate spatial and temporal grains (e.g., river segments, years to decades), scales that are most relevant for fish populations (Fausch, et al., 2002; Torgersen, et al., 2006).
In winter, ice-free areas (i.e., open water) in the Tanana River are thought to be a result of upwelling water that prevent surface freezing, even when ambient temperatures reach −50º C. Tanana River fall chum-salmon spawn in late October and early November, and thereby have limited time for egg incubation (Salo, 1991). Warm, upwelling water would shorten this incubation time, allowing for successful fall spawning. The potential importance of upwelling means identifying open-water areas as an important step for chum-salmon management and research on the Tanana River. However, assessing the location and persistence of these areas through time is a particular challenge for this large and remote system. Although the physical process of upwelling and biological process of fish incubation is thus far unobservable using remote-sensing techniques, areas of open water can be identified using synthetic aperture radar (SAR) imagery (Figure 1). This phenomenon can be observed and monitored through time at intermediate spatial and temporal scales. The goal of this study was to evaluate the potential for SAR imagery to monitor the presence and persistence of openwater areas that are spawning habitat for fall chum salmon in the mainstem Tanana River and model habitat selection from spatial distributions of individuals through the use of radiotelemetry.
RADARSAT-1 images, collected from C-band (5.7-cm wavelength) taken from the standard-beam mode, acquired by the Alaska Satellite Facility (ASF), collected in March 1997, 2000-2003, 2005, 2006, and 2008, proved useful for identifying the presence and persistence of open water through time. Results from the stacked layering of the standard-beam data showed spatially persistent open-water areas in the Big Delta, Alaska, region. Results showed that nearly half of the tagged spawning salmon chose to spawn in mapped open-water areas that accounted for less than 5% of the habitat that was available, demonstrating the preferential use of these areas as spawning habitat. FLIR images of persistent open-water areas mapped from SAR imagery were collected in November 2009 to show surface-temperature patterns and localized hotspots. This allowed for the identification of areas with strong groundwater inputs and complex thermal heterogeneity, creating a suitable thermal environment for egg incubation (Figure 2).
Fish populations carry out life-history events at intermediate spatial and temporal scales, and this scale is said to be the most difficult to survey (Fausch, et al., 2002). Previously, satellite imagery was used to study stream habitat at the basin scale and only to analyze one-dimensional channels that do not show habitat features required by fish (Fausch, et al., 2002). Using SAR imagery, identified in spatially and temporally persistent ice-free areas, is important for fall chum-salmon spawning at the intermediate scale. The information was combined with a more focused intermediate analysis using FLIR imagery that identified thermal heterogeneity within persistent open-water areas. This approach created an opportunity for a view of a system at a watershed scale – the scale for which lands are managed and salmonids carry out life-history events (Rieman and Dunham, 2000). Technological advancements and the use of remote sensing, in part, circumvent spatial and temporal constraints in traditional freshwater ecological studies for assessing fish habitat.
Information on obtaining SAR data is available online at http://www.asf.alaska.edu/. This research was funded through he Arctic Yukon Kuskokwim Sustainable Salmon Initiative, the Alaska Department of Fish and Game-Commercial Fisheries Division, and the Tanana Chiefs Conference.
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