Abstract
Unmanned aircraft systems (UASs) were tested for counting Chinook salmon ( Oncorhynchus tshawytscha) redds as a more accurate, safer alternative to manned helicopter flights. Counting redds from the helicopter was less expensive and time consuming, but of the total redds counted at selected sites with a UAS, an average (± SD) of only 77% ± 14% was counted from the helicopter. A river-wide census of redds was not possible with a UAS because the study area was too large for the single field crew to survey. Simulation analyses were used to compare stratified random sampling (STRS) and sampling proportional to size (PPS) for estimating annual total redd counts from data collected with a UAS. The STRS estimates were more accurate and precise, whereas the PPS estimates, though biased, had 95% CIs that included the observed redd count more frequently. We strongly recommend that researchers conduct simulation analyses to evaluate alternative survey sampling methods if they are considering replacing census counts made from manned aircraft with counts estimated from data collected with a UAS. We conclude that UAS application reduces the risk inherent to manned aircraft flights, but the reduction in risk can come at the cost of estimates of population parameters that can sometimes be inaccurate and lack 95% CI coverage.
Highlights
Salmon redds can be easy to see because they are relatively large and generally appear as regular- or irregular-shaped oval areas that contrast with the undisturbed river bed when viewed from above (Burner 1951; Dauble and Watson 1997)
In British Columbia, Idaho, Oregon, and Washington alone, there were 24 helicopter and fixed-wing aircraft accidents during the past 20 years associated with natural resource monitoring that resulted in 44 individual fatalities (Transportation Safety Board of Canada (TSBC) 2015; National Transportation Safety Board (NTSB) 2015)
The potential for expanding the use of unmanned aircraft is large, especially when the study areas are intermediate or small in size. It is not a simple matter, to conclude that coupling the two methods is the best alternative for conducting natural resource surveys in large study areas such as the lower Snake River and its tributaries
Summary
Salmon redds (i.e., nests) can be easy to see because they are relatively large and generally appear as regular- or irregular-shaped oval areas that contrast with the undisturbed river bed when viewed from above (Burner 1951; Dauble and Watson 1997). Depending on the study area size, redds can be counted by walking, from rafts or boats, or by fixed-wing aircraft or helicopters (Gallagher et al 2007). Other natural resources, made from manned aircraft can be inaccurate when the density of the subjects being counted is high. Total counts of Chinook salmon (Oncorhynchus tshawytscha) redds made along the Columbia River from fixed-wing aircraft were two to three times lower than counts made from photographs of the spawning sites surveyed. In British Columbia, Idaho, Oregon, and Washington alone, there were 24 helicopter and fixed-wing aircraft accidents during the past 20 years associated with natural resource monitoring that resulted in 44 individual fatalities (Transportation Safety Board of Canada (TSBC) 2015; National Transportation Safety Board (NTSB) 2015). The inaccuracy and risk are two reasons for testing alternative methods for counting redds and other natural resources from the air
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