Use of Aerial Photography to Monitor Fall Chinook Salmon Spawning in the Columbia River

Abstract

This paper compares two methods of enumerating the redds of chinook salmon Oncorhynchus tshawytscha and their application to the monitoring of spawning activity. In 1994 and 1995, aerial photographs of fall chinook salmon spawning areas in the Hanford Reach of the Columbia River were digitized and mapped by means of geographical information systems techniques as part of annual population assessments. The numbers of redds visible in these photographs were compared with the counts obtained from visual surveys made from fixed-wing aircraft. The proportions of the total redds in each of five general survey areas were similar for the two enumeration techniques. However, the total number of redds based on aerial photographs was 2.2 times as high as that observed visually in 1994 and 3.0 times as high in 1995. The divergence in redd counts was most evident near the time of peak spawning activity, when the number of redds in individual spawning clusters exceeded 500. Aerial photography thus improved our ability to determine the number of visible salmon redds and to quantify habitat use.