Use of Carcass Recovery Data in Evaluating the Spawning Distribution and Timing of Spring Chinook Salmon in the Chiwawa River, Washington

Abstract

Spawning ground surveys are routinely conducted to assess status and trends for many populations of Pacific salmon Oncorhynchus spp. Biological data from carcasses recovered during spawning ground surveys are used for various stock assessment purposes but may also be useful in comparing the spawning distribution and timing of hatchery- and naturally produced fish. The objective of this study was to measure the distance between redd and carcass locations of spring Chinook salmon O. tshawytscha in the Chiwawa River and assess whether carcass location adequately represents spawning location. We also estimated the redd residence time (d) of hatchery- and naturally produced spring Chinook and compared the spawn timing using both redd and carcass data for hatchery- and natural-origin fish. We determined the redd location, spawn timing, and carcass location of individual hatchery- and naturally produced spring Chinook salmon during spawning ground surveys using passive integrated transponder tags implanted during the adults' migration to the spawning grounds. Differences in mean carcass drift (the distance between redd location and carcass location) were detected between male (4,465 m) and female (150 m) spring Chinook salmon (P < 0.003) but not between hatchery- and naturally produced females (P = 0.32). The mean spawn date and the estimated spawn date derived from carcasses were not significantly different for either hatchery- or natural-origin spring Chinook salmon (P > 0.05). We concluded that the use of carcass data from female spring Chinook salmon to estimate the spawning location and timing is appropriate when surveys are conducted on a consistent weekly schedule throughout the entire spawning period.