Heterogeneity And Bias In Abundance Estimates Of Outmigrating Chinook Salmon In The Chena River, Alaska

Abstract

The objective was to examine bias due to heterogeneity in capture probability (p) in an abundance estimate for chinook salmon (Oncorhynchus tschawytscha) outmigrants in the Chena River, Alaska. A higher proportion of day-marked fish (21/636 = 0.0330) compared to night-marked fish (17/1724 = 0.0098; p $<$ 0.0001, $\alpha$ = 0.05) was recaptured at the lower site in a Cormack-Jolly-Seber experiment with upper, middle and lower sites. Heterogeneity was also likely at the middle site between upper site-marked and unmarked fish. Simulations with heterogeneity confined to the middle and lower sites (i.e., due to inadequate mixing) caused small bias ($<$2.5%) in the upper site abundance estimate. With heterogeneity at all three sites (a subpopulation effect), the upper site estimate had 22.9% to 29.3% negative bias. Because heterogeneity observed in the Chena was probably due to inadequate mixing (related to daytime trap evasion), bias in the upper site estimate was probably small. <p>