Use of PIT Technology to Estimate Sockeye Salmon Escapement in the Wenatchee River Basin

Biologists have relied on visual observations to calculate sockeye salmon Oncorhynchus nerka escapement in the Wenatchee Basin; however, high turbidity levels have an unquantified influence on the accuracy these estimates.We evaluated escapement using passive integrated transponder (PIT) tags and mark-recapture methodology in 2009 and 2010 as an alternative method without visual bias. Detection arrays were installed on both spawning tributaries above Lake Wenatchee - the Little Wenatchee and White rivers. The overall probability of detection at these sites was 0.992 and 0.900, respectively. Over 1,000 PIT-tagged adults were included in analyses each year. Total spawning escapement above Lake Wenatchee was 14,452 adults in 2009 and 21,604 adults in 2010, with precision of the estimate to each tributary better than ± 0.025 SE both years. Results indicate that the visual-based spawning surverys underestimated the proportion of escapement in the White River both years, and suggest that PIT technology may provide a more accurate approach to estimating spawning escapement in turbid systems.