Regurgitation rates of intragastric radio transmitters by adult Chinook salmon and steelhead during upstream migration in the Columbia and Snake Rivers

Regurgitation rates for radio tags gastrically implanted into adult salmon Oncorhynchus spp. and steelhead O. mykiss are difficult to estimate in the wild because most fish are never recaptured to allow inspection of secondary tags. During 1996-2000, 9,006 Chinook salmon O. tshawytscha and steelhead with both radio tags and secondary tags were released near Bonneville Dam on the Columbia River (Washington-Oregon), and 1,764 fish were recaptured in midmigration 460 kin upstream on the lower Snake River. Minimum annual regurgitation rates ranged from 0.4% to 10.9% for spring-summer Chinook salmon (pooled rate = 3.0%; n = 838), from 3.5% to 4.3% for steelhead (pooled rate = 4.0%; n = 881), and from 0% to 5.6% for fall Chinook salmon (pooled rate = 2.2%; n = 45). Fish that lost transmitters retained them a median of 7 d (average = 14.1 d) before regurgitation, and a majority of losses occurred in the lower Columbia River. Transmitter retention was improved by placing rubber bands or a ring of surgical tubing around part of each tag.