Performance of passive integrated transponder tags and radio tags in determining dam passage behavior of adult chinook salmon and steelhead

Passage of adult Pacific salmon Oncorhynchus spp. and steelhead O. mykiss at dams in the Columbia River basin has historically been determined by visual fish counts and radiotelemetry. Increasingly, however, passive integrated transponder (PIT) tags are being used for adult salmonid research and monitoring. Although both radiotelemetry and PIT tag technology provide accurate and cost-effective data under certain circumstances, neither alone meets all needs, and managers need to understand the strengths, weaknesses, and biases of each method. To evaluate the two tagging methods, we tagged over 3,200 adult Chinook salmon O. tshawytscha and steelhead during 2002 and 2003 with both a radio transmitter and a PIT tag as the fish migrated upstream past Bonneville Dam. We compared their performance in measuring upstream passage and fallback (i.e., downstream passage) behavior at each of four main-stem hydropower dams. Counts of fish passage at dams based on data from double-tagged fish were similar between methods (largest difference was 5.1%), but the two methods produced less consistent results for fallback behavior. Visual count escapement estimates based on PIT tags were 1% higher on average than those based on radio tags, suggesting there would be little impact on the management of salmonid populations with a switch from radiotelemetry to PIT tag systems as the primary data source. We also analyzed a broader group of PIT-tagged (but not radio-tagged) fish to simulate the analysis one could expect from PIT tag data without radiotelemetry validation. When compared with PIT tag data from double-tagged fish, data from the PIT tag-only fish showed slight differences in the timing of peak passage. These differences were due, at least in part, to the fact that the fish in each group comprised differing proportions of fish from multiple populations, highlighting the importance of having representative stocks of fish PIT-tagged as juveniles.