Adult Sockeye Salmon Gastrically Tagged Near Spawning Grounds Exhibit Lower Survival Rates throughout the Spawning Period than Externally Tagged Conspecifics

Abstract

Telemetry is a common tool for studying the behavior and fate of migrating adult Pacific salmon Oncorhynchus spp., yet few field studies have compared behavior and fate associated with different tagging techniques. In this study, adult Harrison River (British Columbia) Sockeye Salmon O. nerka were captured in their natal river near spawning areas, radio‐tagged by gastric insertion or external attachment in the dorsum, and released. Tagging occurred on 5 d spread over 3–8 weeks prior to spawning, thus encompassing fish in varying stages of maturity and freshwater residency. Tagged individuals were monitored over the spawning season by using fixed receiver stations and mobile tracking. The probability of fish moving upstream or downstream of the tagging site within 35 h of tagging was a function of tagging date but not tag type. Tag type significantly influenced fate, with almost twice as many externally tagged fish (41.6%; 42 of 101) surviving to reach spawning areas compared to gastrically tagged fish (22.4%; 21 of 94). The number of active externally tagged fish in the Harrison River system was consistently greater than the number of active gastrically tagged fish that received tags on the same date for four of the five tagging dates. External tag attachment may be a better approach than gastric insertion for studies that tag adult salmon near or on spawning areas.