Coded-Wire Tag Sampling: The Case for Electronic-Field Detection

Abstract

A clipped adipose fin served as an effective external mark indicating presence of a coded-wire tag (CWT) in salmon (Oncorhynchus spp.) from the 1960s until the mid 1990s when hatcheries in the Pacific Northwest began mass marking released fish with an adipose fin clip, but not necessarily a CWT. Since then, many CWT sampling programs of commercial fisheries have transitioned to electronic-field detection, while others are still visual-field only, examining snouts from all adipose-clipped salmon, even those without CWTs. Because some CWT salmon are released from hatcheries without any external marks, visual-field only programs also fail to sample these CWTs. In 2012, we used electronic tag detection at a processing plant in Kodiak, Alaska, to scan 1,201 Chinook salmon (O. tschawytscha) caught as bycatch in the US North Pacific groundfish fisheries in the Gulf of Alaska (GOA). Chinook salmon bycatch were also electronically scanned in partnerships with private industry: 3,713 salmon in the 2013–2016 US rockfish fishery in the central GOA, and 611 salmon in testing of salmon excluder devices in 2013 in the central GOA groundfish fisheries. Electronic-field detection increased CWT recovery rates by 20–24% over visual-field detection of adipose-clipped Chinook salmon, and an estimated 64–74% of adipose-clipped Chinook salmon sampled had no CWTs. Visual-field only CWT sampling programs may unnecessarily process large numbers of untagged, adipose-clipped salmon while also recovering fewer CWTs than comparable electronic sampling programs.