Potential Effects of Acoustic Tag Strength Variation on Detection Probabilities of Migrating Fish: Recommended Measurements Prior to Tagging
Michael C. Melnychuk
Abstract.—A common assumption in acoustic or radio telemetry studies is that tag transmission strength is homogeneous for a particular tag type, which in turn supports the assumption that detection ranges or mark–recapture detection probabilities are homogenous among tagged fish. Variation among tags in acoustic intensity could reduce precision in detection probability estimates that do not account for it, and therefore possibly in the precision of survival or abundance estimates. Simple methods are suggested for quantifying variation in tag strength prior to tagging fish and incorporating these measurements into mark–recapture models. At little extra effort to the researcher, these measurements could explain part of the variation in detection probability estimates and therefore could increase the precision of survival or abundance estimates of migrating fish. This potential source of variation in detection probabilities was investigated in a case study with migrating salmon smolts. An index of tag strength was quantified while coded acoustic tags were activated prior to tagging fish. Detection and survival probabilities were estimated with standard mark–recapture methods for the downstream and early ocean migration. A model that included the tag strength index as an additive covariate of detection probabilities had a reasonable level of support compared to a model without the index, suggesting that this source of variation should not be ignored.