Under pressure: Comparing in situ and boat tagging methods using time-to-event analyses

With the increase in telemetry studies over the past decade, improving understanding of how different tagging methods influence the probability of presence in a receiver array is important in maximizing the resulting data. Disappearance from the array may be due to mortality from surgery complications, tag loss, predation, or emigration. Internally tagging fish on a boat can cause barotrauma injuries, increased stress from prolonged handling times, or predation after fish have been released back into the water. Conducting in situ internal acoustic tagging at depth of capture removes barotrauma stresses and simplifies the release method, which may improve fish survival and decrease risk of disappearance from the array. In this study, we used 8 years of acoustic tagging data to determine if the tagging method (in situ versus on the boat) influenced the likelihood of a fish being detected 4 and 6 days after the tagging event. At 6 days after tagging, Kaplan–Meier survival curves revealed that the probability of presence for fish tagged on the boat was 66% compared to 90% for fish tagged in situ. Tagging method was the only variable to significantly affect probability of presence based on Cox proportional hazards models, with fish tagged in situ ~ 75% less likely to disappear from the array compared to fish tagged on the boat at both 4 and 6 days after tagging. Examining tagging methods separately, handling time only marginally influenced probability of presence of boat-tagged fish and no variables had a significant effect on probability of presence of in situ tagged fish. In this study, tagging method was the only variable to significantly affect the probability of presence for internally tagged fish. Other factors had little to no influence, but correlation of variables limited what factors could be included in the models. Implanting internal acoustic tags in situ is not a practical method for every species and for every environment, but given the increased probability of presence demonstrated here, we strongly suggest it be considered where applicable.