Effects of predation tags on growth and stress response in juvenile rainbow trout Oncorhynchus mykiss

Background: Acid-sensitive predation tags have recently been introduced to fisheries management. These predation tags are designed to help eliminate the observation bias of seeing a moving tag and assuming that it is the original fish. Because of the impact of tagging data on fisheries management decisions, and the labor and cost associated with tagging, it is essential that the behavior, growth, and physiology of tagged fish be similar to that of untagged conspecifics. Thus, the objective of this study was to ascertain the impact of these tags on juvenile rainbow trout Oncorhynchus mykiss physiology and growth after implantation. Twenty approximately 13 g rainbow trout were placed into each of five tanks, with ten surgically implanted with dummy acid-sensitive predation tags and 10 control fish not subjected to surgical procedures. Glucose, hematocrit, tag-retention, survival, and growth metrics were collected for 30 days post-surgery. Results Four tagged fish died while no control fish died. Tag retention was 76%, with tags lost in week 2, 3, and 4. Control fish were significantly longer and gained significantly more weight at the end of the experiment. Hematocrit levels for the tagged fish dropped significantly over the course of the trial and were 30% lower than the untagged control fish at the end of 30 days. Glucose levels were highly variable for both treatments. Conclusions The results of this study indicate the negative impacts of predation tagging on the physiology of juvenile rainbow trout. Of particular concern was the long-term anemia experienced by the tagged fish. Results from predation field trials should be interpreted with caution because the tagged fish are likely at a competitive disadvantage compared to their untagged conspecifics.