Growth, survival, and tag retention of steelhead trout (Oncorhynchus mykiss) and its application to survival estimates

Abstract

Steelhead trout, Oncorhynchus mykiss, are known to expel acoustic tags which can negatively bias population survival estimates. Tag burden may also affect the development and behavior of smolts, thereby impacting the results of population and behavioral studies. We monitored the growth, condition, and tag expulsion rate of steelhead trout in similar-sized individuals and used these data to adjust survival rates from an acoustic telemetry study conducted in the Sacramento River. Eighty fish were surgically implanted with tags—40 with cylindrical tags of 9 mm diameter and 21 or 24 mm length (V9, Vemco Ltd) and 40 with a 7 mm diameter and 20 mm length tag (V7, Vemco Ltd)—to examine the impact of tag size on peritoneal retention and survival rate of juvenile steelhead trout. A total of 20 % (16/80) of all tags were expelled by smolts during the 143-day study. Ten V9 tags were expelled between day 18 and day 66. Six V7 tags were expelled between day 21 and day 143. A statistical difference was found for retention rate by surgeon even though the surgeons were of equal experience and received the same training. There were no significant differences in the tag retention rate in relation to the tag/body weight ratio, or in growth (weight or fork length) among the control, V7 or V9 treatment groups over the duration of the study. All individuals survived throughout the experiment. Two methods were used to adjust the survival estimates of an acoustic telemetry data set from the Sacramento River based on the tag retention study. First, a simple individual censorship approach in Program MARK was utilized and next ATLAS, a software program designed to compensate for bias in survival estimates caused by tag failures was used. The results of the adjusted survival estimates were not significantly different from the unadjusted rates suggesting that it may be more important to focus on improving surgical techniques to reduce tag expulsion rather than adjusting survival estimates dependent on the study. The surgical techniques utilized in this study did not have significant impacts on the growth rates of either of the tag treatment groups compared to the control. However, tag retention was an issue regardless of the size and weight of the implanted tag and the size of the steelhead.