Comparison of Electrofishing and Snorkeling Mark-Recapture Estimation of Detection Probability and Abundance of Juvenile Steelhead in a Medium-Sized River

We compared nighttime electrofishing- and snorkeling-based mark-recapture methods for estimating the detection probability and abundance of juvenile steelhead Oncorhynchus mykiss in the Cheakamus River, British Columbia. The reliability of abundance estimates largely depends on the precision and accuracy of detection probability (the fraction of marked individuals detected) as well as a few key assumptions of closed population models that we evaluated in this study. There was minimal bias (-2.5%) in diver estimates of the fork lengths of juvenile steelhead, and the relationship between measured and estimated fork lengths was very precise (r(2) = 95%). With a hierarchical Bayesian model, estimates of the detection probability for smaller juveniles (40-60 mm) ranged from 0.4 to 0.6 with electrofishing and were near zero with snorkeling. In contrast, snorkeling-based detection probability was 0.6 and independent of size for larger juvenile steelhead (> 60 mm) and much greater than that with electrofishing. These results provide strong evidence that there is considerable individual heterogeneity in detection probability driven by fish size for both methods. Owing to these differences, the abundance of age-0 steelhead based on snorkeling was underestimated by 50%, but that of larger, age-1 fish was unbiased and more precise (10-fold) than that based on electrofishing. The use of electrofishing during marking resulted in a substantive reduction in snorkeling-based detection probability during recapture, but the converse was not true. Thus, there is strong evidence of behavioral heterogeneity in detection probability induced by electrofishing, but only when snorkeling is used to detect fish during recapture. The differences in detection probabilities among size-classes and sampling methods were probably driven by differences in concealment behavior, spatial distribution, and fright responses to sampling. Our results indicate that snorkeling is the better way to estimate abundance for larger juvenile steelhead, whereas electrofishing is preferred for smaller fish.