Estimating Habitat-Specific Abundances of PIT-Tagged Juvenile Salmonids Using Mobile Antennas: A Comparison with Standard Electrofishing Techniques in a Small Stream

The use of passive integrated transponder (PIT) tags in ecological studies is increasingly common, but evaluations of their performance and range of application in the field are still emerging. Here, we compare habitat unit–scale abundance estimates of PITtagged juvenile coho salmon Oncorhynchus kisutch and steelhead O. mykiss derived from multiple-pass surveys with a submersible pole-mounted PIT antenna and backpack electrofishing in a small coastal California stream. We found high concordance of methods for coho salmon and age-0 steelhead and moderate concordance for age-1 and older steelhead. Regression analysis of PIT antenna estimates on electrofishing estimates indicated an approximately one-to-one relationship between methods. Regression intercepts for coho salmon and age-0 steelhead were significantly greater than zero, but the differences were small (with an effect equivalent to less than one fish). We found no evidence that pool area, cover area, or cover complexity influenced the relationship between methods for coho salmon and age-0 steelhead, but some evidence that electrofishing may be more effective than PIT antennas for estimating the abundance of age-1 and older steelhead in pools with high cover complexity. Our results demonstrate that surveys using submersible PIT antennas can provide habitat unit–scale estimates of juvenile salmonid abundance similar to those derived from electrofishing in small streams. An advantage of using PIT antennas is that they allow fish abundance estimation without fish recapture and frequent sampling can occur without subjecting study animals to excessive handling stress or mortality. This is a particularly important consideration in studies of small populations, sensitive species, or fish listed under the U.S. Endangered Species Act. One of the challenges of fish ecology is linking the behavior, growth, and survival of individual fish to population dynamics. A useful method for linking individualand population-level *Corresponding author: [email protected] 1Present address: Department of Fisheries and Wildlife, Oregon State University, 104 Nash Hall, Corvallis, Oregon 97331, USA. Received July 30, 2010; accepted June 10, 2011 Published online November 29, 2011 ecological processes is to tag and recapture or relocate individuals across a range of environmental conditions and to track the consequences, in terms of growth and survival, of differential habitat selection (e.g., Bell et al. 2001; Harvey et al. 2005; Ebersole et al. 2006). Passive integrated transponder (PIT) tags have become an increasingly common tool in ecological studies, in part because they provide an efficient and relatively inexpensive method for tagging and tracking large numbers of individual animals. Passive integrated transponder tags consist of a small computer chip, capacitor, and antenna coil encapsulated within 11–32-mm-long glass tubes that are typically implanted within the peritoneal cavity of small fish (Guy et al. 1996; Roussel et al. 2000). When passed near an external energy source, PIT tags transmit a unique numeric code that can be recorded and stored by a data logger. In the last decade, application of PIT technology to ecological studies has helped field researchers gain detailed insight into key processes affecting survival and growth rates of animal populations (e.g., Zydlewski et al. 2006; Brakensiek and Hankin 2007). Although studies using PIT tags are increasingly common, evaluations of PIT tag performance and their range of application in field studies are still forthcoming. Here, we test whether PIT technology can be adapted to obtain habitat-specific estimates of tagged fish abundance in streams by using multiplepass surveys with submersible antenna systems. In fall 2005, we began a study of the seasonal abundance, habitat use, and movements of juvenile coho salmon Oncorhynchus kisutch and steelhead O. mykiss in a small coastal California stream (Stillwater Sciences 2008). We hypothesized that winter conditions would 986 D ow nl oa de d by [ O re go n St at e U ni ve rs ity ] at 1 1: 14 0 6 Ju ne 2 01 2 MANAGEMENT BRIEF 987 cause a recruitment bottleneck for juvenile salmonids within the study stream and that the response to flood disturbance would be mediated by stream cover complexity at the habitat unit scale. To test these hypotheses, we tracked the change in abundance of PIT-tagged fish across a range of habitats over relatively short (e.g., ≤1-month) time intervals during winter. In support of the ecological objectives of our study, we conducted a methodological evaluation to determine whether PIT techniques could estimate in situ fish abundance using relocation data from submersible antennas. This evaluation involved comparing population estimates from multiple-pass surveys with a submersible pole-mounted mobile PIT tag antenna and those obtained by backpack electrofishing. Specifically, our objectives were to determine whether relocation data from a mobile PIT tag antenna could provide abundance estimates of PIT-tagged fish similar to those obtained by multiple-pass electrofishing at the habitat unit scale, and whether the agreement between methods varied by species, age-class, or physical habitat features such as habitat size and instream cover complexity.