Using Redd Attributes, Fry Density, and Otolith Microchemistry to Distinguish the Presence of Steelhead and Rainbow Trout in the Elwha River Dam Removal Project

Abstract

The removal of impassable dams can affect stream fish communities in many ways, including allowing previously separated anadromous and resident life history types to intermingle. Recolonization of steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) into areas that were previously occupied solely by resident conspecifics presents a challenge for monitoring because (1) staging or spawning adults may be difficult to directly observe; and (2) the offspring of the two life history forms are indistinguishable during early life. We evaluated the utility of using redd attributes, fry (age-0) size, and fry density to distinguish the presence of steelhead and Rainbow Trout in the Elwha River, Washington. Redd area and substrate size differed between forms: steelhead redds were significantly larger and had a greater median substrate particle size than Rainbow Trout redds. Fry density was significantly higher in steelhead spawning reaches than in Rainbow Trout reaches during both survey years (2010 and 2011). Steelhead fry tended to be slightly larger than Rainbow Trout fry but not significantly so in either year. Additionally, otolith microchemical analyses indicated that all fry collected from newly colonized habitat were of steelhead maternal origin, and these fish were intermediate in size to the steelhead and Rainbow Trout fry collected prior to dam removal. Thus, fry size may not effectively differentiate parental origin despite the tendency of anadromous parents to be larger and to spawn earlier than residents. Redd attributes and fry density may be useful for evaluating the spatial distributions and relative abundances of steelhead and Rainbow Trout after dam removal. Our results have implications for other dam removal projects and other monitored species.

Received April 13, 2015; accepted July 3, 2015

ACKNOWLEDGMENTS

We thank Keith Denton (National Oceanic and Atmospheric Administration [NOAA], Northwest Fisheries Science Center [NWFSC]), Todd Bennett (NOAA, NWFSC), and Raymond Moses (Lower Elwha Klallam Tribe) for help in collecting and measuring fish; Matt Sloat for discussions, reviews, and suggestions during the early stages of the research; Alex Stefankiv (NOAA, NWFSC) for creating Figure 1; and Sam Brenkman and Pat Crain (Olympic National Park) for sharing their data and observations on Rainbow Trout and steelhead in the Elwha River. We especially thank Andrew Claiborne and Anna Hildebrandt (Washington Department of Fish and Wildlife, Fish Aging Laboratory) for otolith sample preparation and chemical analysis. We are grateful to Polly Hicks (NOAA, NWFSC) for funding support for J.R.M. Funding was partly provided through a grant to T.P.Q. from Washington Sea Grant, University of Washington (NOAA Award Number NA10OAR4170075, Project R/LME-7). We also thank Jim Myers, Krista Nichols, Phil Roni (NOAA, NWFSC), and anonymous reviewers for suggestions that improved the manuscript. The views expressed herein are those of the authors and do not necessarily reflect the views of NOAA or any of its subagencies.