Effects of In-Channel Structure on Chinook Salmon Spawning Habitat and Embryo Production

Robyn L Bilski,Joseph M Wheaton,Joseph E Merz

doi:10.3390/w14010083

Abstract

Adult salmonids are frequently observed building redds adjacent to in-channel structure, including boulders and large woody debris. These areas are thought to be preferentially selected for a variety of reasons, including energy and/or predation refugia for spawners, and increased hyporheic exchange for incubating embryos. This research sought to quantify in-channel structure effects on local hydraulics and hyporheic flow and provide a mechanistic link between these changes and the survival, development, and growth of Chinook salmon Oncorhynchus tshawytscha embryos. Data were collected in an eight-kilometer reach, on the regulated lower Mokelumne River, in the California Central Valley. Nine paired sites, consisting of an area containing in-channel structure paired with an adjacent area lacking in-channel structure, were evaluated. Results indicated that in-channel structure disrupts surface water velocity patterns, creating pressure differences that significantly increase vertical hydraulic gradients within the subsurface. Overall, in-channel structure did not significantly increase survival, development, and growth of Chinook salmon embryos. However, at several low gradient downstream sites containing in-channel structure, embryo survival, development, and growth were significantly higher relative to paired sites lacking such features. Preliminary data indicate that adding or maintaining in-channel structure, including woody material, in suboptimal spawning reaches improves the incubation environment for salmonid embryos in regulated reaches of a lowland stream. More research examining temporal variation and a full range of incubation depths is needed to further assess these findings.

Highlights

The experiment focus was to compare survival, development, and growth of Chinook salmon embryos within and away from the direct influence of inchannel structure. To do this we focused on paired sites, which were selected to represent spawning areas encompassing and deficient of in-channel structure, in the form of large woody debris and boulders
Surface water velocity measurements at sites not associated with in-channel structure were largely homogeneous
Water velocity measurements taken at sites containing in-channel structure were characterized by accelerations along the sides and just downstream of the structures, as well as zones of reduced velocity on the lee side of structures

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Habitat heterogeneity is thought to be positively correlated with biotic production and species diversity [1,2,3]. In-channel structures (e.g., large woody debris, bank irregularities, bedrock outcrops, roots, and boulders) play an important role in maintaining habitat diversity, increasing organic matter retention, and inducing changes in channel morphology [4,5]. In-channel structures change channel morphology by promoting sediment scour and aggregation in alluvial streams and controlling and maintaining the formation of channel features [6,7,8,9], flow regime and sediment supply must be adequate to maximize benefits [10]

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Results

Conclusion