Habitat Selection and Overlap of Atlantic Salmon and Smallmouth Bass Juveniles in Nursery Streams

Abstract

Introduced smallmouth bass Micropterus dolomieu have invaded much of the historic freshwater habitat of Atlantic salmon Salmo salar in North America, yet little is known about the ecological interactions between the two species. We investigated the possibility of competition for habitat between age-0 Atlantic salmon and age-0 and age-1 smallmouth bass by means of in situ observations and a mesocosm experiment. We used snorkel observation to identify the degree and timing of overlap in habitat use in our in situ observations and to describe habitat shifts by Atlantic salmon in the presence of smallmouth bass in our mesocosm experiments. In late July 2008, we observed substantial overlap in the depths and mean water column velocities used by both species in sympatric in situ conditions and an apparent shift by age-0 Atlantic salmon to shallower water that coincided with the period of high overlap. In the mesocosm experiments, we detected no overlap or habitat shifts by age-0 Atlantic salmon in the presence age-1 smallmouth bass and low overlap and no habitat shifts of Atlantic salmon and age-0 smallmouth bass in fall 2009. In 2009, summer floods with sustained high flows and low temperatures resulted in the nearly complete reproductive failure of the smallmouth bass in our study streams, and we did not observe a midsummer habitat shift by Atlantic salmon similar to that seen in 2008. Although this prevented us from replicating our 2008 experiments under similar conditions, the virtual year-class failure of smallmouth bass itself is enlightening. We suggest that future studies incorporate the effects of varying temperature and discharge to determine how abiotic factors affect the interactions between these species and thus mediate the outcomes of potential competition.

Received March 16, 2010; accepted February 28, 2011

ACKNOWLEDGMENTS

We thank Wes Ashe, Paul Damkot, Brett Ferry, Cory Gardner, Dimitry Gorsky, Derek Lucas, James Osenton, Scott Ouellette, Megan Patridge, and Silas Ratten for their assistance in the field. We thank Paul Santavy, Scott Craig, and the staff at the U.S. Fish and Wildlife Service Craig Brook National Fish Hatchery for providing Atlantic salmon fry and logistical support. We thank the Union River Salmon Association for their aid in allocation and stocking of fry. This project benefited from the logistical and professional support of Joe Dembeck and Nels Kramer of the Maine Department of Inland Fisheries and Wildlife, Peter Ruksznis and Randy Spencer of the Maine Department of Marine Resources, and David Huntress and Dana Degraaf of Stantec Consulting Services Inc. We thank Donna Parrish and Joe Hightower for their review of an earlier version of this manuscript. We also thank Dennis DeVries and three anonymous reviewers who provided valuable suggestions to improve the manuscript. Funding was provided by the University of Maine, Maine Agricultural and Forest Experiment Station, U.S. Geological Survey Maine Cooperative Fish and Wildlife Research Unit, Outdoor Heritage Fund, and Maine Department of Marine Resources. This paper is Maine Agricultural and Forest Experiment Station Publication Number 3165. All experimental fish were handled in accordance with protocol A2005–08–01 approved by the University of Maine Institutional Animal Care and Use Committee. Mention of trade names does not imply endorsement by the U.S. Government.