Estuarine Environments as Rearing Habitats for Juvenile Coho Salmon in Contrasting South-Central Alaska Watersheds

Abstract

For Pacific salmon, estuaries are typically considered transitional staging areas between freshwater and marine environments, but their potential as rearing habitat has only recently been recognized. The objectives of this study were two-fold: (1) to determine if Coho Salmon Oncorhynchus kisutch were rearing in estuarine habitats, and (2) to characterize and compare the body length, age, condition, and duration and timing of estuarine occupancy of juvenile Coho Salmon between the two contrasting estuaries. We examined use of estuary habitats with analysis of microchemistry and microstructure of sagittal otoliths in two watersheds of south-central Alaska. Juvenile Coho Salmon were classified as estuary residents or nonresidents (recent estuary immigrants) based on otolith Sr : Ca ratios and counts of daily growth increments on otoliths. The estuaries differed in water source (glacial versus snowmelt hydrographs) and in relative estuarine and watershed area. Juvenile Coho Salmon with evidence of estuary rearing were greater in body length and condition than individuals lacking evidence of estuarine rearing. Coho Salmon captured in the glacial estuary had greater variability in body length and condition, and younger age-classes predominated the catch compared with the nearby snowmelt-fed, smaller estuary. Estuary-rearing fish in the glacial estuary arrived later and remained longer (39 versus 24 d of summer growth) during the summer than did fish using the snowmelt estuary. Finally, we observed definitive patterns of overwintering in estuarine and near shore environments in both estuaries. Evidence of estuary rearing and overwintering with differences in fish traits among contrasting estuary types refute the notion that estuaries function as only staging or transitional habitats in the early life history of Coho Salmon.

Received December 5, 2012; accepted June 10, 2013

ACKNOWLEDGMENTS

This work was completed as part of student graduate research and much appreciation is expressed to University of Alaska School of Fisheries professors Franz Mueter and Megan McPhee for their review and guidance of the dissertation work leading to this manuscript. Kachemak Bay National Estuarine Research Reserve provided staff support, facilities, and equipment for completion of this study. Research team members Angela Doroff, Ori Badajos, and Jasmine Maurer provided much appreciated assistance with equipment, data management, and editorial review. We thank volunteer assistants Janet Fink, Jason Neher, Charles Owens, and Michelle Gutsch. Additional staff and equipment support came from U.S. Fish and Wildlife Service, U.S. Geological Survey Alaska Science Center, and the University of Alaska School of Fisheries and Ocean Sciences. Portions of this work were performed at the Advanced Instrumentation Laboratory, University of Alaska Fairbanks, and we thank Karen Spaleta for her guidance in the laboratory. We especially thank Charles Simenstad for his guidance on the project and review of this manuscript and the two anonymous reviewers for their helpful suggestions. This research was conducted in the National Estuarine Research Reserve System under an award from the Estuarine Reserves Division, Office of Ocean and Coastal Resource Management, National Ocean Service, National Oceanic and Atmospheric Administration. Additional funding was provided by State Wildlife Grants to KBRR, the University of Alaska Fairbanks, and Alaska Experimental Program to Stimulate Competitive Research. All work involving use and handling of animals was completed under the University of Alaska Institutional Animal Care and Use Committee approved sampling plans. Use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.