Estuary Utilization by Juvenile Chinook Salmon

The Problem

Tidal channels in the estuarine emergent marsh provide food and habitat for juvenile chinook salmon.

Little is known about the importance of estuarine habitats for juvenile chinook salmon (Oncorhynchus tshawytscha), hence managers are uncertain of the appropriate levels of protection for such habitats. Research to determine the length of residence and growth of juvenile chinook salmon in the Skagit River estuary will indicate the importance of the estuary to this species, help managers decide on appropriate levels of protection, and support fisheries managers efforts to deflect alternative uses of this habitat in the Skagit River and similar streams. The methodology that we develop or refine in this study should foster similar studies along the Pacific coast thereby enhancing our general understanding of chinook salmon and their habitat requirements.

Objectives

Otolith

The 5-year research goals of the Fisheries and Aquatic Resources Program addressed are to: understand relationships between and among aquatic species habitats; provide science for restoring and maintaining declining species and their required habitat; and investigate aquatic species diversity. This subtask will evaluate the use of otolith (ear stones) microstructure for investigating: (1) age and size at entry to the estuary; (2) length of residence and growth in association with salt marsh or other estuarine habitats; and (3) differences in these life-history traits among different populations of salmon occurring in the same river system.

Methodology

The first two objectives are addressed by analysis of otoliths from juvenile salmon; the third objective requires analysis of otoliths from returning adults. Previous work suggests that checks are deposited on the otoliths of juvenile chinook salmon at various transition points in their life such as at emergence and at entrance to the estuary. Otolith size is positively related to the size of the fish, and daily growth increments typically are deposited on the otoliths. Consequently, we anticipate using back-calculations from otolith microstructure to estimate the length of time and growth between checks, or between a check and the capture date of juvenile salmon. This is a pilot study which will examine chinook salmon in the Skagit River, Washington, in collaboration with the Skagit System Tribal Cooperative. Juvenile salmon will be collected from various reaches of the lower Skagit River throughout the season of use to

Block seine for capturing juvenile chinook salmon in the estuarine emergent marsh, Skagit River, Washington State.

ascertain the river or estuary reaches at which specific checks on the otoliths are formed. Otoliths from these juveniles will be prepared and their microstructure examined and measured with light microscopy. A laboratory study will be used to test the validity of an apparent emergence check, and the effects of water temperature and feeding on the check. This laboratory study also will test whether increments are deposited one per day immediately after the fish have emerged from the gravel (in winter). Recaptures of hatchery fish in the river and estuary will provide a test of daily growth increments later in the season (spring). Conservative estimates for length of residence and growth in the intertidal zone of the estuary will be derived from the juvenile salmon in our samples, but we hope to derive unbiased estimates for these variables from returning adult salmon. Of course, estimates from adults should provide insight into the successful juvenile life histories, which on average may differ substantially from the life histories of juveniles that don't survive to adulthood. Analysis of otoliths from adult salmon will allow us to compare juvenile life histories from the various populations and seasonal runs of salmon within the river.

Highlights and Key Findings

4/19/2002 - The life history types identified from previous analysis of otolith microstructure were classified into one of three primary groups: 1) early-migrating fry using the estuary only as a migration corridor, 2) juveniles rearing in the estuary, and 3) parr using the estuary only as a migration corridor. For the two years analyzed, juveniles rearing in the estuary were the predominant life-history type among salmon caught in Puget Sound. Residency time within the estuary varied from year to year, however, at least 70% of all fish rearing in the estuary resided there for a month or longer. The variation in residency between years is suggestive of density dependence and habitat limitation. In the year of high juvenile density, residency time was shortened and growth limited. In the year of low density, juveniles arrived in the estuary earlier, remained longer, and grew almost 1 cm larger. Mean size at entrance to the estuary was 47mm FL for both years and emigration size ranged from 69 to 77mm FL, with a 4-5 fold increase in weight.

Segment of an otolith from a juvenile chinook salmon captured in the Skagit River estuary. FW indicates growth in freshwater; E indicates growth in the estuary; TZ indicates the transition between fresh water and estuary.

4/30/2001 - Estuarine residence times (assuming 1 increment = 1 day) were distinct between life history types (characterized by increment patterns on otoliths), with average residency for some types up to twice as long as the average for other types. Previously identified checks and life history types (reflecting migrational or sequential habitat use) from juveniles and age-3 adults were further validated with age-4 adults of the same brood year. Increment and check patterns thought to be associated with tributary origin were held true for those age-4 adults collected from spawning grounds. Samples collected after hatching in incubation trays showed daily increments at both 5 and 10°C even though growth, based upon mean increment width, at 10°C was two and a half times greater than at 5°C. Regression of time in days versus the number of increments on otoliths of emerged fry reared at 10°C and collected 3,10 and 21 days post-emergence indicated that the emergence check encompasses one day.

3/28/2000 - Sagittal otoliths have been removed from several hundred juvenile chinook salmon taken throughout the year in the upper river, lower river, intertidal estuarine habitats, and bay. These otoliths have been mounted, prepared, interpreted, and increments measured in preparation for analysis. An experiment was conducted in artificial stream channels and standard fish tanks to determine when a standard emergence check is actually deposited on the otoliths, and how the timing is affected by food level and water temperature. Most of the otoliths from these experimental fish remain to be extracted and analyzed.

Where Are We Headed In 2003

1. Hire and train two new employees in otolith preparation and analysis.

2. Prepare and analyze otoliths from ~550 age-0 juvenile chinook samples collected in 1997 from nearshore habitat of Skagit Bay or in 2000 from different spawning ground areas and the mainstem trap in the Skagit River.

3. Prepare a manuscript describing the juvenile life history types of wild ocean-type chinook salmon in the Skagit River system.

4. Complete analysis to verify the emergence check on otoliths from juvenile chinook salmon.

5. Analyze data from microchemical assays of Sr/Ca ratios to further substantiate associations between specific patterns of otolith microstructure and specific habitats.

6. Using SrCl marked fish held in net pens, test whether otolith increments are deposited daily.

Project Contact

Reg Reisenbichler
U.S. Geological Survey
Western Fisheries Research Center
6505 NE 65th St.
Seattle, WA 98115

Email: reg_reisenbichler@usgs.gov
Phone: 206-526-6282
Fax: 206-526-6654

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