Size-Selective Mortality of Chinook Salmon During Early Marine Residence in the Nisqually Region of Puget Sound

Amy Duarte1, Madilyn Gamble2, David A. Beauchamp2, Adam G. Hansen2

1Humboldt State University, 2School of Aquatic and Fishery Science, University of Washington

Puget Sound Chinook Salmon

Acknowledgments

I would like to thank the Joint Institute for Atmospheric and Ocean Sciences and the School of Aquatic and Fishery Science for making this research possible. My deepest gratitude to the students and staff whose advice and feedback supported me throughout this journey.

References

Duffy, E. J., & Beauchamp, D. A. (2011). Rapid growth in the early marine period improves the marine survival of Chinook salmon (Oncorhynchustshawytscha) in Puget Sound, Washington. Canadian Journal of Fisheries and Aquatic Sciences, 68(2), 232-240.

The abundance and survival of Chinook salmon in Puget Sound has declined over the past 30 years. Populations of Chinook salmon in Puget Sound are listed under the Endangered Species Act. Little is known about sources of marine mortality for these salmon.

Recent work showed a strong positive correlation between the body mass of juvenile hatchery Chinook salmon during offshore residence in July in Puget Sound and overall smolt-to-adult survival (Fig. 1). This indicates that (1) mortality after July is highly size-dependent, and (2) rapid growth during early marine residence is critical for overall marine survival (Duffy and Beauchamp 2011). Whether strong size-selective mortality is also operating during early marine residence by Chinook salmon is an open question. Moreover, the processes that affect growth and survival in different habitats (i.e., estuarine deltas, nearshore, and offshore) during this period have not been evaluated. Determining if, when and where size-selective mortality is occurring during early marine rearing, and identifying the factors affecting growth and survival in different habitats during this period will inform researchers on appropriate restoration efforts for Puget Sound Chinook salmon.

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Fig. 1. Smolt-to-adult survival of Puget Sound Chinook salmon as a function of body mass.

Objectives and ApproachDetermine if, when, and where size-selective mortality is occurring in the Nisqually region of Puget Sound (Fig. 6) by comparing scale based growth history of juvenile Chinook salmon in different habitats through duration of early marine period.

Define critical periods and sizes that influence the overall marine survival of Puget Sound Chinook salmon by comparing size at specific scale circuli for juveniles across spatial and temporal continuum.

Future WorkEvaluating extent of size-selective mortality among diverse watersheds and stocks of Chinook salmon in Puget Sound.Diet analysis of juvenile Chinook salmon to understand how food supply and demand influence their growth and survival.Bioenergetics and food web modeling to further analyze growth and predation factors.

Scale Pattern Analysis

•Scales record growth history of fish by laying down incremental circuli. •Total scale radius correlates linearly with the length of fish allowing us to estimate the size of fish at previous circuli.• Total circuli count can be associated with peak residence in specific habitats under the assumption that sub-yearling salmon are exhibiting the typical life-history strategy of progressively moving from estuaries to nearshore and ultimately to offshore habitats.

Fig. 5. Acetate impression of a Chinook salmon scale. The red line shows the axis used to make scale measurements.Is size-selective mortality operating in Nisqually?

FL(mm)= 38.518+139.65*Radius (mm)R² = 0.65N=190

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Fig. 6. Map of Puget Sound indicating study region.

Fig. 7. The relationship between fork length (mm) and total scale radius (mm) for juvenile Chinook salmon caught in the Nisqually region of Puget Sound during May-July of their first marine year.

Results and DiscussionScales from juvenile Chinook salmon caught in estuary, nearshore and offshore habitats in Nisqually showed a linear relationship between scale radius and fork length (Fig. 7). This indicates that the fork length could be back-calculated from the radius of the scale at specific circuli.Size-selective mortality is most pronounced between nearshore and offshore habitats, given the stronger rightward shift in the frequency curves generated for fish sampled offshore (June and July) at circuli 14 relative to the nearshore curve at circuli 14 (Fig. 8). Given that scale radius corresponds linearly with fork length, we can infer that a large fraction of the smaller fish present nearshore, did not survive to offshore, or did not survive once they got to offshore habitats.

Methods

Fig. 8. The proportion frequency of scale radius measurements to specific circuli for juvenile Chinook salmon from the Nisqually region of Puget Sound. Hatchery fish were provided by local hatcheries. Juvenile Chinook salmon were caught nearshore by beach seine and offshore by purse seine during May-July of their first marine year. The inset figures display the frequency distribution of total scale circuli count from which the median circuli count corresponding to each habitat/life-stage were estimated.

Fig.2. Monthly purse seine sampling conducted in offshore environments.

Fig.3. Fish scales, weight and fork length collection.

Fig.4. Chinook scales (10X magnification).

Field Sampling Fish Processing Scale Mounting and Analysis