Marine survival of coho salmon (Oncorhynchus kisutch) from small coastal watersheds in Northern California
Graduation Date
2015
Document Type
Thesis
Program
Other
Program
Thesis (M.S.)--Humboldt State University, Natural Resources: Fisheries, 2015
Committee Chair Name
Darren M. Ward
Committee Chair Affiliation
HSU Faculty or Staff
Keywords
Humboldt State University -- Theses -- Fisheries
Abstract
California coho salmon Oncorhynchus kisutch populations are at low abundance and factors governing recruitment variability remain unclear. Changes in freshwater habitat that increase juvenile growth and size of salmon outmigrating to sea (smolts) may improve ocean survival. The best data to evaluate this among wild coho salmon populations in California come from life-cycle monitoring (LCM) stations. This study investigated whether marine survival is size-dependent (larger individuals within a cohort have higher marine survival) and whether sites and years with higher growth have higher marine survival across five LCM locations. I tested for size-dependent survival using two techniques: comparing the size of outmigrating smolts and back-calculated smolt sizes from scales of adult fish that survived to return; and using information from fish that were tagged as smolts and survived to return as adults. Analyses comparing smolt sizes back-calculated from adult scales and observed lengths from smolt traps indicated that within-year size-dependent mortality at sea occurred among many outmigrant cohorts, while analyses using smolt lengths of recaptured adult fish tagged as juveniles did not indicate any within-cohort size-selective mortality at sea. Potential explanations for the conflicting results include errors in scale back-calculation; smolts growing in habitats below smolt traps prior to ocean entrance; and fish with alternative juvenile life histories that were unaccounted for in outmigrant sampling surviving and contributing to the adult populations. In regressions across sites and years, marine survival was positively associated with early marine growth measured from the scales of surviving adult salmon and in some instances marine survival was also positively associated with mean fork length of outmigrating smolts. Although size may be an important determinant of ocean survival, this study shows that comparison of back-calculated smolts sizes from scales of adult fish and observed lengths of smolts at an upstream trapping location are unreliable approaches for testing size-selective mortality. This study also provides support for expanding studies at LCM stations to determine how juvenile coho salmon use habitat downstream of migrant trapping locations.
Recommended Citation
Cochran, Sean M., "Marine survival of coho salmon (Oncorhynchus kisutch) from small coastal watersheds in Northern California" (2015). Cal Poly Humboldt theses and projects. 2163.
https://digitalcommons.humboldt.edu/etd/2163
https://scholarworks.calstate.edu/concern/theses/cv43p014q