BIRD PREDATION ON COHO SALMON (ONCORHYNCHUS KISUTCH) IN HUMBOLDT BAY

Author

Colton J. Dixon, Humboldt State UniversityFollow

Graduation Date

Fall 2024

Document Type

Thesis

Program

Master of Science degree with a major in Natural Resources, option Fisheries

Committee Chair Name

Darren Ward

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Daniel Barton

Second Committee Member Affiliation

HSU Faculty or Staff

Third Committee Member Name

Jose Marin Jarrin

Third Committee Member Affiliation

HSU Faculty or Staff

Fourth Committee Member Name

Barbara Clucas

Fourth Committee Member Affiliation

HSU Faculty or Staff

Keywords

Coho Salmon, Oncorhynchus kisutch, Humboldt Bay, Bird predation, Passive Integrated Transponder, PIT tags, Double-crested Cormorants, Phalacrocorax auritus, Caspian Terns, Hydroprogne caspia, Great Blue Herons, Ardea herodias, Great Egrets, Ardea alba, Salmon smolt, Migration, Predation risk factors, Stable isotope analysis, Carbon and nitrogen isotopes, Salmon conservation, Predation ecology, Piscivorous birds, Freshwater Creek, Jacoby Creek, Ryan Creek, Salmon outmigration, Bird diet analysis, Salmon recovery, West Coast salmon decline, Habitat loss, predation, Smolt vulnerability, Avian predation dynamics

Subject Categories

Fisheries

Abstract

Coho Salmon (Oncorhynchus kisutch) populations along the west coast of the United States are declining. In Humboldt County, located in northwestern California, Coho Salmon have been in decline since their federal listing as threatened in 1997, primarily due to habitat loss from logging, agriculture, and urban development. Although these anthropogenic factors caused salmon populations to decline, bird predation may also be hindering population recovery, as evidenced by the recovery of Passive Integrated Transponder (PIT) tags, originally implanted in Coho Salmon, from bird colonies in Humboldt Bay. This study examines predation by Double-crested Cormorants (Phalacrocorax auritus), Caspian Terns (Hydropogne caspia), Great Blue Herons (Ardea herodias), and Great Egrets (Ardea alba) on Coho Salmon smolts from tributaries to Humboldt Bay, including Freshwater Creek, Jacoby Creek, and Ryan Creek. Predation was assessed by comparing the number of PIT tags recovered from bird colonies to the number of tags implanted in juvenile salmon, providing a minimum estimate of the proportion of salmon consumed by birds and highlighting factors associated with increased predation risk. Additionally, stable isotope analysis of carbon and nitrogen in cormorant chick tissues from Sand Island helped evaluate the significance of Coho Salmon in their diet. Finally, bird counts were conducted to assess piscivorous bird activity during the smolt outmigration period. From 3%-18% of tags deployed in smolts each year were found in scans of bird colonies. Smaller smolts are at greater risk of predation, with their migration timing and cohort year also influencing vulnerability to predation. Despite high predation rates, stable isotope analysis and bird behavior suggest that the birds are not focusing on Coho Salmon as a primary food source. These findings underscore the need for further research into the interactions between Coho Salmon and avian predators to support conservation efforts.

Citation Style

APA

Recommended Citation

Dixon, Colton J., "BIRD PREDATION ON COHO SALMON (ONCORHYNCHUS KISUTCH) IN HUMBOLDT BAY" (2024). Cal Poly Humboldt theses and projects. 795.
https://digitalcommons.humboldt.edu/etd/795