Fisheries gear and biological context drive fishing-related incidental mortality in Pacific salmon spawning migrations

Most fisheries inadvertently capture non-target fish species or populations. In addition, fish may interact with and escape fishing gears without being observed. Regardless of whether fish are released or escape, negative impacts from interacting with fishing gears (including stress and injury) may lead to fishing-related incidental mortality (FRIM). However, FRIM is typically difficult to observe and therefore is not easily or well quantified. Further, FRIM is contingent on many biological and environmental factors, which have received little consideration in fisheries management. Specifically, infectious disease is expected to mediate FRIM, but has been studied little in this context. Because of their life histories, Pacific salmon are both particularly vulnerable to FRIM and ideal model animals for its study. This dissertation sought to further the knowledge regarding FRIM in Pacific salmon during their freshwater spawning migrations by conducting in situ studies of capture methods and pertinent biological factors. Salmon received radio-frequency tags following exposure to fisheries gear so that migration survival could be determined. Quantitative PCR was performed on non-lethal biopsies to determine the presence and load of infectious agents, as well as immune system function. Sockeye salmon escaping or released from gillnets experienced elevated FRIM, slowed migration, and reduced spawning success. Although the impact of gears was consistent between years, the incidence of FRIM was reduced for mature fish. The high impact of severe injuries was indicated by low migration survival and the elevated expression of acute phase response genes. Mature Chinook salmon with infections of a blood-borne protozoan parasite experienced elevated mortality, regardless of a simulated fisheries gear exposure. My dissertation indicates that salmon interacting with gillnets in freshwater experience significant FRIM, but biological factors can override these impacts. Future research should seek to estimate the rate of gillnet escapes, identify gears that minimize physical damage, localize the time in migrations when maturity confers resilience, and use repeated sampling to better define the role of infectious agents in FRIM. Fisheries managers could reduce FRIM by prioritizing lower impact gears, promoting fisheries in locations where salmon are more mature and pathogens are sparse, and conducting regular monitoring of infectious agents.Forestry, Faculty ofGraduat