Host-pathogen-environment interactions determine survival outcomes of
adult sockeye salmon (Oncorhynchus nerka) released from fisheries
Abstract
Incorporating host-pathogen(s)-environment axes into management and
conservation planning is critical to preserving species in a warming
climate. However, the role pathogens play in host stress resilience
remains largely unexplored in wild animal populations. We experimentally
characterized how independent and cumulative stressors (fisheries
handling, high water temperature) and natural infections affected the
health and longevity of released wild adult sockeye salmon (Oncorhynchus
nerka) in British Columbia, Canada. Returning adults were collected
before and after entering the Fraser River, yielding marine- and
river-collected groups, respectively. Fish were exposed to a mild
(seine) or severe (gill net) fishery treatment at collection, and then
held in circulating freshwater tanks for up to four weeks at historical
(14°C) or projected migration temperatures (18°C). Using weekly
nonlethal gill biopsies and high-throughput qPCR, we quantified loads of
up to 46 pathogens with host stress and immune gene expression.
Marine-captured fish had less severe infections than river-captured
fish, a short migration distance (100 km, 5-7 d) that produced profound
infection differences. At 14°C, river-collected fish survived 1-2 weeks
less than marine-collected fish. All fish held at 18°C died within 4
weeks unless they experienced minimal handling. Gene expression
correlated with infections in river-collected fish, while
marine-collected fish were more stressor-responsive. Cumulative
stressors were detrimental regardless of infections or collection
location, likely due to extreme physiological disturbance. Because
river-derived infections correlated with single stressor responses,
river entry likely decreases stressor resilience of adult salmon by
altering both physiological status and pathogen burdens, which redirect
host responses toward disease resistance.