Documentation of en route mortality of summer Chum Salmon in the Koyukuk
River, Alaska and its potential linkage to the heatwave of 2019
Abstract
This paper documents a mass en route mortality event of adult summer
chum salmon (Oncorhynchus keta) returning to the Koyukuk River, Alaska
in the Yukon River watershed. In response to reports from local
communities, researchers (including the author) surveyed ca. 315 km of
river on July 26 and 27, 2019 and counted 1,364 dead individuals, but
this likely reflects a small fraction of the true number of fish that
died. We sampled 73 carcasses to confirm death occurred prematurely
prior to complete maturation and spawning, to quantify sex and length.
Visual inspection revealed a substantial fraction exhibited patterns of
fungal growth consistent with secondary infections of skin lesions
caused by the ubiquitous natural bacterial pathogen Flavobacterium
columnare. Water temperatures during the survey averaged 17.1°C and the
water was approximately 85% saturated with oxygen (ca. 8.5 mg/L), which
likely contributed to the stress for upstream migrants. Evidence
suggests size-selective en route mortality as female migrants that died
were 2% and male migrants 5% shorter than individuals that survived to
their spawning grounds on Henshaw Creek. This translates to very strong
estimates of natural selection using standardized selection
differentials, though randomization tests of size data revealed this
observed outcome of selection was expected to occur 25% of the time due
to chance alone. It is unclear whether selection acts on body size
directly or indirectly through correlated phenotypic traits such as run
timing. The mortality event likely underpins the below average returns
of summer chum salmon to the Koyukuk in 2019, suggesting an impact on
spawner abundance. The future consequences of this, or potentially
increasingly frequent, en route mortality events for population
productivity and the extent to which genetic adaptation or adaptive
phenotypic plasticity of migration behavior may facilitate persistence
of these populations is unknown.