Results
Consistent with the flood of local observations of a die-off, we
encountered the first carcasses floating mid-river nearly immediately
upon leaving Hughes. In the ca. 315 km of survey, we counted 1,364 dead
fish.
Sampled carcasses varied markedly in condition and state of
decomposition. None of the carcasses had signs of trauma or post-mortem
scavenging by animals, a pattern that was also widely noted by local
residents. Every individual examined internally for sex had not
completed maturation, indicated by firm testes in males and unovulated
eggs in females (Fig. 3). Approximately 1/3 of the carcasses had signs
of potential fungal growth, consistent with secondary infections
following the growth of Flavobacterium columnare ( James Winton,
USGS, personal communication).
The water temperature in the mainstem Koyukuk River during the survey
averaged 17.1°C (range 16.8 – 17.4). Dissolved oxygen averaged 8.3 mg/L
(Range 7.9 – 8.6). Given that DO saturation at 17.1°C occurs at 9.65
mg/L, our measurements indicate the water was on average only
approximately 87% saturated with dissolved oxygen.
Consistent with size-selective mortality, individuals that died en
route were ca. 4% smaller on average compared to individuals that
survived to the spawning grounds (Table 1). Body size was significantly
and inversely associated with probability of death (Fig. 4,
p<0.0001) Estimates of standardized selection for males was
0.77, and 0.86 for females. These estimates exceed the
99th percentile of estimates reported by Siepielski et
al. and is thus evidence of very strong selection. However, simulation
analysis indicate that our observations of selection could have occurred
approximately 25% of the time due to chance alone, which likely
reflects the natural wide distribution of body sizes in summer Chum
Salmon.