Introduction
Prespawning mortality events in Pacific (genus Oncorhynchus )
salmon are well-documented throughout the ranges of species, but the
mechanisms and consequences of these events for populations and
ecosystems, including people, are not well known (Bowerman et al. 2016).
Prespawning mortality events can occur at various stages of the upstream
homeward migration. Upon reaching the spawning grounds, prespawning
mortality (PSM) appears linked to spawner density and streamflow, which
in turn influences the availability of life-supporting dissolved oxygen
(Sergeant et al. 2017). Our understanding of PSM on the spawning grounds
is largely limited to years with anomalously large returns (Quinn et al.
2007; Tillotson and Quinn 2017) or locations where spawner densities are
influenced by returns of hatchery-produced salmon (McConnell et al.
2018). While our understanding of PSM on the spawning grounds is in its
early phases, most of what we know has been garnered by studies tracking
fish en route upstream during the freshwater phase of the
homeward migration (Hinch et al. 2012).
Substantial work on large watersheds such as the Fraser River (British
Columbia, Canada), Klamath River (USA), and the Columbia River (USA) has
revealed that en route mortality events are the result of complex
interactions between the phenotype of migrants (e.g. run timing and
physiology) and their environment, with water temperature and flow being
master variables (Keefer et al. 2008). As a poignant example, early
migrating sockeye salmon (O. nerka ) in the Fraser River tend to
die at much higher rates than later returning individuals, which appears
to be associated with a mismatch between current warm water conditions
and thermal adaptation in terms of aerobic scope to historical migration
conditions (Eliason et al. 2011). In contrast, later-timed migrants of
Sockeye Salmon died at higher rates in the Columbia River during 2000,
likely due to delayed migration and increased susceptibility to parasite
and pathogens (Keefer et al. 2007). Perhaps the best known example of a
temperature-related disease outbreak is from the Klamath River in
September of 2002, where an estimated 34,056 migrating Chinook salmon
(O. tshawytscha ) died as a result of heavy infections ofIchthyopthirius multifiliis (Ich) and/or Flavobacterium
columnare (columnaris) (Guillen 2003). Of course, a perennial challenge
is disentangling whether pathogen presence and disease is the proximate
agent of mortality or, more commonly in wild fish, a response to other
unquantified stressors. Taken as a whole, strong evidence indicates that
the combination of high water temperatures, accelerated maturation and
senescence, elevated stress, ionoregulatory dysfunction, and disease are
the causal agents behind en route mortality in migrating salmon
(Hinch et al. 2012). Moreover, patterns observed thus far indicate that
populations at the southern margins of the ranges may be most
susceptible to en route mortality given regional warming that
increasingly push water temperatures beyond tolerance thresholds. In
this paper I document, a large-scale en route mortality event by
summer Chum Salmon (O. keta ) returning to spawning grounds on the
edge of the Arctic in the Yukon River watershed, Alaska (Fig.1).
The summer of 2019 will forever be burned in the consciousness of
Alaskans. Most apparent was the extreme air temperature anomalies with
many regions, including the Yukon, experiencing many days with maximum
temperatures in excess of 32°C. Temperatures in July and August were far
above average compared to long-term records (Fig. 2). In addition, ocean
temperatures in Alaska were 5 degrees warmer than average, wildfires
burned throughout the state choking the air with smoke, droughts plagued
rain forests, a lack of sea ice in the Bering Sea, and anomalous
biological patterns were detected across the region (Thoman and Walsh
2019). Beginning on July 20th, the residents living along the Koyukuk
River that have long been the stewards and first responders on their
lands began reporting a large scale die-off of Chum Salmon on social
media outlets. Hundreds of comments and similar observations streamed
in, and on July 26th a dispatch of western-trained
scientists, including the author, traveled to the Koyukuk River drainage
with the goals of i ) documenting the extent of the die-off,ii ) inferring any apparent abiotic or biotic causes of the event,
and iii ) quantifying whether mortality was associated with body
size or sex by comparing to observations of fish that survived to the
spawning grounds.