salmon choose to spawn within a river system has an impact on their relative
Washington’s Chiwawa River, a high proportion of natural-origin spring chinook
salmon chose areas higher in the river’s watershed where better conditions
exist for spawning, while most hatchery origin fish spawned further downstream,
closer to where they were released as juveniles.
recent study that spanned ten years (2004 -2014) looked at the distribution of
chinook salmon carcasses to determine the differences in spawning areas chosen
throughout the river, a tributary of the Wenatchee River, between the
natural-origin fish and their hatchery counterparts.
choice of spawning location also impacted the quality of the redd or nest that
the female salmon built. Hatchery salmon in the lower stretches of the river
spawned in plane-bed channels where they were unable to build redds of the
quality built by natural-origin salmon farther upstream in pool-riffle
study found that the spawning distributions of hatchery- and natural-origin
female spring Chinook salmon differed in the Chiwawa River,” said study author
Michael Hughes, biologist with the Washington Department of Fish and Wildlife’s
Fish Science Hatchery/Wild Interactions Team in Wenatchee. “Given the
geomorphic characteristic differences between channel types, we found that
hatchery females spawning in plane-bed channels were unable to build redds
similar to redds built by their counterparts in pool-riffle channels.”
not surprising, he said, considering that hatchery fish are expected to be
influenced by the locations of their juvenile acclimation and release sites. In
the Chiwawa River, the release site is in the lower river at river kilometer
1.0, which is also an area dominated by plane-bed channels. On the other hand,
the distribution of natural-origin fish is driven by the quality and
availability of spawning habitats, and that generally drives them further
upstream to pool-riffle channels.
constructed in plane-bed channels were smaller in size, shallower, and closer
to stream banks than redds in pool-riffle channels, Hughes said. However, when
hatchery females spawned higher in the system in pool-riffle channels, only
limited differences in redd characteristics were detected between natural and
argue that these observed differences in spawning locations and redd
characteristics could contribute to previously documented reproductive success
differences between hatchery and natural spring Chinook salmon in the Chiwawa
River,” he said.
habitat of hatchery spring chinook salmon and possible mechanisms contributing
to lower reproductive success,” was published online July 31, 2017 in
Transactions of the American Fisheries Society (http://www.tandfonline.com/doi/abs/10.1080/00028487.2017.1336114?journalCode=utaf20). Hughes’ co-author
is Andrew Murdoch, WDFW’s Eastern Washington Science Manager.
the results of differences in spawning distribution and fish size
(natural-origin females are 9 percent larger), redds constructed by hatchery
females were more susceptible to environmental sources of mortality than were
redds constructed by natural females,” the study says. “This study provides
insight into the possible mechanisms responsible for the reported lower
reproductive success among hatchery females spawning in the natural
reproductive success or RRS is defined by the study as “the number of offspring
produced by an individual spawner.”
some cases the lower RRS is due to the rapid adaptation of fish to the hatchery
environment and that hinders their performance when in the natural environment,
the study says. In other cases, the reduced RRS can be attributed to
environmental factors, such as the choice of where to spawn and the resulting
quality of the redd.
the Chiwawa River, the upstream areas are characterized by pristine pool-riffle
channels and the lower areas of the river by “higher-gradient, larger
substrate, plane-bed channel types” with fewer gravel or cobble areas that are
best for spawning. Pool-riffle channels are of moderate to low gradient with
beds of bars, pools and riffles, according to the study.
channels accounted for 58 percent of the river’s habitat (32 kilometers or
about 20 miles) and included most of the river’s best spawning reaches in
upstream areas, while Plane-bed channels accounted for 34 percent of the
river’s habitat (18 km or about 11 miles), mostly in downstream reaches.
largest number of carcasses were found in pool-riffle areas. Some 62 percent of
hatchery-origin fish were found in pool-riffle areas, but 90 percent of the
natural-origin fish spawned in those reaches. Of the fish found downstream in
plane-bed channels, 92 percent were of hatchery-origin.
nearly all of the hatchery fish spawn upstream of the acclimation and release
site at rkm 1, and, in fact, the mean distance upstream for hatchery fish
spawning was 23.63 rkm. That, the study says, indicates that there may be other
factors than just homing to natal locations that contribute to the selection of
said the research is part of a larger study that is examining the reproductive
success of hatchery spring chinook salmon in the upper Wenatchee River basin.
research associated with this study will directly examine the relationships of
habitat quality and individual female fitness data in an attempt to begin to
understand the environmental and/or genetic mechanisms associated with
decreased fitness,” he said.