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Choice Of Spawning Habitat May Result In Lower Reproductive Success For Hatchery Spring Chinook
Posted on Friday, August 11, 2017 (PST)

Where salmon choose to spawn within a river system has an impact on their relative reproductive success.


In 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.


A 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.


That 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 channels.


“Our 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.”


That’s 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.


Redds 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 hatchery females.


“We 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.


“Spawning 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 ( Hughes’ co-author is Andrew Murdoch, WDFW’s Eastern Washington Science Manager.


“As 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 environment.”


Relative reproductive success or RRS is defined by the study as “the number of offspring produced by an individual spawner.”


In 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.


In 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.


Pool-riffle 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.


The 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.


Still, 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 spawning locations.


Hughes 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.


“Future 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.


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