Spawning, Rearing Habitats
Study Follows Juvenile Salmon With Two Life-Histories
Not all juvenile chinook salmon in Idaho rivers stay in their natal waters to rear and that may contribute to their success when returning to spawn as adults.
A recent study followed juvenile spring/summer chinook salmon from the same streams, but with two life history paths. It found that most of the juveniles would move quickly out of their natal, and in many cases infertile, stream to overwinter in a non-natal area downstream.
In a second life history path, juveniles overwinter in their natal stream, beginning their migration to the sea after winter.
Of the two paths -- the first, termed the downstream rearing group (DSR), the second the natal reach rearing group (NRR) -- the NRR group showed up on their downstream passage one month later than the DSR group and returned as adults in smaller numbers.
"There is a lot of recent literature about life history diversity in salmon. We have shown how this diversity can be important to population dynamics and why it needs to be incorporated in life-cycle models and management," said the study's co-author Timothy Copeland, senior research fisheries biologist with the Idaho Department of Fish and Game. "The presence of suitable, accessible rearing habitats downstream from the natal reaches increases population productivity and has great potential value to population recovery, delisting, and future harvest opportunities."
The life history of the DSR salmon suggests that stream restoration for salmon recovery should not just concentrate on habitat where salmon spawn, but should also consider the impacts of habitat management and improvements outside core spawning areas, according to Copeland.
The study, "The Importance of Juvenile Migration Tactics to Adult Recruitment in Stream-Type Chinook Salmon Populations," was published online in October in The Transactions of the American Fisheries Society.
In addition to Copeland, study authors are David Venditti, senior research fisheries biologist, and Bruce Barnett, senior fisheries technician, both with the Idaho Department of Fish and Game.
The authors studied chinook salmon populations that are listed as threatened by the federal Endangered Species Act in tributaries of the Clearwater and Salmon rivers in Idaho, identifying the two migratory tactics among those populations.
Most of the juveniles left the natal stream as subyearlings (the DSR group) in the summer and fall and overwintered somewhere between the natal stream and Lower Granite Dam, which is between 266 and 747 kilometers (about 165 to 464 miles) downstream. The fish, apparently, find better rearing habitat downstream. The new location also gives the DSR fish as yearlings a head start by about one month on their downstream migration to the ocean as spring arrives.
However, the NRR group remained in the natal stream and also began their downstream migration as yearlings. These are the natal reach rearing group and their numbers are far fewer than the DSR group.
"Both groups pass Lower Granite Dam on their way to the Pacific in the spring within a month of each other," Copeland said. "But fish from the DSR group composed the bulk of the spawning population in every case investigated. This implies that use of the accessible habitat on different schedules by juvenile salmon affects their relative fitness and number of adults back to freshwater."
Spawning and rearing habitats are not necessarily the same and there can be a benefit to leaving the natal reach early. A salmon's ability to migrate and disperse to other areas allows them to use a variety of habitats available, increasing population production, he added.
The DSR-type begins with a large numerical advantage: more than an average of 85 percent of the total population of juveniles migrate and overwinter downstream. The DSR smolt-to-adult recruitment tended to be 1.6 times greater than the NRR group. In fact, it appears that the DSR fish derive a delayed survival benefit by moving to more friendly rearing habitat as juveniles.
The NRR group performed better in years when total recruitment was low, depending on the natal stream.
The diversity -- the two life histories -- spreads the risk, even while one of the life histories produces most of the spawning population.
". . .[D]ispersive life history types increase population productivity and resilience, but resident life histories enhance stability, and thus both contribute to population persistence," the report says.
The DSR type should be more important to recovery of the salmon population because there are far more of the fish than the NRR type, but the NRR fish "should buffer the population during periods of low abundance," the report says.
According to Copeland, there are two major implications for salmon managers:
First, because populations may vary in expression of the DSR and NRR tactics, management actions in the migratory corridor or spawning reaches may affect the populations differently.
Second, there is also a big gap in knowledge about how the habitat between the natal reaches and Lower Granite Dam are used for rearing and overwintering by spring/summer chinook salmon in Idaho.
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