Chinook salmon reared in the upper stretches of the Columbia River watershed 250 to 500 years ago used to leave their freshwater habitat and enter the estuary - and possibly even the Pacific Ocean - when they were smaller and younger than most of their contemporary counterparts.
Researchers tracking the life history of salmon long before dams were built on the Columbia say the finding suggests that fisheries leaders may need to manage for a diversity of life histories.
Results of the research have been published in the Canadian Journal of Fisheries.
"The Columbia River estuary is an amazingly productive system and there clearly are advantages for fish to enter into that environment," said Jessica Miller, an Oregon State University ecologist and lead author on the study. "Yet today fish remain in fresh water for a longer period of time - possibly because they must navigate past the dams, and because river flows during their ocean migration have been reduced with the development of the hydropower system.
"Chinook salmon have a more diverse portfolio than other salmon species, which may be one reason some of their populations are doing so well," Miller added. "Managing the resource to retain that diversity seems like a logical strategy."
"We know there are advantages for the salmon to reach a certain size before entering the ocean, especially in avoiding prey," Miller pointed out. "But there may be long-term advantages to having individuals that migrate at a diversity of sizes."
To learn more about ancient salmon runs, the researchers worked with the Confederated Tribes of the Colville in Washington, where they obtained the skeletal remains of salmon from a former archaeological site just downriver from Grand Coulee Dam. The fish, which the scientists dated to 250-500 years ago, were in an area of the Columbia River which is no longer accessible to migrating fish because of the dams.
One goal of the research was to see if fish that used to go upstream of Chief Joseph Dam - the farthest upriver that salmon and steelhead return - had different characteristics than present-day fish. To do this, they looked at the bony structure within the salmon's ears called an "otolith," which accretes calcium carbonate and forms growth rings. By examining the growth rings and isotopes within otoliths, scientists can ascertain the age of a fish, where it lived and sometimes what it has eaten.
"It's pretty amazing that we can look at the otolith of a 500-year-old fish and determine which river it likely originated in and at what size it entered marine waters," said Miller, an assistant professor of fisheries and wildlife who works out of OSU's Hatfield Marine Science Center in Newport.
They do this by analyzing the ratio of strontium-to-calcium isotopes in the otolith. A high ratio indicates a fish has been living in salt water, while a lower ratio suggests recent freshwater history. They also can examine two isotopes of strontium, which can provide information on the river of origin.
"We can also estimate where in the river system they were, because as you move east to west, the rocks get younger and the strontium values change," Miller said. "In most cases, the isotopic signature is extraordinarily revealing."
Miller also was lead author on another study, published in the Marine Ecology Progress Series, which examined diversity of fish runs in modern populations. Focusing on Central Valley (California) Chinook salmon, the study determined that adult fish typically had begun their juvenile migration in two "pulses."
A majority of adults had begun their seaward migration as larger juveniles (75 millimeters or longer), which typically leave rivers in mid-April to May. But the adult sample also contained fish that had begun their emigration as smaller fish (less than 55 mm). Though fewer in numbers, these smaller fish were still significant and typically left rivers in February and March.
"In the Central Valley, the vast majority of hatchery production is focused on larger juveniles, whereas most of the naturally produced fish appear to emigrate at a smaller size," Miller said. "Similar to the variation in adult run timing - which may protect runs against catastrophic floods, drought or disease - variation in the timing of juvenile migration to the ocean may be important for long-term survival."
Other researchers on the Canadian Journal of Fisheries study include Virginia Butler, Portland State University; Charles Simenstad, University of Washington; David Backus, Williams College; and Adam Kent, OSU.
Source: Oregon State University