A study of coho salmon in three small Olympic Peninsula rivers with estuaries show a complex life history that includes juveniles that migrate to sea early in their first year and others that stay in the stream for up to a year before they migrate into the sea where they reside for six or eighteen months.
Biologists have believed that the early out-migrants had not added to the number of adults returning one to two years later, but the study concluded that they do contribute to the number of adults that return to the streams to spawn.
In addition, the study found that some of the juveniles migrate among the three rivers and in and out of salt water before making their final migration to the sea.
This migration diversity is not uncommon. In fact, studies in 2011-2013 have found up to five or more juvenile coho salmon life histories in one river basin, allowing the species to spread the risk of mortality.
However, prior to this study it was thought that the later migrants (spring) were responsible for all returning adults and that early migrants (fall/winter) simply had no impact on smolt-to-adult returns.
As the study says, they were thought to be “’surplus’ to the stream’s carrying capacity.”
“Nomads no more: early juvenile Coho salmon migrants contribute to the adult return,” published in April in Ecology of Freshwater Fish (http://onlinelibrary.wiley.com/doi/10.1111/eff.12144/abstract;jsessionid=32580E232075E5009EC29BF07862F165.f03t01?deniedAccessCustomisedMessage=&userIsAuthenticated=false, is by Todd Bennett, research fishery biologist with the Watershed Program at NOAA Fisheries Science Center in Mukilteo, Wash.; Phil Roni, Watershed Program manager at NOAA’s Northwest Fisheries Science Center in Seattle; Keith Denton, fisheries scientist at NOAA Fisheries; Michael McHenry, fisheries habitat biologist/manager in the Natural Resources Department of the Lower Elwha Klallam Tribe; and Raymond Moses, project biologist in the Natural Resources Department of the Lower Elwha Klallam Tribe.
Contributors to the field work also included the Weyerhaeuser Corporation, the Washington Department of Fish and Wildlife and the Washington Department of Ecology.
Bennett said the study is a byproduct of another project that looked at the survival of juvenile coho in streams where habitat had been enhanced (East Twin River and Deep Creek) compared to a control stream with no habitat enhancement (West Twin River). The rivers flow directly into the Strait of Juan de Fuca on the northern edge of Washington’s Olympic Peninsula.
“Because the project has been in place for about ten years, we were able to see several generations of adult returns from the juvenile tagging,” he said. “We noticed immediately the large fall outmigration that occurred every year and wanted to see if those fish contributed to the adult return or were in fact ‘surplus’ fish.”
Instead of being “nomads” that do not contribute to the adult population, the study found that early migrants, that is the migrants who left the river in the fall and winter of their first year, contributed 37 percent of the returning adults. Of this 37 percent, half spent two winters in salt water and returned as generally larger adults, according to the study.
On the other hand, mean survival for the spring migrants (the later migrants) was more than three times higher than the fall/winter early migrants, largely due to body size as juveniles when they enter salt water, the report said. However, the size of a smolt when entering salt water may not be the only factor impacting survival. Early migrants “may encounter entirely different environmental conditions than those that enter in the spring: temperature, food availability and predator interactions may influence survival.”
In addition, movement among streams could affect SAR rates. The authors observed both stream swapping by juveniles and straying among adults.
Finally, they found that the early migrants returned to the stream as adults about 16 days later than the fish that remained in the stream and migrated out to the sea later.
“It’s becoming apparent that this phenomenon occurs in other streams and may represent a significant portion of the adult return,” Bennett said. A recent study in Oregon is seeing similarities in streams with estuaries. That study, he said, is “transferrable in that the methods could be used in small streams throughout the range of the species, which would in turn show how variations of the early migrant life history occur across the range.”
While some streams have no estuaries, Bennett pointed to the Salmon River in Oregon and Winchester Bay in Oregon that have well-established estuaries. “In the northern part of the range, such as Alaska, we see all different types of streams – high gradient, low gradient, with and without estuaries. It would be great to do this type of project across the whole range (north-south) and include all types of streams. I think we’d see even more life histories emerge,” he said.
The knowledge that a significant segment of a coho salmon population once known as nomads but now known to contribute to adult returns will likely have an impact on management.
Today, SAR estimates for coho salmon in these streams is calculated using just the spring smolt counts and some form of adult census. The estimates do not account for juveniles leaving the stream early.
“Our results indicated that traditional methods of spring-only smolt enumeration may underestimate juvenile survival and total smolt production, and also overestimate spring smolt-to-adult return (SAR),” the report says.
On the other hand, the contribution of the adult return from the early fall/winter migrants is highly variable, according to the report, and so the traditional calculations for SAR would also be highly variable. In addition, predicting a higher number of early migrants could be overly optimistic if the SAR rate turns out to be lower than expected. This complicates management.
“If harvest rates are based only on spring SAR, they could be set higher than is sustainable for many populations,” the report says. The authors say that predictions could be better refined by incorporating metrics other than smolt numbers. Those metrics could be size (the proportion of coho greater than 70 millimeters in length, for example) and the proportion of early to later migrants leaving the stream.