Chinook study focuses on off-channel areas

It has long been known that healthy habitats off the main channels of rivers is important to migrating juvenile salmon and steelhead, even if for very short periods of time, and more off-channel habitat improvement projects should be pursued to preserve a diversity of life histories, according to a recent study.

While not all juvenile fish stop on their migration to the sea to feed and grow in these off-channel habitats, the study finds that those fish that do stop can spend just a few hours in these refuges or they can hang out for up to 25 days. It’s possible that some of the fish will stop at multiple off-channel sites on their migration.

Just 80 square kilometers (about 31 square miles) of a potential 344 square kilometers (about 133 square miles) of off-channel habitat are available in the tidal freshwater of the Columbia River from river kilometer 58 (36 miles from the Columbia River mouth) upstream to Bonneville Dam at river kilometer 234 (145 miles), the area of the study that was published in the Canadian Journal of Fisheries and Aquatic Sciences.

“This study is the first to estimate residence times for juvenile salmonids specifically in off-channel areas of tidal freshwater and, most importantly, residence times for chinook salmon expressing a life history of overwintering in tidal freshwater,” said Gary Johnson, research scientist with the Pacific Northwest National Laboratory in Portland and one of the study’s authors. “The findings support restoration of shallow off-channel habitats in tidal freshwater portions of the Columbia River.”

“Residence times of juvenile salmon and steelhead in off-channel tidal freshwater habitats, Columbia River, USA” was published online January 6, 2015. In addition to Johnson, authors are Gene Ploskey with PNNL in North Bonneville, Wash., Nichole Sather with PNNL in Sequim, Wash., and David Teel with NOAA Fisheries’ Northwest Fisheries Science Center in Manchester, Wash. All are fisheries biologists.

Johnson said the researchers documented two life history strategies for juvenile salmon that at some point in their migration used off-channel tidal freshwater habitats of the Columbia River:

1--active migrations by upper river chinook salmon and steelhead during the primary spring and summer migration periods, and

2--overwinter rearing in tidal freshwater habitats by coho salmon and naturally produced chinook salmon mostly from lower river sources.

With 13 salmon and steelhead stocks in the Columbia River basin listed under the federal Endangered Species Act as threatened or endangered, one of the strategies developed for recovery is restoring “flood plains wetlands that were historically connected to off-channel habitats in the tidally influenced” areas of the lower Columbia River estuary “under the premise that restoration of shallow, off-channel habitats will improve survival and increase life history diversity,” the study says.

Juvenile upper river chinook salmon and steelhead use the off-channel tidal freshwater shelter during their spring and summer migrations while actively migrating. In addition, coho salmon and naturally-produced chinook salmon that are mostly from lower Columbia River sources use the sites as overwintering refuges.

The researchers used acoustic receivers in the Sandy River delta in 2007 and 2008 to collect data about residence times. In addition, they sampled juvenile yearling chinook salmon and coho salmon in January 2010 and February 2011 in the Sandy River delta and added Cottonwood Island downstream near Longview, Wash. in 2012 to determine residence times for the fish.

What they found in the earlier tagging study was, as a percentage of the total run, yearling chinook salmon used tidal freshwater off-channel habitat the most, with 8.1 percent of the total run of juveniles using an off-channel site in 2007 and 9.3 percent in 2008. In 2008, 4 percent of steelhead used off-channel sites. 3.6 percent of subyearling chinook salmon used the sites in 2007 and 6.1 percent did in 2008.

The surprising finding of the study was the time juvenile fish that were tagged upstream of Bonneville Dam spent in the off-channel sites.

Residence time was short, with yearling chinook salmon showing a median residence time of 2.5 hours in 2007 and 2.6 hours in 2008. Yearling chinook lengths were 134 millimeters (5.3 inches) in 2007 and 158 mm (6.2 inches) in 2008. Subyearling chinook stayed a little longer, 3.0 hours in 2007 and 3.4 hours in 2008. Median lengths were 104 mm (4.1 inches) and 116 mm (4.6 inches). Yearling steelhead (median length of 215 mm, 8.5 inches) stayed just 2.5 hours.

The 2010-2012 study, which had a different study design and targeted yearling chinook and coho salmon, found that fish stayed longer in the off-channel sites. In that study, median residency lasted 11.6 days to 25.5 days for chinook and 11.2 days for coho. These fish likely used the shallow freshwater habitat in order to feed and grow, the study said. In addition, these are minimum residence times and that implies that the fish were not active migrants.

“We surmised that the fish entered tidal freshwater as subyearlings late in the previous summer or fall, overwintered, and emigrated as yearlings,” Johnson said of the short stops at off-channel habitats made by overwintering juveniles. “The residence times are minimums because we don’t know how long the fish was in the area before it was captured and tagged.”

Many of the fish likely stopped at multiple off-channel sites during their migration, the study said.

Overall, the study illustrates “that the full range of the chinook salmon juvenile life histories in the Columbia River is not easily described using a simple subyearling-yearling dichotomy.” The study does suggest that the juveniles in the 2010-2012 study entered the lower Columbia River estuary as subyearlings and eventually migrated to the ocean as yearling fish.

The residence times observed in this study support the importance of freshwater off-channel habitats in the lower Columbia River estuary, the study says. “The amount of off-channel habitat available, therefore, is important to support the diversity of life history strategies expressed by juvenile salmon,” it concludes.