Study evaluates juvenile salmon in Columbia tidewater

Juvenile chinook salmon density in shallow habitats downstream of Bonneville Dam is largely due to time of year, but density does differ across habitat types.

The best habitats, according to a recent study, included those with higher percentages of tree cover, acceptable dissolved oxygen levels and higher emergent vegetation.

The study looked at 63 months of data (2007 – 2012) from two tidal freshwater areas in the Columbia River – the Sandy River delta just east of Portland and a stretch of mainstem river downstream (river mile 68 to 88) that the researchers call the lower river reach.

According to study author Nichole Sather, research fishery biologist with the Pacific Northwest National Laboratory, Marine Science Laboratory, there is a strong seasonal trend in the abundance and size distribution of juvenile chinook salmon that is consistent across years in the Sandy River delta, and the proportions of genetic stock groups were similar across years, but differed across seasons.

In the lower river reach, genetic stocks of juvenile chinook salmon did not differ across seasons, nor did they differ among the three different habitat strata, Sather said. Densities were greatest in off channel habitats and lowest in main channel.

“When this study was initiated, very little was known about juvenile salmon use of tidal freshwater habitats, especially in the Columbia River estuary,” Sather said. “We selected a diversity of habitats to understand how juvenile salmon use these areas. In 2009, the research scope was expanded to include an additional study area to provide context to our findings.”

The initial study used different spatial and temporal sampling approaches which allowed the researchers to examine different aspects of juvenile chinook salmon life history, she added. Intensive monthly sampling in one area yielded information on seasonal and inter-annual patterns of abundance, size, and genetic stocks. A random stratified sampling approach in the second study area allowed the researchers to examine characteristics related to different habitat types – main channel, off channel, and wetland channel.

Sather’s co-authors are Gary Johnson, research fishery biologist at PNNL, Marine Science Laboratory; Gary Johnson, research fishery biologist, PNNL; David Teel, geneticist, NOAA Fisheries Northwest Fisheries Science Center; Adam Storch, research fisheries biologist, Oregon Department of Fish and Wildlife; John Skalski, statistician, Columbia Basin Research School of Aquatic and Fishery Sciences, University of Washington; and Valerie Cullinan, statistician, PNNL, Marine Science Laboratory.

Researchers observed in the two study areas that the greatest densities of salmon occurred during spring and winter.

“Low salmon densities during summer and fall in the shallow-water habitats could be the result of higher water temperatures and lower water surface elevations in comparison with winter and spring,” the study says.

During the summer, salmon density in the main channel study area was constrained by “thermal constraints of the wetland channel of off-channel habitats.”

“Salmon densities were generally higher in off-channel habitats than in wetland channel and main-channel habitats,” the study says.

“We found significant positive associations between juvenile salmon density and percent tree cover, dissolved oxygen level, and percent emergent vegetation,” the study says.

Trees provide shade in shallow areas to cool water. Shoreline vegetation supports salmon through “inputs of terrestrial invertebrates as well as through the production of detritus,” the report says.

Dissolved oxygen was similar across all habitat types, but the researchers found that concentrations were lower in wetlands channel sites than in main-channel and off-channel habitats.

The researchers found that several species of juvenile salmon occupied a range of tidal freshwater habitats, Sather said. Juvenile chinook salmon were the most predominant salmonid captured and were captured during all seasons. Other salmonids captured included chum, coho, and steelhead.

“As expected, most of the chinook salmon that occupied habitats in both the SRD and LRR were from populations with fall and summer/fall adult run timing, since those populations make extensive use of shallow estuarine habitats for juvenile rearing,” the study says. However, the Sandy River delta also contained 11 percent spring-run fish and the lower river segment had 7 percent.

The Sandy River delta percentage of spring chinook was during the fall, with 63 percent. Overall in the delta, unmarked chinook salmon were the most predominant group of salmonids captured, Sather said.

“We did distinguish catches of hatchery groups. For example, unmarked chinook salmon accounted for 69 percent of the salmon catch in the SRD study area, and marked chinook comprised 7 percent,” Sather said. “While marks indicate hatchery origin, unmarked fish can include both natural and hatchery origin fish.”

Winter samples in the Sandy River delta were mostly of Spring Creek fall stock, while spring and summer stocks were mostly upper Columbia River summer/fall fish, and fall is dominated by Willamette River spring chinook stock (these fish are in the Sandy River due to a history of hatchery stock transfers), according to the study.

Although the study wasn’t intended to evaluate restoration efforts in the Columbia River because the areas studied had not been the subject of restoration, it may have implications for future restoration efforts, according to Sather.

“In terms of implications for restoration, we note that juvenile salmon occupy a diversity of habitats and suggest that restoration strategies target a diverse suite of habitats to support a range of salmonids life histories,” she said.

The study says it is also important to protect existing habitats and to ensure habitat availability “year-round to support the recovery of endangered salmon populations in the Columbia River basin.”