Juvenile Salmon Ecology of Tidal Freshwater Wetlands in the Lower Columbia River

The tidal freshwater portion of the lower Columbia River includes the steep banks of the Columbia River Gorge, urbanized areas near Portland and lower gradient areas bordering the estuary. Floodplain habitats in downstream river reaches are generally flatter with higher tidal ranges, while sites in the upper reaches are generally steeper with greater variability in water levels.

Until the recent decade, much of the research for the lower Columbia River have focused on the estuarine section and neglected the tidal freshwater section. Currently, there are several monitoring programs filling previous knowledge gaps by assessing status and trends of ecosystem condition, critical uncertainties in juvenile salmon outmigration patterns for habitat use, and effectiveness of individual and cumulative habitat restoration actions. This presentation will describe findings from several studies to discuss juvenile salmon ecology in this region of the lower river. We will focus on aspects important for consideration in designing restoration actions in this region, including the following sample of study findings:

Timing and locations of habitat use:

• During April through August sampling, abundance of juvenile salmon peaked during April-June.
• Juvenile Chinook salmon were the most abundant salmon and both marked and unmarked Chinook were encountered at all sites.
• Juvenile coho were more prevalent in the upper reaches of the lower river, while unmarked juvenile Chinook dominated lower reaches.
• Small numbers of chum were found at most sites.

Juvenile salmon prey and diet:

• Across sites and time, prey were more abundant in samples collected nearshore and near emergent vegetation relative to samples collected in deeper, open water.
• Juvenile Chinook consumed primarily aquatic fly larva and pupa (Diptera) at sites.
• Results from mean selectivity analyses indicate a high selection for Dipterans vs. Cyclopoida and Cladocerans, even though the latter were more abundant.

Spatial differences in salmonid condition:

• Chemical contaminant concentrations were highest in juvenile Chinook salmon in urbanized and downstream areas.
• Contaminant levels in stomach contents show a similar trend.

Site condition and inundation patterns potentially affecting salmon use:

• Vegetation species richness generally increased in the middle reaches, while the composition of wetland plant types remained consistent. 
• Vegetation community boundaries did not change in response to inter-annual water level variability.
• Differences in water levels between years affected vegetation cover and potential for fish access and feeding.
• Water temperatures are unsuitable for salmonids after June.