Subyearling Chinook salmon diets in Lower Columbia River estuarine habitats

The Lower Columbia River and Estuary is critical rearing habitat for
juvenile Pacific salmon. Extending from the river mouth to Bonneville Dam
235 river kilometers upstream, the Estuary has been altered by dams,
dikes, and habitat loss due to deforestation and wetland removal. Since
2008, a science group has monitored five sites to identify the long-term
status and trends in Lower Columbia River and Estuary juvenile salmon
rearing habitat. Here, we address predominantly natural origin juvenile
Chinook Oncorhynchus tshawytscha diets and identify spatial and temporal
trends in prey consumption, stomach fullness, energy consumption, and the
metabolic costs associated with fish size and water temperatures. Juvenile
Chinook diets consisted mainly of corophiid amphipods, chironomid
dipterans, and cladocerans, with other insects filling in most of the
remainder of their diets. Juvenile Chinook salmon diets were stable, and
stomach fullness and caloric intake was comparable among the sites where
most salmon were collected. Juvenile Chinook salmon were frequently in
water temperatures above fitness thresholds. Higher water temperatures
raise metabolic rates, so increased foraging will be necessary for growth
in rising water temperature regimes. Reduced growth, earlier migration,
and prey production timing mismatches are near term possibilities.
Juvenile salmon rearing resiliency in the estuary can be aided by
maintaining sufficient river discharge levels for salmon, and by restoring
habitat and habitat connectivity to the mainstem channel.