Ongoing research conducted by the Skagit River System Cooperative1 observes a density
dependent relationship occurring within the Skagit delta on wild ocean type Chinook. The
finding provides a solid basis for advocating delta restoration, especially considering the history
of approximately 80% habitat loss within the delta (Collins and others 2001).
In addition to our density dependence research, we have been studying juvenile Chinook otolith
microstructure2 in order to identify the juvenile life history type of individual fish and estimate
their growth and residence by habitat type. We recently completed analyses that show the
relationship of delta residence on later performance (measured as growth rate in Skagit Bay) by
wild ocean type Chinook originating from within the Skagit River.
The density dependence study results indicate that the relationship between freshwater wild
juvenile Chinook population size and wild juvenile Chinook abundance in estuarine river delta
habitat is density dependent (asymptotic) (Figure 1). This result supports the idea that present
day Skagit delta habitat capacity is inadequate for outmigrating delta-rearing Chinook.
Conversely, the proportion of the total wild juvenile Chinook population in Skagit Bay that
bypasses rearing in delta habitats and migrates directly into Skagit Bay (we define this life
history type as fry migrant) increases with wild smolt outmigration levels above 2,500,000
(Figure 2). This finding indicates that at least some of the density dependence occurring in the
delta results in the displacement of juvenile Chinook out of delta rearing habitats and into Skagit
Bay early in the year (usually in February or March) at a very small size (~40 mm fork length).
The findings provide a solid biological basis for advocating restoration to delta capacity for
rearing ocean-type Chinook.