Based on several of the presentations made at the workshop, and the early discussions of this working group, we formulated two hypotheses on which a subarctic Pacific U.S. GLOBEC effort could be based. They are as follows:
Hypothesis 1 (H1): Ocean survival of Pacific salmon is determined primarily by survival of juvenile salmon in coastal regions, and is affected by interannual and interdecadal changes in Gulf of Alaska physical forcing.Hypothesis 2 (H2): Variation in size-at-age of returning salmon is determined largely by interdecadal and interannual variation in physical conditions and productivity of the oceanic realm of the subarctic Pacific, and may show density dependence.
These two hypotheses imply a first-order independence of mortality and growth processes. H1 implies that the survival of salmon populations is principally determined by coastal conditions, early in the juvenile phase. H2 suggests that final size (weight) of those individuals which survive the early juvenile phase in coastal waters is reflective of growth occurring during their final and/or penultimate year, when they are feeding in the oceanic realm of the subarctic Pacific. Density dependence of growth rate may occur if early juvenile mortality is low, a large number of salmon survive to exploit a common resource in the oceanic realm, and competition for that resource occurs. In this way, growth rates in the later phase are coupled to mortality rates in the early phase.
The working group realized that detailed, process-intensive studies of both the oceanic and coastal realms of the subarctic Pacific would not be feasible with the resources likely to be available for a U.S. GLOBEC study. Process-oriented research and surveys in a focused coastal study were designed to address H1. These are more feasible, with limited resources, than the larger-scale process studies in the deep ocean that would be needed to address H2. However, in order to connect a coastal Gulf of Alaska ecosystem study to climate forcing, we recommend that some specific limited biological and physical observations be obtained at the basin (gyre) scale. Figure 3 provides a diagram (cartoon) of the types of observations needed to make that connection; specifics of this diagram are described in the later sections on monitoring, modeling and process studies.
