Using this approach, PATH scientists identified two key uncertainties that have the strongest effects on survival and recovery of Snake River spring/summer and fall chinook: extra mortality of nontransported fish and the relative post-Bonneville survival of transported fish compared to post-Bonneville survival of nontransported fish. Bonneville Dam is the last of eight dams that smolts pass on their way to the ocean (“BON” in Fig. 1).
Extra mortality is defined as any mortality occurring outside the juvenile migration corridor that is not accounted for by the other terms in the life cycle model used for retrospective and prospective modeling (i.e., terms for stock productivity and carrying capacity, mortality in dams and reservoirs, and estuarine/ocean mortality affecting all salmonid populations). Because many of the changes that may account for historical patterns in extra mortality all happened around the same time (e.g., Fig. 2) there is uncertainty about which of these factors (or mix of factors) influences extra mortality. Therefore, PATH formulated three alternative hypotheses about the source of this extra mortality:
Extra mortality can only be inferred from other measured quantities; it cannot be
directly measured. This makes it difficult to monitor changes in extra
mortality resulting from an experimental action, and thus to test alternative
hypotheses. Nevertheless, extra mortality is still an important construct
because (a) it helps to design experimental management actions that address its
potential causes; and (b) it is needed to simulate the range of effects of
alternative experimental actions to assess their relative risks and benefits.
In the PATH modeling framework, the ratio of these two values is known as “D”. Like extra mortality, D cannot be directly measured, but must be inferred from other measured quantities (e.g., Transport:Control ratios and in-river survival estimates from transportation studies for spring/summer chinook). Differences in the assumptions used to estimate D led to alternative hypotheses about both historical and future D values, for both spring/summer and fall chinook.
IMPLICATIONS FOR SELECTING A LONG-TERM MANAGEMENT ACTION
Reducing these key uncertainties can help to identify the long-term management action that is best able to recover the stocks. There is an interaction between extra mortality hypotheses and the D value: forecasts of recovery are generally more sensitive to the extra mortality hypotheses if D is a high value. If D is high, fewer transported fish die below Bonneville Dam. Other factors causing extra mortality of all fish are then required to explain historical declines in overall survival. If D is low, post-Bonneville mortality of transported fish is sufficient to explain most of the observed historical declines in overall survival rates, and extra mortality factors affecting all fish become less important. This suggests that we should not measure D without also narrowing down the extra mortality hypotheses, and vice versa.