4.2 - Biological Processes

Non-fishing and fishing mortalities occur independently and at different times of the year. For most stocks, non-fishing mortality is composed entirely of natural ocean mortality and is assessed at the beginning of each year. For Columbia River stocks that spawn upriver from dams, additional non-fishing mortality (called pre-spawning mortality or inter-dam losses) is assessed after fishing mortality to account for losses between dams (Fig. 4.2).

Fig. 4.2 Assessment of mortalities during one year.

Natural ocean mortalities are age specific, but not stock specific. For example, age two fish from all stocks have the same natural ocean mortality, or survival, rate. Thus, at the start of each year, ocean run sizes for each cohort are updated as follows:

OcnRun(s,a) = Cohort(s,a)OcnSurvRt(a) [4.1]

where

• OcnRun(s,a) = ocean run size of stock s, age a
• Cohort(s,a) = cohort size of stock s, age a at the start of the year
• OcnSurvRt(a) = natural ocean survival rate for age a.

In 1982, when these cohort analysis procedures were begun (undocumented), it was decided to use stepped values of mortality by age. The values chosen are given in Table 4.1 The mean of the values used for ages three through five is 20% (similar to the 17% used in the Argue paper). The 40% continues the stepped progression.