4.4 - Fishing Mortality

Fishing mortality rates are estimated using cohort analysis based on coded-wire-tag (CWT) recoveries and are stock, age, and fishery specific. The estimation procedure is explained in more detail in the next section.

Two types of fishing mortality rates are distinguished. "Exploitation Rates" are expressed in terms of total coastwide abundances, not regional abundances. Thus, an exploitation rate of 0.10 for a given stock, age, and fishery means that 10% of the coastwide abundance of that stock/age cohort is harvested in the given fishery. "Harvest Rates" refer to fishing mortality rates in terminal areas where the regional abundance (i.e., true terminal run) of the stock is known.

Mortalities associated with fishing activities are assessed in two phases-preterminal and terminal-corresponding to the two primary life history phases of each cohort-immature and mature. Within each phase there are legal harvests and incidental mortalities. Incidental mortalities are caused by (1) the inadvertent capture of sublegal sized fish during fisheries targeting on chinook salmon (called shaker mortalities) and (2) the inadvertent capture of sublegal- and legal-sized chinook salmon during fisheries targeting on other salmon species (called chinook non-retention, or CNR, mortalities).