1.2 - General Description

The PSC Chinook Model was developed by the PSC Chinook Technical Committee to examine alternative management approaches to implement the PSC chinook rebuilding program (the next section contains a brief history of the model). The model is capable of simulating a large number of years, stocks (hatchery and natural), and fisheries (troll, net, and sport) (Table 1.1 and Table 1.2). A key feature of the model is the interaction between stocks through annual catch ceilings imposed upon fisheries that harvest multiple stocks. As stocks rebuild or decline at different rates over time, relative harvest rates in ceilinged fisheries also change. Single stock models cannot simulate this type of interaction.

Simulations are divided into two time periods: (1) a calibration period; and (2) a management simulation period. The calibration period runs from 1979 through the last year for which model parameters can be estimated (usually one year behind the current year). The simulation period runs from the current year to any future year (usually about 10-15 years in the future). The PSC Chinook Model produces information to help evaluate the effects of changes in brood year survival rates and several management actions:

• pre-recruitment (i.e., age one) survival projections
• pre-spawning survival (i.e., inter-dam losses)
• enhancement activities
• catch ceilings (catch quotas)
• harvest rate strategies
• size limits.

Production parameters for both hatchery and natural stocks are estimated from historical data. Ocean survival rates for ages one through five are assumed fixed (at 0.5, 0.6, 0.7, 0.8, and 0.9, respectively) for all stocks. Survival rates to age one (also called Environmental Variability, or "EV," scalars) are estimated during the calibration process. Other parameters are estimated by a technique known as "cohort analysis" or "virtual population analysis." This type of analysis involves the reconstruction of an annual series of abundance estimates using catch and escapement data and making assumptions about natural and incidental mortalities. Once each cohort has been reconstructed, the following parameters are estimated:

• Cohort size for each age class at the beginning of each year
• Age specific harvest rates for each fishery
• Maturity schedule for all ages
• Estimates of incidental fishing mortalities.

Management changes are evaluated by changing key parameters, such as future catch ceilings or harvest rates, and rerunning the model. In the QuickBasic(TM) version of the PSC Chinook Model, parameters are changed by opening appropriate ASCII data files and changing the appropriate data fields. This process also involves changing file names in control files. A 25 year simulation with 30 stocks and 25 fisheries takes three to five minutes using a PC computer with a 486 microprocessor. Output data are displayed by downloading to data files which must be imported into other analysis programs, such as a spreadsheet.

CRiSP Harvest allows the operator to change parameters and view results interactively. Parameter values can be changed by using the mouse or keyboard. Results can be presented in graphical form on the screen immediately after a simulation run (graphs can also be printed), or can be downloaded to data files for archiving or further analysis.