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Estimating Fishing Mortality Rates
Parameters are estimated by a technique known as "cohort analysis" or "virtual population analysis". A cohort, in this context, is the total production which results from the escapement of a single year class from a particular group of fish. This type of analysis involves the reconstruction of an annual series of abundance estimates using the following data:
- Catch at age data from fisheries of interest
- Assumptions regarding incidental mortality rates and losses associated with these catches
- Escapements at age data
- Expansion of escapements to account for straying and pre-spawning mortality rates for some stocks
- Assumed rates of natural mortality
- Assumptions regarding the maturity of fish in the catch (i.e., differentiating between terminal and preterminal fisheries).
Data from CWT experiments are employed to produce a profile of harvest and escapement for the entire production of a stock. Data are analyzed through a backwards-stepping procedure, beginning with the oldest age class (assumed to be age five). Escapement, an estimate of pre-spawning mortality (when appropriate) and the terminal catch (including associated incidental mortality) are added to produce a mature run size for that age class. The ocean catches of that age class, associated incidental mortalities, and the cohort size of the next older age class are added to compute the size of the population immediately prior to fishing. This sum is then divided by the survival rate (1 - natural mortality) to give the cohort size for that age class. These calculations are summarized in Fig. 4.8.
Fig. 4.8 Calculations used in cohort analysis.
Once each cohort has been reconstructed, the following parameters can be estimated:
- Cohort size for each age class at the beginning of each year
- Age specific harvest rates for each preterminal and terminal fishery
- Maturity schedule for all ages
- Estimates of incidental fishing mortalities.
At this stage of development, CRiSP Harvest is a forecasting model and does not estimate parameters. It relies completely on parameters estimated by the PSC Chinook Technical Committee.
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CRiSP Harvest Manual, Chapter 4. Theory
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