Rick-Methot-NOAA
commented
10 months ago The calculations do not assume that the first year entered is unfished. For each year, value that is entered, SS3 creates an expected value from the simple calculation:
vbio = (SSB_current + 1.0e-06) / (SSB_virgin + 1.0e-06);
Then with Q=1, this value is treated as the absolute level of depletion in that year.
There is no need to enter a time series or even >1 observation. The code works with a single value for the depletion survey.
If the user believes that an unknown level of historical catch has depleted the stock to 80% of unfished level at some early year then the setup would:
- set the depletion obs to start_yr, 0.80
- add an initial eq catch with an arbitrary value
- set lambda for the initial eq catch to nil value
This will cause SS3 to estimate the initial F. The value of the initial F will be mostly influenced by the depletion survey value, plus some influence from the size/age comp in early years.
When the first year, value is something like 1950, 1.0 then it is usually because the user wants model to
e-perl-NOAA
Describe the solution you would like.
I think the current implementation of the depletion survey option assumes the first year entered is at unfished. It would be great to have a flexible definition of what the first year is relative to unfished.
For instance, in a model that does not have a full time series of catch, but the user has a guess of depletion in the initial year of available catches, it would be great to be able to specify the survey with that depletion in the first year of the model.
As an example, say catches start in 1975 forward, but there were removals before that time. The user may want to presume the stock is at, say 75% of unfished in 1975, thus the model can move forward with that depletion assumption, instead of it being assumed unfished in 1975.
Describe alternatives you have considered
The tricky part of this I see is assuming what the age/length structure is in that initial year if it is not unfished. Current ways of doing this is to use the equilibrium catch approach to calibrate a model's population structure and achieve a certain depletion value in a specified year.
If this can be automated in the model, it could be much easier for users to apply.
Statistical validity, if applicable
No response
Describe if this is needed for a management application
No response
Additional context
No response