F multiplier - Public Forums - Stock Synthesis

Hi Dan,

I'm assuming the other two reference points are the Biomass (Bratio) target and MSY. A group of values associated with each one are reported at the end of the DERIVED_QUANTITIES with labels like "Fstd_Btgt", "Fstd_SPR", and "Fstd_MSY". The F-multipliers used in the search for these values are reported in the Fmult column of the "SPR/YPR_Profile" in Report.sso as well as various tables in Forecast-report.sso following comments like "find_target_SPR", "Find_target_SSB/Bzero", and "Find_MSY_catch". Those tables in Forecast-report.sso aren't currently read by any r4ss function, but it wouldn't be hard to create one to do so.

-Ian

Hi Dan.  Yours is a good question from which to recap these controls.  I'll augment Ian's response.

While F is a simple concept in a biomass dynamics model with a single fishing fleet, the concept of "F" as a single number is very incomplete when there are multiple fleets, some with length-based and/or dome-shaped selectivity.  In SS, the F multiplier is multiplied first by fleet-specific relative F's to get the F for each fleet, then that fleet-specific F is multiplied by the age-selectivity to get F at age.  Depending on the choice of selectivity pattern, these age-specific F's may or nay not peak at 1.0 and can even exceed 1.0 in some circumstances.  The realized F-at-age for each fleet is (F multiplier) * (relative F) * (age-selectivity).  In addition to what Ian described, you can see the results under Exploitation in report.sso.

While F multiplier is always done as just described, the user has various options for the reporting of the realized F.  These are in starter.ss and described in the manual.

Thanks Ian and Methot for your detailed response. That's very clear and helpful.

Hi,

it seems like a silly question but I ask. J

We can read on page 15 of the manual:
The SPR approach to measuring fishing intensity was implemented because the concept of a single annual F does not exist in SS.

But there are F_msy, F_age and F_mean. Thus, we can calculate the annual F and can compare F_mean with F_msy. Why not?