Mid-year abundances and SSB

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Megumi Oshima, modified 5 Years ago.

Mid-year abundances and SSB

Youngling Posts: 9 Join Date: 5/14/19 Recent Posts

Hello all,

Can anyone tell me the equation for how SS calculates the mid-year numbers-at-age? I tried Ny.5,a = (Ny.0,a - Cy.0,a)*e^(-Ma/2) because in the model, catch is taken from the first half of the year but I am getting numbers that are higher than what SS outputs in the report file. It seems that I am missing some component of mortality. I don't think it has to do with discards because none of the fleets have discard mortality specified.

Also, when calculating spawning stock biomass, is everything converted to ages first? I tried N-at-length*maturity-at-length*fecundity-at-length*weight-at-length but I am an order of magnitude (and some extra) off from the report file SSB.

Cheers,

Meg

Richard Methot, modified 5 Years ago.

RE: Mid-year abundances and SSB

Youngling Posts: 222 Join Date: 11/24/14 Recent Posts

Hello Meg. Here is some information that can help.

Mid-year numbers at age is not done according to Pope's approximation. The mid-year numbers come from Ny.5,a = Ny.0,a *e^(-Za/2) where Za = Ma+sum(Fa)

SS does work entirely in a numbers-based system. So maturity at length and fecundity at length are used with the distribution of length at age to calculate the reproductive output at age for females. You can find this output labelled as "fecund" in the table labelled AGE_SELEX in report.sso

Then calculation of spawning biomass (really it is total reproductive output) uses the vector of numbers at age and Z at age to calculate the female numbers at the time of spawning. Those numbers are multiplied by Fecund to get total reproductive output. Time of spawning comes from your entry of spawn_month near top of the data file. In a two-sex model, this reproductive output is for females only. In a one-sex model, it is for the total asexual population.

Hope this helps. Check the Methot and Wetzel (2013) paper for more detail on calculations.

Rick

There are some intermediate quantities included in report.sso that can help you

On Tue, May 14, 2019 at 3:20 PM Megumi Oshima <VLab.Notifications@noaa.gov> wrote:

Hello all,

Can anyone tell me the equation for how SS calculates the mid-year numbers-at-age? I tried Ny.5,a = (Ny.0,a - Cy.0,a)*e^(-Ma/2) because in the model, catch is taken from the first half of the year but I am getting numbers that are higher than what SS outputs in the report file. It seems that I am missing some component of mortality. I don't think it has to do with discards because none of the fleets have discard mortality specified.

Also, when calculating spawning stock biomass, is everything converted to ages first? I tried N-at-length*maturity-at-length*fecundity-at-length*weight-at-length but I am an order of magnitude (and some extra) off from the report file SSB.

Cheers,

Meg

--
Megumi Oshima Stock Synthesis Virtual Lab Forum https://vlab.noaa.gov/group/stock-synthesis/discussions-forums-/-/message_boards/view_message/6609218 VLab.Notifications@noaa.gov

Richard D. Methot Jr. Ph.D.
NOAA Fisheries Senior Scientist for Stock Assessments

Mobile: 301-787-0241