Many SS3 models include multiple fleets and include separate female and male stock dynamics. However, the `openMSE`

OM object includes only a single stock and single fleet. When imported into `openMSE`

, output from an SS3 model is first converted to an ‘MOM’ object which reproduces the predicted stock and fleet dynamics from the SS3 model in `openMSE`

(see Importing MOM from SS3).

When an `MOM`

object is converted by the `SSMOM2OM`

function to an `OM`

object, the overall fleet characteristics are calculated from the individual fleets within the `MOM`

object, and the stock characteristics are calculated as averages from the sex-specific values in the SS3 model.

This page provides details on how the sex-structured, multi-fleet SS3 model is converted to a combined-sex, single-fleet `OM`

object.

## Biological Properties

### Unfished Recruitment (R0)

The equilibrium recruitment to age-0 in the unfished state is calculated as the sum of the sex-specific R0s from the SS3 model.

### Natural Mortality

Natural mortality for the combined sex model is calculated as the mean of the sex-specific M-at-age values in the SS3 model.

### Weight-at-Age

Weight-at-age for the combined sex model is calculated as the mean of the sex-specific weight-at-age values in the SS3 model.

### Length-at-Age

Length-at-age for the combined sex model is calculated as the mean of the sex-specific length-at-age values in the SS3 model.

### Fecundity-at-Age

Fecundity-at-age in the combined model is taken from the female component of the sex-specific SS3 model.

### Maturity-at-Age

Maturity-at-age in the combined model is taken from the female component of the sex-specific SS3 model.

### Growth Parameters

Following above, the growth parameters for the von Bertalanffy growth model are calculated as averages of the sex-specific values in the SS3 model.

### Length-Weight Parameters

The 2 parameters of the length-weight relationship are calculated as averages from the sex-specific values in the SS3 model.

## Fleet Characteristics

### Selectivity-at-Age

The overall sex-specific fishing mortality-at-age is calculated as:

\[F_{a,i,y} = \sum_n^N{S_{a,i,n,y} f_{y,n}} \] where \(F_{a,i,y}\) is fishing mortality for age \(a\), sex \(i\), in year \(y\), \(S_{a,i,n,y}\) is sex- and fleet-specific selectivity-at-age, and \(f_{y,n}\) is the apical fishing mortality for fleet \(n\) in year \(y\).

Overall selectivity-at-age for sex \(i\) then: \[V_{a,i,y} = \frac{F_a,i,y}{\max{F_a,i,y}}\] where \(\max\) represents the maximum value.

Finally, the combined selectivity-at-age is calculated as the average of the \(I\) sex-specific values: \[V_{a,y} = \frac{V_{a,i,y}}{\sum_i^I{V_{a,i,y}}}\]

### Retention-at-Age

Overall retention-at-age is calculated with the same method described above, except using the sex- and fleet-specific retention-at-age values predicted by the SS3 model.

### Fishing Mortality-at-Age

Overall fishing mortality-at-age is calculated by multiplying the apical fishing mortality in each year (calculated as the maximum of \(F_{a,i,y}\) across \(a\) and \(i\)) by the overall selectivity-at-age schedule.