Calculate spawning stock biomass at F = 0 (SSB0)

`get.ssb0()` computes spawning stock biomass under zero fishing mortality (often referred to as SSB0), based on natural mortality, maturity, weight, and fecundity-at-age, accounting for seasonal structure and the timing of spawning. Optionally, a non-zero fishing mortality (`FM`) can be supplied to obtain spawning biomass under that F instead.

Usage

get.ssb0(
  M,
  mat,
  weight,
  fecun = 1,
  asmax,
  ns,
  spawning,
  R0,
  indage0,
  season,
  FM = NULL
)

Arguments

M: Natural mortality-at-age (and, if applicable, season), typically given as a vector or matrix of instantaneous natural mortality rates.
mat: Maturity ogive, giving the proportion mature at each age (and season, if relevant).
weight: Mean weight-at-age (and possibly season), used to convert numbers of mature fish into spawning biomass.
fecun: Fecundity-at-age (and possibly season). Defaults to `1`, in which case fecundity is assumed to be proportional to weight and maturity only.
asmax: Integer giving the maximum age class (or the number of age classes) used in the calculation.
ns: Integer giving the number of seasons per year.
spawning: Indicator (e.g. logical or integer vector of length `ns`) identifying the spawning season(s) within the year.
R0: Unfished recruitment level used to scale the numbers-at-age under F = 0. Depending on the implementation, this may be used to convert per-recruit quantities into absolute biomass.
indage0: Integer index specifying the recruitment age (age at which new recruits enter the age structure).
season: Integer or vector specifying the season(s) for which SSB is evaluated; typically aligned with `spawning` and the seasonal structure.
FM: Optional fishing mortality-at-age (and season). If `NULL` (default), fishing mortality is set to zero and SSB0 is computed. If non-`NULL`, the function returns spawning biomass under the combined mortality `M + FM`.

Value

A numeric value giving spawning stock biomass under F = 0 (SSB0), or under the supplied `FM` if not `NULL`. The exact scaling (per recruit vs. absolute biomass) depends on how `R0` is used internally.