ssb: Calculate or return the Spawning Stock Biomass

In flr/FLCore: Core Package of FLR, Fisheries Modelling in R

Calculate or return the Spawning Stock Biomass

Description

The calculated Spawning Stock Biomass (SSB) of a fish population is returned by this method. SSB is the combined weight of all individuals in a fish stock that are capable of reproducing. In some classes this is calculated from information stored in different slots, while in others ssb() is simply an slot accessor. When the later is the case, the corresponding replacement method also exists.

For an object of class FLStock, the calculation of SSB depends on the value of the 'units' attribute in the harvest slot. If this is in terms of fishing mortality (units(harvest(object)) == 'f'), and assuming an object structured by age, then SSB is calculated as

SSB_{y} = \sum\nolimits_{a} N_{a,y} \cdot e^{-(F_{a,y} \cdot Hs_{a,y} + M_{a,y} \cdot Ms_{a,y})} \cdot W_{a,y} \cdot T_{a,y}

where N_{a,y} is the abundance in numbers (stock.n) by age (a) and year (y), F_{a,y} is the fishing mortality (harvest), Hs_{a,y} is the proportion of fishing mortality before spawning (harvest.spwn), M_{a,y} is the natural mortality (m), Ms_{a,y} is the proportion of natural mortality before spawning (m.spwn), W_{a,y} is the mean weight at age in the stock (m), and T_{a,y} is the proportion mature at age in the stock (mat). For FLStock objects with other dimensions (area, unit), the calculation is carried out along those dimensions too. To obtain a global value please use the corresponding summing method. If the harvest slot contains estimates in terms of harvest rates (units(harvest(object)) == "hr"), SSB will be computed as

SSB_{y} = \sum\nolimits_{a} N_{a,y} \cdot (1 - H_{a,y} \cdot Hs_{a,y}) \cdot e^{-(M_{a,y} \cdot Ms_{a,y})} \cdot W_{a,y} \cdot T_{a,y}

where H_{a,y} is the harvest rate (proportion of catch in weight over total biomass).

Usage

ssb(object, ...)

## S4 method for signature 'FLStock'
ssb(object, ...)

## S4 method for signature 'FLBiol'
ssb(object, ...)

Arguments

object

Object on which ssb is calculated or extracted.

Details

Objects of the FLBiol class do not contain any information on catch or fishing mortality, so a call to ssb() will only correct abundances for natural mortality to the moment of spawning. The method can also take information on catches or fishing mortality and use them when calculating abundances at spawning time. An FLQuant named either 'catch.n', 'f', 'hr' or 'harvest' can be used. The first three are self-explanatory, while for the last units must be either 'f' or 'hr'. The quantities should refer to total yearly values, as the value in the 'spwn' slot will be used to calculate what fraction of fishing mortality to apply.

Value

An object, generally of class FLQuant.

Author(s)

The FLR Team

See Also

FLComp

areaSums

Examples

data(ple4)
# SSB from FLStock
ssb(ple4)
biol <- as(ple4, "FLBiol")
# SSB from FLBiol, abundances corrected only for M
ssb(biol)
# Provide catch-at-age, F or HR to correct N
ssb(biol, catch.n=catch.n(ple4))
ssb(biol, f=harvest(ple4))
ssb(biol, harvest=harvest(ple4))
ssb(biol, hr=catch.n(ple4) / stock.n(ple4))

flr/FLCore documentation built on May 4, 2024, midnight