`R/selectivity_funcs.R`

`double_sigmoid_length.Rd`

A hump-shaped selectivity function with a sigmoidal rise and an independent
sigmoidal drop-off. This drop-off is what distinguishes this from the
function `sigmoid_length()`

and it is intended to model the escape
of large individuals from the fishing gear.

double_sigmoid_length(w, l25, l50, l50_right, l25_right, species_params, ...)

w | the size of the individual. |
---|---|

l25 | the length which gives a selectivity of 25%. |

l50 | the length which gives a selectivity of 50%. |

l50_right | the length which gives a selectivity of 50%. |

l25_right | the length which gives a selectivity of 25%. |

species_params | A list with the species params for the current species.
Used to get at the length-weight parameters |

... | Unused |

The selectivity is obtained as the product of two sigmoidal curves, one
rising and one dropping. The sigmoidal rise is based on the two parameters
`l25`

and `l50`

which determine the length at which 25% and 50%
of the stock is selected respectively. The sigmoidal drop-off is based on the
two parameters `l50_right`

and `l25_right`

which determine the
length at which the selectivity curve has dropped back to 50% and 25%
respectively.

As the size-based model is weight based, and this
selectivity function is length based, it uses the
length-weight parameters `a`

and `b`

to convert between length and weight.