R/selectivity_funcs.R
double_sigmoid_length.RdA 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, a, b)
| 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%. |
| a | the multiplier of the length-weight function. |
| b | the exponent of the length-weight function. |
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 is also necessary to supply the
length-weight parameters a and b.