A sigmoid shaped selectivity function. Based on two parameters l25 and
l50 which determine the length at which 25% and 50% of the stock is
selected respectively.
sigmoid_length(w, l25, l50, species_params, ...)Vector of selectivities at the given sizes.
Vector of sizes.
the length which gives a selectivity of 25%.
the length which gives a selectivity of 50%.
A list with the species params for the current species.
Used to get at the length-weight parameters a and b.
Unused
The selectivity is given by the logistic function
$$S(l) = \frac{1}{1 + \exp\left(\log(3)\frac{l50 -l}{l50 - l25}\right)}$$
As the mizer 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
$$l = \left(\frac{w}{a}\right)^{1/b}$$
gear_params() for setting the selectivity parameters.
Other selectivity functions:
double_sigmoid_length(),
knife_edge(),
sigmoid_weight()