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. 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.

sigmoid_length(w, l25, l50, species_params, ...)

## Arguments

w the size of the individual. 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