sigmoid_length: Length based sigmoid selectivity function

In mizer: Dynamic Multi-Species Size Spectrum Modelling

Length based sigmoid selectivity function

Description

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.

Usage

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

Arguments

w	Vector of sizes.
l25	the length which gives a selectivity of 25%.
l50	the length which gives a selectivity of 50%.
species_params	A list with the species params for the current species. Used to get at the length-weight parameters a and b.
...	Unused

Details

You would not usually call this function directly. Instead, set the sel_func column in gear_params() to "sigmoid_length" and provide the l25 and l50 values as additional columns. setFishing() will then call this function automatically when calculating the selectivity array.

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}

Value

Vector of selectivities at the given sizes.

See Also

gear_params() for setting the selectivity parameters.

Other selectivity functions: double_sigmoid_length(), knife_edge(), knife_edge_length(), sigmoid_weight()

Examples

# Selectivity at weight given l25 = 10 cm, l50 = 15 cm
# using length-weight parameters a = 0.01, b = 3
sp <- list(a = 0.01, b = 3)
w <- c(1, 10, 100, 500, 1000)
sigmoid_length(w, l25 = 10, l50 = 15, species_params = sp)

mizer documentation built on Aug. 31, 2026, 5:08 p.m.