## Selectivity

An age-specific fishing mortality rate is often treated as the product ofa full ('fully selected') fishing mortality rate F and selectivity at age sa. Selectivity measures both availability (presence of fish in the fishing area) and vulnerability (susceptibility to fishing gear). Its value is scaled to that of the most selected age or ages, for which sax 1. Selectivity may be estimated for each age, but often a parametric function of age or length is used, typically the logistic equation

with parameters •q1 controlling the slope and "q2 determining the location of inflection (age at 50% selection). If selectivity is not monotonically increasing, but rather dome shaped, a common function is the double-logistic,

with parameters •q1 and "q3 controlling slopes of ascending and descending portions of the curve, respectively, and "q2 and determining the locations of inflection. The curve

Age or length

Figure 3 Example of logistic (solid) and dome-shaped (dashed) selectivities.

Age or length

Figure 3 Example of logistic (solid) and dome-shaped (dashed) selectivities.

of eqn [30] often requires rescaling to achieve a maximum of one (Figure 3).

Selectivity models may include time dependence to account for changes in fishing methods. For example, location parameters (^2 or ^4) could be treated as functions of time. A more common approach lets selectivity vary among periods of time, yet remain constant within periods, to account for changes in fishing regulations.

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