Fish Population Dynamic and Stock Assessment (2+1)

5.1.4. Sigmoid shaped selection curve

In the fixed gear, the entry of larger fish is prevented but it permits the escapement of smaller fish in relation to size. By contrast, mobile gears allow all the smaller size fish to escape through meshes and larger fish become more susceptible to selection. As size of fish increase, the percentage of fish retained also increases. The selection process produces “S” shaped selectivity curve.

The point where 50% of the fishes are retained and 50% escape, is known as the 50% selection size ($$L_{50}$$ ). The steepness of the curve indicates the efficiency of the selection access. A shallow curve that gradually changes between $$L_{25}$$ and $$L_{75}$$ depicts a poor selection access, whereas a curve that very quickly changes from 0% to 100% selection (a vertical line) indicates optimum selection. The small selection range produces a vertical line selection and this is said to approach “Knife edge” pattern.