Fish Population Dynamic and Stock Assessment

5.1.2. Selection Process and Selectivity

According to Parrish (1963), selection in fishing can be defined as “any process that gives rise to differences in the probability of capture among the members of the exploitable body of fish”. Such a general definition allows for the consideration of both between and within species selection during the different stages of the catch process.

In fact, the catch process (and hence selection) can be thought of as being divided in three distinct phases:

- probability that the occurrence of fish belonging to a single or different species coincides in time and space with the use of the fishing gear;

- probability that fish belonging to a single or different species encounters the fishing gear provided they are present when and where the gear is used (i.e., that fish are accessible to the gear);

- Probability that fishing gear retains fish belonging to a single or different species provided they have encountered it. (i.e., that fish are vulnerable to the gear).

The first two phases are essentially dependent on fish distribution and behavioural patterns, while in the latter the specific characteristics of the fishing gear play a fundamental role.

When between-species selection is considered, capture will depend mainly on the behaviour displayed by each species towards the fishing gear, while in the case of within-species selection the retention of a fish will be driven by its specific characteristics (age, length or girth). In this case, selection is often taken as synonym of length selection, in spite the fact that where meshes are concerned, selection is essentially a girth/mesh-opening related process. Selectivity is no more than the quantitative expression of selection.

Unlike what happens for trawl codend selection studies gill-net selection studies have the drawback of the lack of knowledge on the structure of the population encountering the gear (with the obvious exception of the direct estimation studies). As a consequence, selectivity estimates are based on the comparative fishing with gillnets of different mesh sizes (the so-called indirect technique), while keeping constant the other physical characteristics of the gear. Furthermore, some basic assumptions are usually taken into consideration, the most important of which is the Baranov’s “Principle of Geometric Similarity”, which states that if selection depends only on the relative geometry of the fish and meshes then all selection curves are similar. Therefore, the selectivity will be the same for any combination of fish length and mesh size for which their ratio is constant (Hamley, 1975), that is to say that all meshes are equally efficient for the length class they catch the best.

Fishing selectivity can be defined as the ability to target and capture fish by species, size or sex during harvesting operations, allowing all incidental by–catch to be released unharmed.

Selectivity play a major role in the development of a sustainable and economically viable fishery. The results of selectivity experiments can be befitted to the fishermen to allow them to capture only targeted fishes and ensures the essential return of juvenile fishes. Most fishing gears, for example, trawl gears are selective for the larger sizes, while some gears (gill nets) are selective for a certain langth range, only thus excluding the capture of very small and very large fish. This property of fishing gear is called “Gear Selectivity”.