Fish Population Dynamic and Stock Assessment

5.2.4. Methods of measuring the selectivity of trawls

1) Covered cod-end

To determine the size and distribution of the total fish population that comes in contact with the gear in a particular fishing area, attach a small mesh cover over the experimental net’s cod-end. The small mesh cover retains the small fish that escape from the cod-end and so the combined catch of the gear and cover of the cod-end provides, a measure of the total population encountered. If series of different experimental cod-ends are performed with a small mesh cover, this gear design will enable the estimation of selectivity of cod-ends of different sized meshes and shapes.

2) Twin Trawl

Two trawl nets are dragged by a single trawler simultaneously. One trawl is composed of a small mesh cod-end used to obtain an estimate of the total population. The other trawl is used as an experimental gear. The selectivity of the test cod-end is determined by analysing the length frequency distribution of the catches from both the test cod-end and small mesh cod-end. Biasness is comparatively less in these methods.

3) Trouser Trawl

A standard trawl is divided into two sections by a vertical panel. Two cod-ends are attached, one on each side of the panel. Comparison of the length frequency distribution of the fish in each codend allows calculations of codend selectivity.

4) Alternate Hauls

This method requires identical hauls to be made alternatively with an experimental trawl and a control trawl. To ensure the validity and reliability of the data, each pair of hauls must be similar in all aspects and in the number of hauls made.

5) Parallel Hauls

This method requires that two vessels fish on the same ground at the same time. The experimental gear is towed by one vessel and the control gear is towed by the other vessel.