Trawl Gear Selectivity

Rhode Island Sea Grant

The University of Rhode Island
URI Fisheries Center

Traditional fisheries management often uses a minimum fish length as a way to protect species from overfishing. The length set is usually based on size at maturity – allowing fish to reproduce at least once or twice before it becomes legal size. To eliminate the catch of undersized fish, fishermen can use different mesh sizes to control the size of the fish caught. This is particularly true of trawl nets, where the final catch can be selected in the bag or cod end of the net. Divers have confirmed that much of the escapement from a trawl net occurs in that final section of the net. The mesh size in the cod end determines, to a large extent, the selectivity of trawl gear.

Covered cod end experiment. Fish that escape through the mesh size in the cod end are captured in the small mesh cover. Those normally would be called “escapees”.

The desired mesh size for the size of the fish is determined by performing at sea comparisons of different mesh sizes. One technique that is often used is the covered cod end experiment where the mesh size to be tested is used in the net itself, but the entire cod end is covered with a finer mesh net kept reinforced with hoops. The selectivity of the gear can then be determined by comparing the sizes of the fish in the cod end with those of the fish in the cover.

The fish from each section are measured and put into a table and then plotted to create a selection curve. When the fraction retained is plotted against each corresponding length group, it appears that the points are following a sigmoid curve, which reaches 1.00 (100% retention) at a certain length and which approaches 0.00 (0% retention) at a certain small length. This sigmoid curve is called the “gear selection ogive“.

The easiest mathematical expression to describe the gear selection ogive is the so called “logistic curve”. The size of fish that is retained is considered to be a probability.

Fisheries scientists often use the length at which 50% of that size fish is retained as a marker point. That means 50% escape and 50% are retained in that size mesh. That is referred to as the L50. This must be calculated for each species since often times fish behavior affects the retention. Sometimes the shape of the mesh will also affect retention (such as diamond or square mesh).

For the complete article and original pdf, click here: Trawl Gear