Improving selectivity of trawls

Background and Objective

The common functionality of trawls is to herd fishes in front of the trawl. These fishes swimm in front of the trawl and flee from the approaching trawl until they are exausted and fall back into the trawl. They are collected at the end of the trawl, which is called codend.

We investigate the selective properties of currently used trawl and codends and work on innovative concepts to further improve their selectivity.

Main topics are:

1. codend-selectivity

At latest inside the codend, the animals give up to avoid net contact. Small fishes can escape through meshes if these meshes are big enough.

The mesh size is often (but not in all cases) adapted to the desired size of the target species. During the last years, the mesh shape was also taken into consideration: square mesh netting is frequently used for target species with round-oval body cross section (roundfish, such as cod or withing).

Further improvement of meshsize and mesh shape, as well as the usage of alternative netting material can help to adapt the selectivity to the need of fishery management and fishery.

2. Multispecies selectivity

In most cases, several species live in a fishing area. Those species differ in terms of growth, behaviour and morphology - most relevant for selectivity is body cross section. A codend, optimized for roundfish species, has a high catchability for flatfish species due to their very wide body shape. In the Baltic for example, some flatfish species are wanted (e.g. turbot). Nevertheless, most flatfish are caught as unwanted bycatch. 

We work on solution to sort wanted and unwanted species under water. We call this "multispecies selectivity". To achieve such selectiviy, we need to develop additional sorting devices and to integrate them into the trawl.

3. alternative selectivity concepts

Usually, a trawl catches larger individuals of a given species, while smaller individuals can escape through the meshes. This results in a change of the age structure of the population. Amongst other effects, the high reproductive potential of large individuals can't be used optimal.

We work on alternative concepts for trawl selectivity, where - for example - old individuals have a chance to escape from the trawl.

Approach

Basis of investigations and development is a deep understanding of the bahaviour and morphology of relevant species. Amongst others activities, we conduct intensive underwater observation in trawls. Using this knowledge of animal behaviour we are able to design specific selection devices and to integrate them in the trawl in a way that unwanted species can use them to escape.

This work and the test of new gear designs are conducted on different research vessels (in most cases FRV "Clupea" and FRV "Solea"), as well as on commercial vessels.