Gillnet modifications to reduce bycatch of harbor porpoises

Gillnets are passive fishing gears that belong to the oldest and most frequently used gears worldwide, providing income and food for millions of people. They are most used in small-scale and artisanal coastal fisheries and operated from small boats often less than 12 m in length. Gillnet fisheries provide approximately 20% of the global catch of consumption fish.

Gillnets are easy to handle, very fuel efficient due to their passive nature, have almost no impact on the sea bottom and are very size selective. The operating principle is very simple: a net is set vertically in the water column like a curtain, marked with buoys on the water surface and left to soak for a given time.

Fish do not see the very thin filaments of the netting and get entangled. To obtain the catch, the net is hauled in and fish are removed. Often, the net is directly set again afterwards. The main drawback of gillnets is the incidental bycatch of marine megafauna, including small toothed whales (odontocetes) like harbor porpoises (Phocoena phocoena).

Several populations of odontocetes are classified as “endangered” with bycatch playing a major role among other reasons. Odontocetes echolocate at high frequencies, but seem to be unable to sufficiently classify gillnet netting as impenetrable barriers, i.e. they entangle and drown. Increasing the acoustic detectability of gillnets for odontocetes by making the netting highly acoustically visible could reduce the bycatch of harbor porpoises and other odontocetes, given that the animals actively echolocate in the direction of the net.

Within this thesis, an optimal acoustic reflector was systematically identified ( Paper I ), the acoustic properties of gillnets were determined for various gillnet modifications using this optimal reflector ( Paper II ) and a first commercial trial to assess the effect of the reflectors on bycatch of harbor porpoises was carried out ( Paper III ).

In Paper I, optimal acoustic reflectors that substantially increase the acoustic reflectivity of gillnets were identified across a large frequency range, and thus for many odontocetes species, through a systematic simulation study. Best results were achieved for small acrylic glass spheres. The simulation results were experimentally verified for selected objects in an acoustic tank.

A single acrylic glass sphere of approximately 8 mm in diameter has almost the same acoustic reflectivity as an airfilled table tennis ball which is five times larger in diameter and gives a very strong echo. A single sphere also has a higher or equal acoustic reflectivity as the area of a gillnet at 130 kHz, the echolocation frequency of harbor porpoises.

The spheres have almost the same density as seawater, should thus be almost neutrally buoyant and hence not significantly influence the hydrodynamic properties and catch efficiency of the modified gillnet. Paper II describes the angle-dependent acoustic properties across a large frequency range of a nylon gillnet and a gillnet made from natural fiber, and modifications of these gillnets.

The nets were modified with different numbers of acrylic glass spheres per m² of netting. Acoustic reflectivity was quantified in terms of area backscattering strength (Sa) and target strength (TS). Acoustic spatial patterns were visualized in echograms. Gillnets modified with acrylic glass spheres have a higher acoustic reflectivity than the standard nets, even when equipped relatively sparsely with acrylic glass spheres.

The standard nets become less acoustically visible when ensonified from an angle, while the gillnets equipped with spheres largely stay equally visible or become even more visible with increasing inclination. Furthermore, the spheres create a clear spatial pattern that could aid harbor porpoises to perceive the gillnets as impenetrable barriers.

In Paper III the first pilot fishery trial using a gillnet equipped with acrylic glass spheres was carried out in the Turkish Black Sea turbot fishery to quantify the efficacy of bycatch reduction of the modified gillnet. Ten pairwise hauls were carried out, each with a modified and a standard gillnet.

The gillnet with acrylic glass spheres caught less harbor porpoises than the standard gear (2 vs. 5 animals) and there was no difference in catch of demersal species such as thornback ray (Raja clavata) or turbot (Scophthalmus maeoticus). As only ten hauls were carried out, there was low statistical power and the difference in bycatch of harbor porpoises was not statistically significant.

Nevertheless, the results are a promising step forward and form the basis for further improvement and upcoming large-scale fishery trials.