Sustainable brown shrimp fishery - is pulse fishing a promising option?

In standard trawl fisheries, beam trawls with bobbin ropes are used to target brown shrimp. This fishing technique negatively impacts the marine environment and thus conflicts with protection requirements in marine protected areas, especially in the Wadden Sea national parks. A significant aspect in developing ecologically sustainable fisheries is therefore the development of measures to reduce the adverse environmental impacts of shrimp fisheries. Pulse fishing with electro-trawls is currently being discussed as a promising option to mitigate the disadvantages of traditional beam trawling by reducing seabed contact and enhancing selectivity. However, fishing by means of electric currents in the ocean is principally prohibited. Only since 2009 does an exemption permit the HOVERCRAN’s use as a commercially applicable system by 5 % of a countryʼs beam trawl fleet (in terms of flatfish and shrimp vessels).

Mode of operation

In pulse fisheries, the mechanical stimulation by traditional bobbins is (partially) replaced by electrical stimulation with electrodes. The electric pulses provoke a flight response (tail-flip) in brown shrimp which forces the animals to jump into the water column and makes them accessible to the fishing net. Short electric pulses with a low frequency (0.25 ms and 4.5 Hz) are used to selectively provoke a reaction from shrimp without stimulating unwanted bycatch organisms such as other invertebrates or fish.

Impact on the seabed

Compared to flatfish beam trawls, shrimp beam trawls are lighter, which lessens their negative impact on the seabed. However, animals living in and on the seabed are not affected by the penetration depth of the gear alone, but among other things also by the direct removal of organisms. Sensitive habitat-forming biotic communities such as Sabellaria reefs or sea cypress populations are almost extinct in some of their original habitats, e.g. the Wadden Sea, where much of the shrimp fishery takes place. The reason for this decline is discussed controversially, in particular with regard to the impact of multiple trawling and the mechanical destruction of benthic species.

Shrimp catches

Shrimp fishery with electrical trawls is less dependent on abiotic factors such as light intensity and water turbidity than the fishery with standard beam trawls. Fishing efficiency strongly depends on the configuration of the gear. In its original hovering configuration with raised ground rope (HOVERCRAN), the pulse trawl caught fewer large shrimp than the conventional beam trawl. This loss was compensated by reducing mesh sizes in the top panel as this prevented the tail-flipping shrimp from escaping through the meshes of the top panel. With this setting, a similar catch weight of shrimp was obtained as with standard beam trawls. In configurations where the electrical pulse was combined with additional bobbins in the ground rope, catches of large shrimp were even higher than with standard beam trawls. Set-ups using 9 or 11 bobbins (compared to 36 bobbins in a conventional beam trawl) caught 10 % more shrimp. A combination of a standard beam trawl with electrical pulses resulted in a 50 % increase in shrimp catches.

Bycatch

Ideally electrical stimulation as applied in the pulse fishery selectively provokes a flight response in shrimp without affecting other benthic species. However, for most of the typical bycatch fish (e.g. dab, plaice, sole1), a substitution of the mechanical stimulus of commercial beam trawls by an electrical stimulus alone did not result in reduced bycatch rates. But electrical pulses in combination with a ground rope that was raised by 10-15 cm achieved the desired results. In the original HOVERCRAN configuration (without bobbins), bycatch (fish and invertebrate species) was reduced by 35 %. But in a setting of a pulse trawl in combination with 11 bobbins, the bycatch rate was only reduced by 15 %. In addition, preliminary results of investigation on various settings of shrimp pulse trawls (without sieve net2) in Belgium and the Netherlands revealed considerably higher bycatch rates for some species, compared to traditional beam trawls with sieve nets. As these results only cover part of the investigation period, it is advisable to wait for the final results to become available before the technique is subjected to a final evaluation. In another investigation, bycatch rates of a pulse trawls equipped with 10 bobbins were reduced by 15 % compared to a standard beam trawl without sieve net.

Comparison with flatfish pulse trawls

The basic principle of a pulse trawl for shrimp differs significantly from that for flatfish, as pulses are low in voltage and frequency. While theses pulses are sufficient to induce muscle contractions in shrimp, they induce only general behavioral reactions (flight response) of fish. Ideally, this flight reflex enables the fish to escape underneath the net. Pulses as applied in flatfish trawls have higher electric field strengths, higher pulse frequencies (40-45 Hz) and a longer duration of exposure. They lead to muscle cramps that make the flatfish contort upwards in a U-shape, which makes the animal easily accessible to the approaching fishing net.

No injuries of fish and invertebrates

No injuries of fish and invertebrates have been observed in bycatch organisms of shrimp pulse trawls, contrary to findings in flatfish pulse trawls. No mortality was observed in typical bycatch species after exposure to electric fields such as those used by shrimp pulse trawls. Some on-going studies further investigate whether or not non-target fish and invertebrates suffer injuries from exposure to shrimp pulses.

Opportunities and risks

Ecological opportunities of shrimp pulse trawls (depending on the setting) are reduced bycatch rates and less damage to the seabed and the associated benthic organisms. Savings in fuel consumption that lead to substantially increased profits in flatfish pulse fishery are of minor importance for shrimp pulse trawling as towing speed and towing resistance are small compared to flatfish trawls.

On the other hand, this technology entails considerable risks to the environment. It is only a small step from an increased efficiency in capturing brown shrimp to overfishing of the entire stock if a large-scale introduction of pulse fishery is not accompanied by the implementation of effective management measures (such as effort limitation, e.g. by means of fleet reduction), scientific monitoring as well as measures of surveillance and control. Especially a combined use of pulse trawls with standard bobbins in the ground rope requires clear rules, provided that pulse fishery will be allowed for more than 5 % of a nation’s beam trawl fleet.

Conclusion

As a preliminary conclusion, based on todayʼs knowledge, the use of pulse trawls in shrimp fisheries should only be allowed in settings with raised ground rope and without the application of additional bobbins. Moreover, a comprehensive legislative framework is strictly necessary in order to regulate the admissible technical settings of pulse trawls. Measures of surveillance and control have to be unequivocally defined in order to offset any possible increase in efficiency by means of effort limitations.