Pulse fishing and its effects on the marine ecosystem and fisheries

This research project was carried out by Wageningen Marine Research at the request of and with funding from the Ministry of Economic Affairs for the purposes of Policy Support Research Theme ‘Sustainable Fisheries' (project no. BO-20-010-078). Authors: Adriaan Rijnsdorp, Dick de Haan, Sarah Smith, Wouter Jan Strietman.

This report summarises the knowledge on the effects of pulse trawls used in the North Sea fishery for flatfish and brown shrimp. The report describes the electrical characteristics of the pulse trawl systems currently used (potential difference over electrode pairs, pulse frequency, pulse width, duty cycle and dimensions of the gear). The shrimp pulse applies a low frequency pulse that invokes a startle response (tailflip) in shrimps. The sole pulse applies a higher frequency that invokes a cramp response that immobilise the fish species facilitating the catching process.

Electrical stimulation changes the species selectivity of the trawl. The catch efficiency of the pulse trawl for sole is higher, and the catch efficiency for plaice and other fish species is lower, when expressed in terms of the catch rate per swept area. It is uncertain whether the pulse trawl has a better size selectivity (reduced bycatch of undersized fish), but all experiments show that the bycatch of benthic invertebrates is substantially reduced. Applying electrical stimulation in the fishery for brown shrimp, offers a promising innovation to reduce the bycatch of fish and benthic invertebrates, while maintaining the catch rate of marketable sized shrimps. The reduction in bycatch depends on the design of the net, in particular the specifics of the groundrope. In ecological terms, the replacement of the tickler chains with electrodes and a lower trawl footprint constitute a highly positive contribution to diminishing the impact of trawling on the North Sea benthic ecosystem.

A number of laboratory experiments have been carried out in which a selection of fish species and invertebrate species have been exposed to electrical stimuli to study possible adverse effects. The maximum pulse treatment applied exceeded the strength of the pulse used by the fishery. Electrical stimulation did not cause direct mortality during exposure. Exposure to the sole pulse stimuli invoked vertebral fractures and associated haemorrhages in roundfish species (cod), but not in flatfish species (sole, plaice, dab) or seabass. Shrimp pulse exposure did not invoke fractures in roundfish or flatfish species. The results suggest that fractures are restricted to the larger size classes of cod that are retained in the net, whereas smaller cod that can escape through the 80mm meshes did not develop fractures even when exposed to high field strength. The fracture incidence in cod increases with field strength and decreases with pulse frequency. Fracture incidence varied between experiments.

Samples of cod and whiting taken on board of commercial pulse trawlers fishing for sole showed vertebral fractures and associated haemorrhages similar to those observed in the laboratory experiments and suggest incidence rates of around 10% and 2% in cod and whiting, respectively. Further studies are required to study the relationship between the fractures and the body size and determine the differences in fractures across fish species.

Histological examination of fish exposed to pulse stimuli in laboratory experiments, did not reveal other abnormalities in species examined, except for a small haemorrhage in 2 of the 25 exposed plaice, and a significant increase in melanomacrophage centres in the spleen of cod exposed to the shrimp pulse. No adverse effect could be detected on the electro-sense organ used in food detection behaviour of small-spotted catshark. In an experiment exposing brown shrimp and ragworms to a sole pulse showed no consistent adverse effects, but shrimps that were exposed 20 times during a 4 day period to a sole pulse showed an increased mortality.

No studies have been done on the effect of pulse stimulation on the functioning of the benthic ecosystem. Although the laboratory experiments suggest that fish and invertebrates resume their normal behaviour after exposure, no information is available on for instance the threshold levels at which the functioning of species is being adversely affected. Also little is known on the effects of electrical stimulation on the development of eggs and larvae. One experiment exposing early life stages of cod (egg, larvae, early juveniles) to a pulse stimulus exceeding the pulse used in the fisheries did not find an increase in developmental abnormalities in exposed animals, but observed a reduced hatching rate and an increased mortality in 2 out of the 8 experiments. No adverse effects were observed in sole eggs and larvae.

For the consistent interpretation of the experimental results, a mechanistic framework is required which is built on fundamental knowledge about how electricity affects the physiology of the organism. Such a framework should provide an explanation why for instance species and size classes differ in their sensitivity for developing vertebral fractures, and should explain how electrical stimulation affects the species- and size-selectivity of the pulse trawl. In 2016, a 4-year research programme commissioned by the Dutch ministry of Economic Affairs has started with the aim to develop such a framework and provide the scientific basis to assess the long term impact of the commercial application of pulse trawls in the North Sea. This project will also tackle the knowledge gaps concerning the effects of electrical stimulation on the functioning of the benthic ecosystem.