Flatfish pulse fishing - Research results and knowledge gaps II

This report is an extension and translation of the report of Quirijns, F., Strietman, W.J., Van Marlen, B., Rasenberg, M. 2013. Flatfish pulse fishing: research results and knowledge gaps. IMARES Report C193/13.

Knowledge about the effects of pulse fishing is spread over various reports and policy documents; a complete and accessible overview of knowledge is lacking. This report provides an overview of the research results and knowledge gaps in pulse fishing.

Pulse fishing is a relatively new fishing method: in this fishery, fish are caught by means of electric pulses. Those pulses cause muscle contractions in fish, that consequently are startled from the seabed and caught in the net. The application of pulse technique in commercial fishing is relatively new and electric fishing is not allowed in Europe through EC Reg. 850/1998. Consequently pulse fishing is done under derogation from the EU and questions were asked on the ecosystem effects of pulse fishing. The fishery study group Pulse and SumWing (part of the fishery study group Flatfish) asked IMARES and LEI to make a summary of the available knowledge and knowledge gaps on the effects of pulse fishing. This resulted in the report of Quirijns et al. of 2013. The Dutch ministry of Economic Affairs asked IMARES to update this report in 2015 with the latest results. This report provides the up to data account on current knowledge on the effects of pulse stimulation of marine organisms and marine ecosystems, and summa-rises the effects on the catch efficiency and selectivity (landings and discards), management of the fish-ery and CO2 emission. At the end of the report, gaps in knowledge are identified.

  • Dab: no lesions were observed in the fish, neither analysed directly after, nor when analysed five days after the exposure to electrical stimulation.
  • Preliminary sampling of roundfish caught by commercial pulse trawl gear showed that about 10% of the cod and 2% of the whiting showed a fracture of the vertebral column.
  • Cod: laboratory experiments showed that the probability of fractures in spinal column occurs in mar-ketable sized fish but not in cod that are small enough to escape through the meshes of the net. The injury probability increase with field strength and decrease with pulse frequency;
  • Dogfish: behavioural response but no injuries observed when exposed close to electrodes. Effects on the proper functioning of their electro receptor organs after exposure to the electric fields was not studied;
  • Benthic invertebrates: some species did not respond to pulse (spisula and starfish); other species did (razor clam, shrimp, common crab and rag worm). Mortality increase, if at all, was low (3-7% for ragworm, common crab and razor clam), and food intake and behaviour recovered after exposure;
  • Marketable plaice and sole: the pulse gear has similar catch levels of marketable sole as the conven-tional beam trawl, but lower ones of marketable plaice;
  • Plaice and sole quality and survival: the species caught with pulse trawl are damaged to a lesser extent than those caught with a conventional beam trawl. A significant higher survival rate for plaice was found for the pulse trawl after 192 hours of observation in comparison with a conventional beam trawl;
  • Discards: the catch level of undersized fish and benthos that are usually discarded is distinctly lower (30-50% fewer fish discards, 48-73% fewer benthic species);
  • Fuel consumption: a pulse trawl is towed at a lower speed than the conventional beam trawl, result-ing in reduced fuel consumption and emissions of C02 and other greenhouse gases.
  • Penetration of gears: Seabed bathymetry changes between ~ 1 and 2 cm and is further increased by
  • higher trawling frequencies. The tickler-chain trawl affected seabed bathymetry to a greater extent than the pulse trawl, when comparing depth differences at 320 kHz. Depth differences were < 6 mm
  • 6 of 39 Report number C091/15
  • for both gears at a 25% probability (of occurrence of a certain depth difference), but the changes to the seabed bathymetry at a 50% probability were up to 20 mm for tickler-chain trawling and 14 mm for pulse trawling.
  • Shrimp and ragworm: No significant increase in mortality or injuries . Shrimp demonstrates tail flip (depending on frequency) and the ragworm demonstrates squirming reaction (independent of the frequency). Increase in severity of virus infection in shrimp at highest electrical field strength (200 V m-1).

Despite the large number of studies that have been carried out, several topics need more investigation:

  • Indirect (or: delayed) mortality;
  • Non-mortal effects;
  • Effects on reproduction;
  • Long term effects on species that encounter pulse trawl gear and on their populations;
  • Minimum and maximum values for pulse characteristics (is there a ‘safe range’?);
  • Effects of pulse fishing on early life stadia of marine organisms that reproduce in shallow water;
  • Effects on seabed, substrate and water column: can the use of pulse result in dissolving toxic chemicals?

The EU STECF stated that control and enforcement issues should be resolved before the number of ves-sels using pulse trawls can be increased. In 2012, procedures for control and enforcement were devel-oped in close cooperation with producers of pulse fishing gears, fishermen’s organisations, the Dutch Ministry of Economic Affairs, representatives of control agencies and scientists, but they are not applied in detail in practice yet.