The effects of electric pulse stimulation on cultured adult cod, continued research 2013

The IMARES study of 2010 on cultured adult and juvenile cod showed that vertebral injuries occurred in 50-70 % of the adult cod at pulse amplitude of 30 to 60 V (de Haan et al., 2011). No injuries were observed in juvenile cod. In a similar ILVO-study on adult cultured cod conducted in June 2013 in Tromsø, Norway no injuries were observed at pulse settings similar to the IMARES application of 2010 and at settings twice the IMARES maximum amplitude rating. To investigate the origin of the different outcome an experiment with the ILVO and IMARES systems was conducted in the period between 9 and 14 October 2013 using the cultured adult cod from the IMR Austevoll hatchery, with which vertebral injuries were observed. Both IMARES and ILVO pulse generators were equivalent to the equipment used in the earlier studies.

In this experiment a total of 83 adult cod were exposed, 53 specimen were exposed at 60 V and 30 specimen twice above the maximum commercially applied amplitude (120 V). In 5 cases vertebral injuries occurred of which 4 at 120 V and a single one at 60 V. When the type of stimulus is taken in account this single injury at 60 V amplitude represents an injury rate of 4.5 % of 22 exposed fish and 13 % at 120 V (4 out 30 specimen). Despite the same electrical parameters were set, none of the 21 fish exposed to the ILVO pulse generator was injured. All injuries occurred using the Delmeco TX68 pulse shape.

The post exposure reaction of the fish exposed at 60 V amplitude seemed less strong than in the IMARES study of 2010. In this study some of the fish accelerated out the holding cage into the main tank, while this did not occur in the present study. A second observation was weaker tail marks indicating vertebral injury, while the fish of 2010 with similar injuries had very strong and large tail marks. The post exposure reaction of the fish exposed at 120 V included electro-narcosis and some became stunned. After dissection parts of the fish were taken to the University of Ghent for detailed analysis on the morphology of the dissected fish.

The present results confirmed the recent ILVO outcome of 2013 and showed that the origin of the conflicting outcome must have been related to differences in morphology of the fish. Vertebral injuries could have been related to deviations in the vertebrae related to unknown changes in the methods of the culture, which could have caused differences in the muscular system, or mineral content of the vertebrae. However the mineral content is expected to be similar for all fish of Austevoll, as all fish used in the experiments between 2008 and 2013 from the Austevoll hatchery had the same exclusive rearing history, in which larvae are fed with zooplankton in the first stages of the culture. The fish used in the Tromsø experiment were reared according the intensive rearing technique, which involves nutritionally enriched rotifers (Brachionus plicatilis) and brine shrimps (Artemia salina) in the very early stadia of the fish. This seems to exclude a particular rearing method as the minor number of injuries observed in the present study is related to fish reared according the exclusive method.

Seasonal effects in the cultured fish could be excluded as the present study was executed in the same period as in the 2008 study and a month earlier than the study of 2010.

Adult cod can get injured in electrified beam trawls as observed in the landings in the past years and this is a sensitive aspect in the legislation and perception of this new technique. Continued research could focus on sampling the occurrence of vertebral injuries in cod in the commercial landings and of catches of the observed discard programme and on discriminating a specific seasonal relationship and on analysing fish’ morphology related to the injury.