Pulse trawl fishing: characteristics of the electrical stimulation and the effect on behaviour and injuries of Atlantic cod (Gadus morhua)

Abstract

In the North Sea flatfish fishery, electric pulse trawls have been introduced to replace the conventional mechanical method. Pulse trawls reduce the fuel consumption, reduce adverse impact on the ecosystem but cause injuries in gadoids. We describe the design and electrical properties of pulse trawls currently in use and study the behavioural response and injuries in cod exposed to electrical pulses under controlled conditions. Pulse trawls operate at an average power of 0.7 kW m−1 beam length and a duty cycle of ∼2%. The electric field is heterogeneous with highest field strength occurring close to the conductors. Cod were exposed to three different pulse types for a range of field strengths, frequencies, and duty cycles. Two size classes were tested representing cod that escape through the meshes (11–17 cm) and market-sized cod that are retained in the net (34–56 cm). Cod exposed to a field strength of ≥37 V m−1 responded by moderate-to-strong muscular contractions. Some of the large cod (n = 260) developed haemorrhages and fractures in the spine, and haemal and neural arches in the tail part of the body. The probability of injuries increased with field strength and decreased when frequency was increased from 100 to 180 Hz. None of the small cod (n = 132) were injured and all survived. The field strength at the lateral boundaries of the trawl was too low to inflict injuries in cod.

Reducing bycatch in beam trawls and electrotrawls with (electrified) benthos release panels

“…Over the entire trial period, 52 cod individuals (0.438 6 0.014 m) were caught in the experimental trawl, and 7.7% were injured by the electric cramp stimulus. A dark discoloration of the skin, as well as the spinal injuries were the same as previously reported for this species (de Haan et al, 2016;Soetaert et al, 2016a,b).…”

“…This incidence was slightly lower than the previously reported 9-11% (van Marlen et al, 2014) and may result from the reduced electric field strength (de Haan et al, 2016) or the reported strongly variable sensitivity of this species (Soetaert et al, 2016b).…”

“…Four out of 45 cod exposed caught by electrotrawls were reported to have paravertebral haemorrhages (van Marlen et al, 2014). This was confirmed in laboratory experiments showing 0-70% spinal injuries in cod exposed near the electrodes (Soetaert et al, 2016a;de Haan et al, 2016). However, no effects were demonstrated in an identical experiment with the non-gadoid roundfish European seabass (Dicentrarchus Labrax L.) (Soetaert, 2015b), dogfish (Scyliorhinus Canicula L.) and dab (Limanda limanda L.) (de Haan et al, 2015).…”

Radiological changes during fracture and repair in neural and haemal spines of Atlantic cod (Gadus morhua)

“…The vertebral column of cod consists of 52 (±2) vertebrae and can be subdivided into 4 distinct anatomical regions: cervical, abdominal, caudal and ural (Fjelldal et al., ). The location of the currently observed fractures, and calluses are in the caudal region, the same region that lordosis is most frequently observed in cod (Fjelldal et al., ), and where vertebral fractures occur when applying electric stimuli (de Haan et al., ), indicating involvement of muscular contractions. Geometric modelling has shown that there is an even flexibility throughout the vertebral column in cod, except in the ural region, where flexibility increases (Fjelldal et al., ).…”

“…However, more acute pathologies may occur. For instance, electric stimuli applied during electrofishing (reviewed in Snyder, ) and electrotrawling (de Haan, Fosseidengen, Fjelldal, Burggraaf, & Rijnsdorp, ; Soetaert et al., ) can give muscular haemorrhage and acute spinal injuries such as vertebral fracture in Atlantic cod ( Gadus morhua ) (de Haan et al., ). In fish, the published literature on bone fracture all focuses on the mechanism during fracture repair in fin rays (Sousa, Valerio, & Jacinto, ; Takeyama, Chatani, Takano, & Kudo, ).…”