Ocean Science Consulting (OSC) Ltd. Managing Director, Dr Victoria Todd, presented at the 2017 PEPANZ conference, held in New Zealand (NZ), on March 23^rd 2017. The conference brings together companies, scientists and researchers to help members of the PEPANZ achieve the safe, sustainable, and profitable development of NZ’s offshore oil and gas resources.

To help contribute to the sustainability of the industry, Dr Todd reflected on the current research on interactions between sharks and submarine power cables. Dr Todd concluded with recommendations for further research to fill knowledge gaps that would better-inform mitigation measures and Environmental Impact Assessments (EIAs). By following Dr Todd’s suggestions, this will allow offshore oil and gas stakeholders to make more sustainable and environmentally-sound decisions that would reduce the effect, if any, of submarine power cables on elasmobranch behaviour.

The research that supported Dr Todd’s request for future study can be found below, in a short, but very thorough review of the literature relating to sharks and power cables.

Interactions between sharks and cables in the ocean

Cables

Globally, there are numerous submarine power cables that transmit power out to major islands or between countries, for example the Spain-Morocco Interconnection which runs two submarine High Voltage Alternating Current (HVAC) cables for 26 km across the Mediterranean, or the Basslink between mainland Australia and Tasmania which includes 290 km of High Voltage Direct Current (HVDC) submarine cables that run across the Bass Straight (Valenza & Cipollini 1995; Basslink 2012). Submarine cables are also used to transfer electricity across estuaries or to near-shore islands over shorter distances.

Elasmobranch electroreception

The morphology and sensitivity of the elasmobranch electroreceptive system varies across species, and reflects the vast interspecific variation in habitat, prey and foraging strategies amongst elasmobranchs (Raschi 1978; Tricas 2001). Experiments with dusky smooth-hounds (Mustelus canis Mitchell, 1815) have demonstrated sensitivity to electric potentials as weak as 5 nV cm^-1 (Kalmijn 1982). Juvenile scalloped hammerheads (Sphyrna lewini Griffith & Smith, 1834), sandbar sharks (Carcharhinus plumbeus Nardo, 1827) and neonatal bonnethead sharks (Sphyrna tiburo Linnaeus, 1758) have all demonstrated behavioural responses to thresholds as low as 1 nV cm^-1 (Kajiura & Holland 2002; Kajiura 2003). It is worth noting that whilst nV cm^-1 is not a standard unit according to the International System of Units (SI), it is used typically in the field of elasmobranch electrosensitivity, and facilitates the comparison between the weak electric fields induced around power cables and the extreme sensitivity of elasmobranchs.

Marine organisms produce weak bi-polar electric fields as part of the process of osmoregulatory ion exchange with seawater. Electrical signals are also produced through direct movement of muscles or firing of nerves (Potts & Hedges 1991; Wilkens & Hofmann 2005; Kimber et al. 2011). Capacity of the elasmobranch electrosensory system to detect very weak, low frequency bioelectric fields enables its use in the detection of the bioelectric fields produced by prey.

Round stingrays (Urolophus halleri Cooper 1863) have been shown to use electroreception to detect the bioelectric fields partially produced through the ventilator movements of the gill slits and spiracles of conspecifics during the mating season. Males use electroreception to detect buried females to mate with. Females use electroreception to locate other buried females, possibly to find refuge for less receptive females (Tricas et al. 1995; Sisneros & Tricas 2002).

Elasmobranchs have also been shown to use electroreception in the detection of predators. Embryos of both the oviparous clearnose skate (Raja eglanteria Bosc, 1800) and brown-banded bamboo shark (Chiloscyllium punctatum Müller & Henle, 1838) have been shown to cease moving when exposed to electrical fields corresponding to natural signals produced by their potential predators (Sisneros et al. 1998; Kempster et al. 2013).

Seawater’s electrolytic properties mean that motional electric fields are induced as it flows through the Earth’s magnetic field (von Arx 1962; Manoj et al. 2006). It is accepted generally that elasmobranchs use such fields to navigate the oceans on a large, possibly global, scale (Klimley 1993; Klimley et al. 2002; Meyer et al. 2005). Whilst it has been shown that sharks can detect changes in geomagnetic fields, it is not yet clear whether the mechanism of detection of these fields stems from induction-based electroreception or direct magnetoreception through magnetite-based receptors (Montgomery & Walker 2001; Meyer et al. 2005; Molteno & Kennedy 2009).

Concerns over responses of elasmobranchs to cables

There is well-documented evidence of elasmobranchs avoiding strong, artificial magnetic fields (O’Connell et al. 2010; O’Connell et al. 2011). There is also evidence of elasmobranchs avoiding very large electrical voltage potentials. Strong magnetic and electrical fields are thought to irritate and potentially overwhelm elasmobranchs’ electroreceptive system, although the upper parameters of the electrosensory system have yet to be determined (Howard 2011).

It has been suggested that elasmobranch bycatch could be reduced with electricity-based deterrent devices that would irritate and deter elasmobranchs but not target teleost species. Although electricity-based shark repellent devices for divers have been developed, as yet, there has been little success in the development of commercially-viable bycatch reduction devices due to various practical considerations and problems with habituation (Stoner & Kaimmer 2008; Howard 2011).

There have also been instances where intended deterrents have instead acted as attractants, suggesting that elasmobranchs’ responses to strong magnetic or electric fields is context-dependent and extremely variable between species (Huveneers et al. 2013; O’Connell et al. 2014; Porsmoguer et al. 2015).

If induced electrical fields are at the higher end of the electroreceptive spectrum, a deterrent effect could be observed. Whilst the area of impact will be restricted to a corridor along or across the cable, migration routes travelling over the cables could be disrupted if an avoidance response is caused (Wilson et al. 2010; Normandeau et al. 2011). Conversely, if induced electrical fields are in a similar range to those of a species’ prey, cables may elicit an attraction and foraging response (Wilson et al. 2010; Kimber et al. 2011). Many electroreception experiments involve the use of artificial fields that approximate those of prey and to which elasmobranchs generally show a foraging response (Kalmijn & Weinger 1981; Kalmijn 1982; Gardiner et al. 2012). This could lead to poor foraging success within the vicinity of cables, particularly in benthic species that forage for buried prey and are particularly reliant on their electrosensory system. This may eventually have implications on population fitness if an area is used as a nursery ground, for example (Wilson et al. 2010; Kimber et al. 2011).

It is possible that any negative behavioural effects of underwater cables would diminish over time as animals either habituate or learn to avoid an area of poor foraging success; however, this is only likely to be the case in populations that are relatively sedentary. Migratory populations or species may be more heavily impacted (Guttridge et al. 2009).

Current research into submarine power cable impacts

Research into the effects of submarine power cables on elasmobranchs is currently very limited (Boehlert & Gill 2010; Kimber et al. 2011; Normandeau et al. 2011; Gill et al. 2014). Whilst a number of literature reviews and reports on the theoretical effects of anthropogenic ElectroMagnetic Fields (EMFs) on elasmobranchs have been published, dedicated studies are currently extremely limited. Only one known field study has been conducted, which found inter- and intra-specific variation in responses (Gill et al. 2009; Gill et al. 2014). One known set of laboratory studies have been carried out, which found that 198 A DC power cables prompted a small investigative behavioural response, whereas 75 A AC power cables had no significant behavioural effect (Orr 2016). Whilst the set of laboratory studies only covered a few specific cases, they provide the foundation for further research, which is clearly required, and give some indication that submarine power cables are unlikely to have a strong impact on benthic elasmobranchs.

For more information on submarine power cable impacts on marine biodiversity, specifically marine mammals, please refer to OSC’s online encyclopaedia. This resource hosts a wealth of information on subjects relevant to offshore operations and marine mammal mitigation services.

References