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Introduction
Use of submarine power cables
Submarine power cables are required in order to transport electricity across bodies of water, for example, from offshore renewable energy developments (OREDs), or to near-shore islands. Such power cables can be either alternating current (AC), where the electrical current switches polarity at a specified frequency, or direct current (DC), where the electrical current remains constant. AC cables are generally used over shorter distances whereas DC cables are usually more cost-effective at distances over 40 km, but may also be preferable when bringing DC generated electricity to shore from OREDs.Electrical current flowing through the submarine power cables generate magnetic fields around the cables. Electrically-conductive seawater flowing through these magnetic fields in turn induces weak electric fields that run parallel to the cables. Further details on the generation of these weak electromagnetic fields (EMFs) can be found in Chapter Two. Whilst the EMFs around submarine power cables are very weak, they generally still fall within the exceptionally sensitive detection range of the elasmobranch electrosensory system.
Electrosensory system of rays
Elasmobranchs have an extremely sensitive electrosensory system and dusky smooth-hounds (Mustelus canis Mitchell, 1815) have been shown to respond to fields as low as 5 nV cm-1 (Kalmijn, 1982). Due to the high accuracy of this sense over a short-range, the electrosensory system is important in the localisation of hidden prey, as well as conspecifics and in some cases, the detection of predators (Kalmijn, 1971, 1982; Tricas, et al., 1995; Kempster, et al., 2013). The importance of and reliance upon the electrosensory system varies across species, and is largely dependent on niche (Kajiura, et al., 2010).The location of electrosensory pores has also been linked to the habitat and prey of different elasmobranch species. Studies comparing electrosensory pore numbers and distribution between ray species have found inter-specific differences, predominantly linked to differences in foraging behaviours (Raschi, 1978; Jordan, 2008; Jordan, et al., 2009a; Bedore, et al., 2014). In general, as shown in Figure 1.2, sharks generally have clusters of pores around the head and mouth, whereas rays have pores that radiate around the periphery of the wings, as well as around the head and mouth (Zakon, 1986; Tricas & Sisneros, 2004; Gardiner, et al., 2012). Consequently, rays often have longer electrosensory canals than similar-sized sharks, and the positive correlation between the length of canal and ampullary sensitivity suggests that in general, rays have more sensitive electrosensory systems than comparable shark species (Bodznick & Boord, 1986; Tricas & New, 1998).Considering that the dorsally-located eyes of rays restricts their ability to see ventrally, the higher sensitivity of rays’ electrosensory system and the distribution of electrosensory pores around the periphery of their wings is not surprising. Thus, benthic-feeding rays, such as the New Zealand eagle ray (Myliobatis tenuicaudatus Hector, 1877), are highly reliant on their electrosensory system in the detection and pin-pointing of prey buried in the sediment, and consequently likely to be impacted by the induced EMFs around submarine power cables (Tricas, 2001; Le Port, 2003). Benthic elasmobranchs are also significantly more likely to actually encounter submarine power cables and their associated EMFs, adding to the potential likelihood of impacts.
Expected impacts of submarine power cables on rays
The EMFs around many submarine power cables fall within the detection range of elasmobranchs, and consequently, there is concern that the EMFs may have a behavioural impact on benthic elasmobranch species that encounter them. Ray species may be particularly susceptible, given the even more specialised morphology of their electrosensory systems. Of notable concern is the potential for the EMFs to deter rays and prevent them from crossing cable zones, or to attract rays and act as phantom prey.The use of electric fields or permanent magnets to deter elasmobranchs, particularly from fishing gear or humans in the water, has been studied with mixed results. In certain cases the magnets which were supposed to repel sharks were, in fact, found to attract them, and other studies observed rapid habituation to the initially-repulsive electric fields or magnets (Howard, 2011; O’Connell, et al., 2011, 2014b; Porsmoguer, et al., 2015). Since these systems aim to overwhelm the electrosensory system of elasmobranchs, the electric and magnetic fields used tend to be at the upper end of the electrosensory detection range, usually above 10 µV cm-1 (Howard, 2011; Kimber, et al., 2011). The EMFs around submarine power cables are usually well below the upper end of the electrosensory detection range, and so are less likely to cause avoidance behaviours.The EMFs may, however, be similar in strength to those produced by potential prey items and may attract elasmobranchs by acting as phantom prey, particularly in the presence of olfactory cues that may be produced by actual prey nearby (Kalmijn, 1972; Kimber, et al., 2011). The few studies that have quantified electric potentials around prey species present measurements in units that are not directly comparable to those discussed here. However, one of the rare converted measurements indicates that induced electric fields associated with bivalves and small crustaceans are generally less than 1000 nV cm-1 at 1 cm from the prey item (Haine, et al., 2001; Kimber, et al., 2011). Whilst attracting elasmobranchs may be considered a less concerning impact than deterring elasmobranchs, if attraction happens regularly and on a wide geographical scale, the potential negative impacts of EMFs around submarine power cables could range from reducing fitness at an individual through to a population level.
Choosing experimental parameters
Ideally, cables similar to those proposed for the Kaipara Marine Turbine Generation Project would be used (refer to Section 1.3.1), however, running cables rated at 350 A over a very short distance raises serious safety concerns, in addition to the prohibitive cost of sourcing such cables and the equipment that could safely generate close to 300 A of current for a sustained amount of time.In view of these safety constraints, the experiments in this chapter constitute a preliminary investigation and were run at a much lower electrical current than would be found in the field, but that would still produce EMFs around the cable that would be within the rays’ electrosensory detection range. If negative effects were found at a low current of 30 A, then effects would clearly also be likely at higher currents, however if negative effects were not observed at 30 A, further investigation at higher electrical currents would be required.As 2.4 m s-1 is the maximum recorded water flow through the Kaipara Harbour channel, a lower water flow rate through the tank was required, as this would more realistically reflect the flow rates that occur most regularly, rather than the extreme, which only occurs periodically (during spring tides).Due to the size and design of the experimental tank, it was unfeasible to achieve a consistent circular water flow higher than 0.2 m s-1. However, it was found that at velocities above 0.15 m s-1, eagle rays struggled to settle on the bottom and remain stationary, and were instead slowly swept around the tank. To avoid this unnatural stress for the animals the water flow through the experimental tank was set at 0.12 m s-1.
EMFs around the chosen parameters
Shows the electric fields expected to be induced around a 30 A submarine power cable by seawater flowing perpendicular to the cable at a velocity of 0.12 m s-1. Despite the lower electrical current through the power cable than would be the case in the field, the EMFs are still within the detection range of elasmobranchs. It was not possible to verify whether the expected, calculated fields were equivalent to those present. However, the calculated induced electric fields in Chapters Four and Five were verified and found to be accurate, so the calculations in this chapter are also expected to be accurate.Expected induced EMFs around a 30 A submarine power cable by seawater flowing at a velocity of 0.12 m s-1 perpendicular to the cable, and at several distances from the cable. EMFs were calculated using Equations 2.1 and 2.3. If cable is resting on the seabed, 0° is perpendicular to the seabed (directly above cable) and 90° is parallel to the seabed. Burying or half-burying the cable does not affect the EMF strengths.As discussed in Chapter Two, an animal – in this case an eagle ray – is electromagnetically conductive, and will induce EMFs as it swims through the magnetic field around a submarine power cable. The EMFs induced by the animal will depend on its swimming speed and the angle at which it crosses the cable. The EMFs induced by an animal will be additive or subtractive to those already induced by any seawater flow, depending on whether the animal is swimming against or with the water flow.In the case of no seawater flow across the cable and therefore through the magnetic field around it, theoretically no EMFs would be induced, except any induced by an animal swimming through the magnetic fields. However, the movement of the animal in the tank is likely to create some movement of seawater, and so some very weak EMFs are likely to be generated, though much weaker than those induced in experiments with seawater flow.
Study species: New Zealand eagle ray
The New Zealand eagle ray (Myliobatis tenuicaudatus Hector, 1877) is one of the most commonly found benthic elasmobranchs in New Zealand (Harthill, 1989). It is commonly found in estuarine environments, including the Kaipara Harbour. M. tenuicaudatus is thus highly likely to encounter the cables from the Kaipara Marine Turbine Generation Project, and may be susceptible to any effects from induced electric fields around the cables.
taxonomy
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata: vertebrates
Class: Chondrichthyes: cartilagnous fish
Subclass: Elasmobranchii: sharks, skates and rays
Order: Rajiformes: skates and rays
Family: Myliobatidae: eagle rays and manta rays
Genus: Myliobatis Cuvier, 1816
Species: Myliobatis tenuicaudatus Hector, 1877: New Zealand eagle ray tenuicaudatus may be synonymous with the southern eagle ray (M. australis Macleay, 1881) found in southern Australia (Francis, et al., 1987; Last & Stevens, 2009).
Morphology
New Zealand eagle rays have an olive green, yellow or dark brown dorsal surface with blue or light grey markings that differ between individuals and a white or pale yellow ventral surface (Cox & Francis, 1997; Davis, 2010). They have a rounded, fleshy rostrum and eyes protruding from the dorsal surface, just anterior to two large spiracles, which are used for gill ventilation and for creating a hydraulic jet used for foraging (Gregory, et al., 1979). The mouth is ventrally-located, and eagle rays’ teeth have evolved into fused broad crushing plates in each jaw that are used to break open the shells of their larger prey (Ayling & Cox, 1982).Tenuicaudatus has wide, pointed pectoral fins that are simultaneously flapped vertically like wings to swim. These features make M. tenuicaudatus easily distinguishable from other native New Zealand ray species – the short-tail stingray (Dasyatis brevicaudata Hutton, 1875) and the thorntail stingray (D. thetidis Ogilby, 1899) – which both undulate their more rounded wings to propel themselves forward (Cox & Francis, 1997; Taylor, 2000).
Range and habitat
Tenuicaudatus is found around the North Island of New Zealand and as far north as Norfolk Island and the Kermadecs, though some specimens have also been recorded as far south as Kaikoura on the South Island (Francis, et al., 1987; Cox & Francis, 1997). If M. tenuicaudatus is indeed conspecific with M. australis, then their range is more widespread and also extends to southern Australia and Tasmania, which would further bolster the likelihood of these rays encountering submarine power cables (Last & Stevens, 2009).New Zealand eagle rays are found in coastal waters and on the inner continental shelf, from shallow waters (less than 5m) down to 100m. Their depth preference appears to vary with season, and they may migrate to deeper waters in the winter months, though there may also be some latitudinal migration, both of which increase the likelihood of these rays encountering submarine power cables (Harthill, 1989). However, observations in the Whangateau estuary, northern New Zealand, indicate that some individuals do overwinter in shallow coastal areas (Le Port, 2003).
Diet and feeding
Tenuicaudatus frequently feeds on prey species that are known to burrow at least 20cm down into the sediment, indicating a high reliance on their electrosensory system to detect them. This is reflected in the morphological arrangement of the ampullary canals of the electrosensory system, which are largely concentrated on the ventral surface (Raschi, 1978; Tricas, 2001; Le Port, 2003; Jordan, 2008). Eagle rays access these prey by taking water in through their dorsal spiracles and forcefully jetting it out of their ventrally-located gills and mouth, clearing the substrate underneath. This foraging behaviour leaves behind distinctive feeding pits, sometimes with an imprint of the ray’s body around the pit (Gregory, et al., 1979; Le Port, 2003).tenuicaudatus feeds on a variety of benthic invertebrates, seemingly predominantly over soft substrata. An ontogenetic shift in prey preference has been observed in a study of eagle ray stomach contents, with smaller individuals mainly consuming shrimps and smaller hermit crabs, but mediumsized individuals shifting towards gastropods and crabs, which also tend to be larger in size. The largest eagle rays in the study also consumed bivalves and polychaetes (Harthill, 1989). The study focused on eagle rays found over soft, sandy substrata so may be biased in its conclusions, as M. tenuicaudatus is also found over rocky reef substrata where it feeds on a wide variety of gastropods (Taylor, 2000; Le Port, 2003). Eagle ray feeding patterns can cause significant disturbance to the sediment and can have an important, albeit localised, impact on the structure and density of communities within the substratum (Hines, et al., 1997).Eagle rays in the Whangateau estuary were found to feed during both day and night, with some evidence of increased foraging intensity at night, which may simply reflect an avoidance of daytime disturbances from boats and swimmers or may indicate a preference for feeding at night. Regardless of time of day, foraging intensity increased at high tide and foraging activity was linked to tidal cycles (Le Port, 2003).
Predators
In New Zealand, resident killer whales (Orcinus orca Linnaeus, 1758) are known to regularly predate on native ray species, and are probably the main natural predator of M. tenuicaudatus (Visser, 1999). Other natural predators of M. tenuicaudatus include great white sharks (Carcharodon carcharias Linnaeus, 1758) (Duffy, 2003).Whilst M. tenuicaudatus is not the target of any commercial fisheries, it is taken as fishing bycatch, predominantly by inshore trawls around the upper North Island as well as in Danish seine nets, set lines and drag and set nets. It is also caught by recreational anglers. M. tenuicaudatus is currently classified as a species of least concern by the IUCN (Duffy, 2003).
Life history and reproduction
New Zealand eagle rays are thought to reach maturity unusually slowly relative to other myliobatids with females probably reaching sexual maturity at around 18 years and males at around 8 years. Males are easily identifiable through the presence of two claspers underneath the tail, which are particularly prominent once sexual maturity has been reached (Harthill, 1989; Le Port, 2003).New Zealand eagle rays are viviparous and their embryos are aplacental, but little else is definitively known about their reproductive cycle (Harthill, 1989; Cox & Francis, 1997). It is thought that they follow an annual reproductive cycle and that parturition, ovulation and mating probably all take place during late winter and spring (Le Port, 2003).Some size segregation has been observed with juveniles generally found deeper than adults. There has also been some evidence of sexual segregation through winter observed in the Leigh area. As with size segregation, this is often observed in elasmobranchs and may relate to uses of certain areas as nursery grounds by females (Harthill, 1989).
1. General Introduction
1.1 Introductory overview
1.2 The role of marine sources in meeting renewable energy demands
1.3 New Zealand as a specific case in the drive for marine-based renewable energy
1.4 The elasmobranch electrosensory system
1.5 Elasmobranchs and power cables
1.6 Working with elasmobranchs in a laboratory setting
1.7 Objectives and structure of this thesis
The Generation of Electromagnetic Fields around
2. Submarine Power Cables
2.1 Introductory overview
2.2 Electricity generation and transmission
2.3 Submarine power cables
2.4 A worked example
2.5 Electromagnetic fields through a tidal cycle
2.6 Summary
3. A Preliminary Investigation into the Impacts of Submarine Power Cables on Benthic Rays
3.1 Abstract
3.2 Introduction
3.3 Methods
3.4 Results
3.5 Discussion
4. The Impacts of Submarine Alternating Current Power Cables on Benthic Shark
4.1 Abstract
4.2 Introduction
4.3 Methods
4.4 Results
4.5 Discussion
5. The Impacts of Submarine Direct Current Power Cables on Benthic Sharks
5.1 Abstract
5.2 Introduction
5.3 Methods
6. General Discussion
6.1 Overview
6.2 Effects of submarine power cables on select elasmobranch species
6.3 Impacts of submarine power cables on benthic elasmobranchs
6.4 Summary of conclusions
References
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THE POTENTIAL IMPACTS OF SUBMARINE POWER CABLES ON BENTHIC ELASMOBRANCHS
