The emerging field of electrosensory and semiochemical shark ...

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6 C.P. O’Connell et al. / Ocean & Coastal Management xxx (2012) 1e10 Fig. 5. The positioning at 1 s intervals (black dots) of one juvenile sandbar shark (Carcharhinus plumbeus) measured over a 2 h duration (1 h lead weight control and 1 h EPM). a) The positioning at 1 s intervals (black dots) of one juvenile C. plumbeus to three lead ingots (gray triangle) suspended in the tank for a duration of 1 h b) Experimental treatment examining the swimming pattern of one C. plumbeus at 1 s intervals in response to three EPM ingots (gray triangle) suspended in the tank for 1 h. Significantly fewer shark interactions occurred within 100 cm of the electropositive metal bars (b), in comparison to the lead ingots (a). Illustration taken from Brill et al. (2009). neodymiumeironeboron magnets significantly reduced total elasmobranch and capture of several elasmobranch species: Atlantic sharpnose shark (Rhizoprionodon terraenovae) and smooth dogfish (M. canis). For the inshore longline experiment, results demonstrated that neodymiumeironeboron magnets had no impact on elasmobranch capture whereas the bariumeferrite magnet significantly reduced total elasmobranch capture, in addition to the capture of the blacktip shark (Carcharhinus limbatus) andthesouthernstingray(D. americana). Teleosts, such as red drum (Sciaenops ocellatus), Atlantic croaker (Micropogonias undulatus), oyster toadfish (Opsanus tau), black sea bass (Centropristis striata), and the bluefish (Pomatomus saltatrix), showed no hook preference in either hook-and-line or longline studies. This study concluded that magnet-induced repellent behaviors may be a species-specific phenomenon and the magnetic characteristic known as the axis of polarization, similar to findings in Robbins et al. (2011), may be the key component to finding an effective magnetic shark repellent (Fig. 6b). 2.3. Beach net applications Fig. 6. a) This illustration represents the 50 mm magnetic rare-earth magnetic discs which reduced Galapagos shark (Carcharhinus galapagensis) depredation rate by 50%. The magnetic flux along the bait ranged from 372 to 1474 G. Illustration taken from Robbins et al. (2011). b) This illustration represents the grade C8 bariumeferrite permanent magnet which significantly reduced overall elasmobranch capture in inshore longline trials. The axis of polarization extended vertically, over the entire bait and had a maximum flux of 3850 G. Illustration modified from O’Connell et al. (2011b). Neither image is drawn to scale. Beach nets are devices used to minimize the potential interaction between predatory shark species (e.g. great white shark- Carcharodon carcharias, tiger shark-Galeocerdo cuvier, and bull shark-Carcharhinus leucas) and beachgoers (Cliff and Dudley, 1992; Dudley, 1997). Although these nets are successful at minimizing this interaction, experimental evidence demonstrates that local elasmobranch populations have plummeted (Dudley, 1997; Stevens et al., 2000; Dudley and Cliff, 1993). As a means to examine the potential utility of ceramic magnets to exclude sharks from netted areas, O’Connell et al. (2011a) conducted a captive study where lemon sharks (Negaprion brevirostris) were individually subjected to a fence-like apparatus containing both a control (clay bricks) and magnetic (grade C8 bariumeferrite magnets) opening. Results demonstrated that ceramic magnets were efficient at manipulating the swimming behavior of N. brevirostris; however, barrier trials and tonic immobility trials demonstrated that habituation to the magnetic fields occurs due to repeated stimulation over a short duration (Fig. 7). It is uncertain how these results would relate to experiments conducted on wild sharks, but was deemed unlikely to be as high as the frequency of Please cite this article in press as: O’Connell, C.P., et al., The emerging field of electrosensory and semiochemical shark repellents: Mechanisms of detection, overview of past studies, and future directions, Ocean & Coastal Management (2012), http://dx.doi.org/10.1016/ j.ocecoaman.2012.11.005
C.P. O’Connell et al. / Ocean & Coastal Management xxx (2012) 1e10 7 Fig. 7. The total behaviors from three lemon sharks (Negaprion brevirostris), with the treatment variables (C¼Control; M ¼ Magnet) listed on the x-axis. a) The total avoidance behaviors (p < 0.001) of the three sharks in trial one and trial two. b) The total number of entrances (p < 0.001) of the three sharks in trial one and trial two. Illustration taken from O’Connell et al. (2011a). interaction seen in this experiment and thus is unknown if sensory habituation would occur. 3. Factors affecting repellent success Prior to drawing conclusions from the current body of research pertaining to electrosensory repellents and as a means to understand inconsistent results, it is imperative that scientists monitor a variety of biological and environmental parameters which may influence repellent success. Although it is important that scientists analyze the basic behavioral responses of elasmobranchs toward various electrosensory stimuli, studying parameters that may alter success will give scientists valuable insight as to methods and conditions which will yield the most successful implementation. 3.1. Satiation The availability of prey can be influenced by a variety of factors, including: climate change (e.g. Loeb et al., 1997; Polovina, 1996), fisheries (e.g. Bearzi et al., 2006), and predator abundance (e.g. Connell, 1998). With changing prey densities, predator satiation may concurrently change which may greatly influence predator behavior (Williamson, 1980). To determine how the level of satiation could effect electrosensory repellent success, several laboratory studies were conducted (Stoner and Kaimmer, 2008; Tallack and Mandelman, 2009). Stoner and Kaimmer (2008) demonstrated that EPM repellent effectiveness on S. acanthias was found to be highly correlated with food deprivation where increasing levels of food deprivation (2 and 4 days) resulted in decreased repellent success. Similarly, Tallack and Mandelman (2009) demonstrated that the responsiveness of S. acanthias to EPM stimuli was inversely proportional to the level of food deprivation. Therefore one influential factor which may be pertinent to the success of electrosensory deterrents is animal satiation. It is unclear how these laboratory findings would translate to the field and future experimentation is needed; however, if these findings hold true, situations where the level of satiation may be highest (e.g. seasonality of prey abundance, lack of fishing pressure, healthy ecosystem) may increase the likelihood of repellent success and thus be pertinent to future implementation strategies. 3.2. Water visibilityeturbidity and light intensity Little is known about the effect of water visibility on elasmobranch sensory allocation; however, preliminary experimentation with the bull shark (C. leucas) illustrates that increased turbidity and low water visibility conditions increase the repellent effectiveness of ceramic magnets (O’Connell et al., submitted for publication). Similar to where blind humans show enhanced auditory localization capabilities in comparison to sighted humans (Lessard et al., 1998; Muchnik et al., 1991; Rice, 1970), it may be possible that the electrosensory capabilities of elasmobranchs are altered in conditions when vision is compromised, therefore maximizing the effectiveness of electrosensory repellents. Upon further consideration, although turbidity may drastically alter the extent of the visual field of elasmobranchs and may yield differences in sensitivity to electrosensory repellents, the electrosensory differences due to variations in light intensity (e.g. day versus night) may be minimal due to the light-reflecting layer behind the retina known as the tapetum lucidum (Gruber and Cohen, 1985). This structure maximizes light available for lowlight conditions. Therefore although future studies should not neglect light intensity, turbidity may be a more influential factor in electrosensory repellent success and may provide pertinent information as to the locations which will yield the most success (e.g. turbid coastal environments). 3.3. Presence of conspecifics and heterospecifics Intra- and inter-specific competition is highly reported in the literature (Crombie, 1947; Connell, 1961; Polis, 1981; Stiling et al., 1984; Munday et al., 2001) and illustrates that the presence of conspecifics and/or heterospecifics induces a competitive mentality, often leading to an abrupt change in behavior. Brill et al. (2009) conducted a laboratory experiment which examined the feeding response of juvenile sandbar sharks (C. plumbeus) attwo different density levels (n ¼ 7; n ¼ 14) toward an EPM alloy. During high-density trials (e.g. 14 sharks), the deterrent effects were minimal and short-lived; however, C. plumbeus demonstrated a sensitivity to EPM alloys during low-density trials and therefore the researchers concluded that competition plays a role in the effectiveness of EPM alloys. (Brill et al., 2009). In a more recent study conducted by Robbins et al. (2011), the efficacy of seven rareearth (neodymiumeironeboron) magnet, two ferrite magnet, and two EPM (NdePr: neodymiumepraseodymium) alloy configurations on reducing the depredation rate of the Galapagos shark (C. galapagensis) on baited lines was examined. During high density levels (greater than 3 sharks) results illustrate that EPMs were ineffective; however, in low density trials behavior was exploratory and cautious. With these findings, this study concluded that conspecific density greatly impacted repellent success and suggested that these repellents may be better suited for recreational Please cite this article in press as: O’Connell, C.P., et al., The emerging field of electrosensory and semiochemical shark repellents: Mechanisms of detection, overview of past studies, and future directions, Ocean & Coastal Management (2012), http://dx.doi.org/10.1016/ j.ocecoaman.2012.11.005
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