Diving Behavior

Much of the diving behavior of elephant seals on the continental shelf is consistent with the hypothesis that selection has acted to minimize the probability of encounters with near-surface predators such as white sharks.

From: Great White Sharks , 1996

Diving Behavior of Elephant Seals: Implications for Predator Avoidance

BURNEY J. LE BOEUF , DANIEL E. CROCKER , in Great White Sharks, 1996

Discussion

Much of the diving behavior of elephant seals on the continental shelf is consistent with the hypothesis that selection has acted to minimize the probability of encounters with near-surface predators such as white sharks. Diving in waters over the shelf, relative to the open ocean, is characterized by fast swimming, a high dive rate, short surface intervals and dive durations, and swimming at the bottom. Juveniles and adult females cross the shelf rapidly and do not linger. Newly weaned pups learn to swim and dive at night in shallow water close to shore.

Although behaving similarly in some respects, breeding-age males deviate significantly from this predator-avoidance pattern. They spend considerable time in crossing the continental shelf. They enter waters near the rookery repeatedly and rest on the water's surface during the breeding season. The risk-taking behavior of males may promote their reproductive success, but they evidently pay a high price, incurring more shark bites during the breeding season than do females. It is possible that this sex difference is due to males escaping from the predator with only a wound, while the smaller adult females more readily succumb. It is difficult to test this possibility directly, but the following points indicate it to be false. First, the adult sex ratio is skewed to males, being on the order of 1 male to 2–4 females. White shark predation may be responsible, in part, for this difference. Second, the annual survival rates of each age group of breeding-age males is about the same, 55% (Le Boeuf, 1974), suggesting that young, smaller males do not succumb to the predator more frequently than do larger males. Third, adult males, despite their great size, did not display significantly more shark wounds than younger males, despite their greater size. The incidence of shark wounds in adult males and males of age 7, 6, and 5 years, respectively, was 50%, 23.8%, 22.6%, and 3.6%, which is similar to the relative frequency of males in the colony during the study period, 41%, 21.9%, 27.5%, and 10%, respectively. We conclude that the mating strategy of breeding-age males puts them in great jeopardy of being injured by white sharks.

Newly weaned pups going to sea for the first time are the other category of elephant seals that may be especially vulnerable to shark predation. They must learn to swim and dive on the edge of the high-risk zone. To what extent gaining aquatic experience at night or in sheltered coves is effective in lowering predation is unknown. The highest annual mortality rate is evident in young-of-the-year during their first trip to sea, which extends from April to September–October. Observations of shark attacks and shark-bitten juveniles, however, are rare in April on both the Farallones and Año Nuevo (Le Boeuf et al., 1982; Ainley et al., 1981, 1985).

Adult female white sharks appear to move south in spring and breed below Point Conception during summer (Klimley, 1985b). If young elephant seals move north to the Gulf of Alaska and across the North Pacific, as older juveniles do on their third and fourth trips (Le Boeuf et al., 1993; Le Boeuf, 1994), they are doing so at a time when white sharks appear to have left the area. Moreover, the seals are moving into northern latitudes, where white sharks are far less common and are replaced by killer whales as the most significant predator (Jefferson et al., 1991). There are no reports of white sharks north of the Queen Charlotte Islands, Canada (Klimley, 1985b). If white sharks are moving south in spring, then juvenile mortality should be higher at southern California rookeries such as San Miguel Island than at central California rookeries. This is apparently not the case (Le Boeuf and Reiter, 1988; Le Boeuf et al., 1994) and can be explained if the sharks do not feed while breeding. Shark attacks on juveniles, age 1–3 years, are most common in October and November on the Farallones (Klimley et al., 1992), which suggests that the young-of-the-year may be most vulnerable when returning to the rookery in the fall. Tracking the movements of both groups of animals will aid in determining the role of white shark predation on the annual mortality rate of elephant seals.

What alternative hypotheses might explain the differences between on-shelf and off-shelf diving? The seals might simply be in a hurry to get to deep water to begin foraging. Assuming that the seals do not forage on the shelf, there is no point in lingering, except possibly to take a more direct route to their foraging destination. Males appear to do this, because they migrate farther north than females, some of them for great distances. From the other perspective, the differences may be due to foraging over deep water, which produces slower dive rates, longer surface intervals, and longer dive durations than those observed in dives over the shelf when the seal is in transit.

The diving pattern of other pinnipeds may provide a perspective on the role of predation in shaping the northern elephant seal's diving pattern. The closest relative of the northern elephant seal is the southern elephant seal Mirounga leonina, which breeds on sub-antarctic islands and mainland sites (Laws, 1994). Southern elephant seals have only one major predator, the killer whale (Voisin, 1976; Carrick and Ingham, 1962; Ling and Bryden, 1981; Lopez and Lopez, 1985). Arrival of the whales at some rookeries coincides with breeding and the entry of weaned pups into the sea (Condy, 1978; Condy et al., 1978). Despite different predators and environments, the general diving patterns of both species are similar (Hindell et al., 1991; Le Boeuf and Laws, 1994). Campagna et al. (1995) reported that postbreeding adult females in Patagonia crossed the broad continental shelf quickly at a mean horizontal swim speed of 1.6 ± 0.1 m/sec, suggesting that they were not feeding. Like northern elephant seals, all females had significantly higher diving rates and shorter dive durations on the shelf than off it, and they dived along the substrate bottom on their way to the shelf break.

It is not clear whether other phocids, with or without near-surface predators, exhibit a diving pattern that may have functions other than foraging. The diving patterns of only a few of them have been described (e.g., Weddell seal Leptonychotes weddelli) and none have been examined from this perspective.

Otariid species, whose diving behavior has been studied [extensively in lactating females (e.g., Gentry and Kooyman, 1986)], do not exhibit the behaviors associated with avoidance of a near-surface predator. Northern fur seals Callorhinus ursinus, whose feeding range overlaps that of the northern elephant seal, spend 74% of the time at sea resting or active at the surface (Gentry et al., 1986). Most otariids swim at the surface during transit, "porpoising" to breathe, for example, California sea lions Zalophus californianus (Peterson and Bartholomew, 1967) and South African fur seals Arctocephalus pusillus (Kooyman and Gentry, 1986). Sea otters Enhydra lutris also spend long periods at the surface, much of it in the high-risk zone. Of these animals, sea otters are attacked and killed (Miller and Collier, 1981; Ames and Morejohn, 1980), but they are not eaten (see Chapter 28, by Ames et al.). A similar low preference for fur seals is suggested by the absence of fur seal remains in shark stomachs. California sea lions are consumed by white sharks (Ainley et al., 1981, 1985), but not as frequently as elephant seals and harbor seals.

This analysis, although speculative, underscores gaps in our knowledge of the relationship between white sharks and northern elephant seals and suggests directions for future research. Knowledge of the habits of either animal will enhance our understanding of this predator-prey relationship.

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Marine Life

David H. Cushing , ... Matthew R. Siskey , in Encyclopedia of Ocean Sciences (Third Edition), 2019

Elasmobranchs

Pelagic sharks are taxonomically diverse and are primarily comprised of Carcharhiniformes (e.g., silky sharks Carcharhinus falciformis, oceanic whitetip sharks Carcharhinus longimanus, and blue sharks Prionace glauca), Lamniformes (e.g., great white sharks Carcharodon carcharias, shortfin mako sharks Isurus oxyrinchus, porbeagle sharks Lamna nasus), and Myliobatiformes (e.g., manta rays Manta spp.). Most species are found between temperate and tropical latitudes, but historically, spatial trends in the abundance of certain species were commonly observed. For example, in temperate waters blue sharks (Prionace glauca) were historically one of the most abundant pelagic elasmobranchs and were subsequently replaced by oceanic whitetip sharks in sub-tropical and tropical latitudes (Bonfil et al., 2008; Howey-Jordan et al., 2013). Some elasmobranchs exhibit complex, partially-pelagic life histories, and/or display habitat shifts when they reach certain sizes/ages (Hoyos-Padilla et al., 2014). Great hammerhead sharks (Sphyrna mokarran) and tiger sharks (Galeocerdo cuvier ), for example, associate both with shallow coastal and open ocean habitats. Many pelagic species are known to perform extensive, sometimes trans-oceanic migrations. Both oceanic whitetip and tiger sharks display diving behaviors in excess of 800  m (Vaudo et al., 2014; Howey et al., 2016). The primary function of this behavior is thought to be foraging; however, it may also serve as a navigation and/or thermoregulatory strategy (Howey et al., 2016).

Some species of pelagic elasmobranchs, such as shortfin mako sharks and salmon sharks, display regional endothermy, and are thus able to maintain internal body temperatures above that of the surrounding water column. This adaptation is thought to allow for greater swimming speeds, increased efficiency of food digestion, and thermal niche expansion (Newton et al., 2015). It is therefore unsurprising that pelagic elasmobranchs house some of the most powerful, fastest swimming species in the ocean; shortfin mako sharks, for example are able to reach swimming speeds of around 40   km per hour.

Aside from the filter feeding whale shark (Rhincodon typus) and basking shark (Cetorhinus maximus), pelagic elasmobranchs feed on a wide diversity of prey items which typically include squid and smaller teleost fishes (e.g., mackerels, herrings, and tunas). Although many species are considered apex predators (e.g., great white shark), pelagic elasmobranchs span most trophic levels, making them one of the most trophically diverse group of fishes on the planet. These diverse foraging strategies suggest that pelagic elasmobranchs provide many important services to pelagic food webs, from top-down control of lower trophic levels by apex predators to bottom-up nutrient cycling as filter-feeding species die and decay.

Over the last 60   years, pelagic elasmobranchs have suffered greatly at the hands of commercial fisheries exploitation (Dulvy et al., 2008). Many species have been captured to support the Asian fin trade in addition to being incidentally captured as bycatch in commercial longline fisheries for tuna and swordfish (Kitchell et al., 2002; Baum and Myers, 2004). Despite many pelagic elasmobranchs having received international protection through governing bodies such as the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), shark fisheries still operate in many regions of the world, suggesting these fish populations should be closely watched to avoid depletion or collapse.

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Trophic Relationships of Coastal and Estuarine Ecosystems

J.J. Vaudo , M.R. Heithaus , in Treatise on Estuarine and Coastal Science, 2011

6.08.9.6 Open Coastal Waters

Several coastal sharks spend considerable amounts of time in the water column and are often not associated with benthic habitats. Lamnids and some large carcharhinids are the main groups of sharks that fall into this category. Because of their large size, these sharks tend to be apex predators, feeding on large fishes, small sharks, and for some species, such as the great white shark, marine mammals. In many of the more pelagic species, cephalopods are also an important prey item (e.g., blue sharks (Tricas, 1979) and salmon shark (Kubodera et al., 2007)).

One of the more notable trophic interactions involving elasmobranchs in this habitat realm is the predator–prey relationship between large sharks and marine mammals, in particular great white sharks and pinnipeds. Seasonally, great white sharks in the eastern Pacific and South Africa gather at pinniped rookeries to take advantage of plentiful prey (Anderson and Pyle, 2003; Martin et al., 2005; Domeier and Nasby-Lucas, 2007). Although the predatory behaviors and factors affecting successful predatory events have been examined at these locations (Tricas and McCosker, 1984; Tricas, 1985; Martin et al., 2005; Hammerschlag et al., 2006; Martin et al., 2009) and pups appear to be targeted by the sharks (Martin et al., 2005; Laroche et al., 2008 ), the numerical impact of great white sharks on these pinniped populations remains unknown. However, it appears that pinnipeds at these sites respond to the presence of great white sharks by altering their behavior. For example, many of the diving behaviors of northern elephant seals Mirounga angustirostris off California are consistent with hypotheses of avoiding predation (Le Boeuf and Crocker, 1996), and at Seal Island in False Bay, South Africa, adult Cape fur seals Arctocephalus pusillus pusillus return to the island under the cover of darkness, while seal pup and shark numbers are highest at dawn (Laroche et al., 2008).

Great white sharks are not the only shark species with the potential to influence pinniped populations. Off Sable Island, Canada, predation by sharks on harbor seal Phoca vitulina pups and mature females accounted for at least 50% of the decline in production observed between 1993 and 1997 and is responsible for much of the decline in this seal population (Lucas and Stobo, 2000; Bowen et al., 2003). Moreover, in Prince William Sound, Alaska, empirical data and theoretical models suggest harbor seals and Stellar sea lions Eumetopias jubatus underutilize resources in deep water to avoid predation by Pacific sleeper sharks (Frid et al., 2007, 2008, 2009). Models of sleeper shark–harbor seal interactions also suggest the potential for a trophic cascade in the northeastern Pacific (Frid et al., 2008). Model removal of sharks from this system results in seals shifting their foraging behavior to exploit profitable deeper water fish, releasing shallow water fish from predation. Because of the densities of harbor seals in the northeastern Pacific, loss of sharks from this system has the potential for causing substantial changes in the teleost community.

The importance of trophic cascades induced by coastal elasmobranchs in open coastal waters has also been suggested based on a combination of time-series and experimental data. Myers et al. (2007) proposed that overfishing of large sharks in the open coastal waters of the northwestern Atlantic started a cascade that has resulted in the collapse of the North Carolina bay scallop population. The loss of large sharks from coastal waters is thought to have released cownose rays from predation, resulting in a population expansion, and the larger migrating ray population caused the collapse of the bay-scallop population in seagrass habitats through consumption. The declines of large-shark population are widely acknowledged and the ability of cownose rays to cause major habitat disturbances has previously been observed (e.g., Orth, 1975; see section 6.08.9.3) and field experiments have shown that cownose rays can collapse local scallop populations (Peterson et al., 2001; Myers et al., 2007). Whether predatory release has caused an increase in cownose rays, however, has drawn criticism. In addition to some of the questions raised previously, there is some degree of mismatch in the timing of the decrease of bay-scallop populations (1980s) and increases in ray catches (1990s). In the late 1980s, prior to the reported increase in rays, scallop losses caused by a red-tide outbreak resulted in well-below-average scallop recruitment for several years (Summerson and Peterson, 1990; Peterson and Summerson, 1992). As a result, the recent impacts of cownose rays may not be the sole cause of scallop-population collapse but may exacerbate continuing declines. Despite criticism of this example, given the worldwide declines in large coastal shark populations, the possibility of such a cascade highlights the importance for research on the importance of shark predation and risk effects, as well as their role in driving habitat.

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Marine Mammals and Ocean Noise

D. Wartzok , in Encyclopedia of Ocean Sciences (Second Edition), 2009

Nonauditory Effects

Direct nonauditory effects of sound have not been demonstrated in marine mammals, but some have been observed in other animals or humans or in vitro. Humans exposed to intense sounds can experience dizziness (Tullio phenomenon), nystagmus, and neurological disturbance in the absence of hearing loss.

Acoustic resonance

Tissues associated with air-filled cavities can be subjected to shear forces when those cavities resonate. Two important factors for resonance are the relationship between the dimensions of the cavity and the wavelength of the sound and the tuning or amplification of the resonance. The latter is described by the Q value, with a high Q indicating greater resonance amplitude. The resonance frequency of beluga and bottlenose whale lungs has been determined to be 30 and 36   Hz, respectively, with relatively low Q values of 2.5 and 3.1, respectively. Thus, at these low frequencies, there is a modest amplification of the resonance magnitude. The resonance characteristics of other air-filled cavities have not been measured but the consensus is that the magnitude of the resonance is not great enough to cause tissue damage.

Rectified diffusion and activation of micro-bubbles

Bubble growth either through acoustically driven rectified diffusion or acoustic activation of micro-bubbles could lead to symptoms similar to decompression sickness in human divers. The basic requirement in either model is that the tissues be highly supersaturated. In some deep-diving marine mammals, such as beaked whales, supersaturation has been calculated to exceed 300%. Beaked whales that stranded subsequent to naval sonar activities have shown bubbles in a number of tissues. However, the current understanding of the exposures of the whales that stranded is that the received sound levels were too low to cause bubble growth or activation (demonstrated at 210   dB re 1   μPa in vitro). A more likely explanation is that a behavioral response to lower received levels initiated the cascade of events resulting in the stranding. Studies in right whales, a species not involved in naval sonar stranding incidents, have shown that received levels of 133   dB re 1 μPa of an unfamiliar signal will cause abrupt changes in diving behavior.

Blast injury

Blast injuries to marine mammals have only rarely been observed. The best-documented case is that of humpback whales that died within 3 days after detonation of 1700–5000-kg Tovex (a trinitrotoluene (TNT) clone) blasts. The mechanical traumas in the whale ears were consistent with classic blast injuries in humans including round window rupture, ossicular chain disruption, bloody effusion in the ear region, and bilateral periotic fractures. These traumas result from eruptive injury during the rarefactive portion of the shock wave when inner ear fluid pressures are much greater than ambient.

Stress

Both high-intensity, short-duration stimuli and long-term exposure to much lower levels of noise can result in elevated levels of stress. Some studies conducted in humans have shown that exposure to chronic noise elevates neuroendocrine and cardiovascular indices of stress and results in diminished performance on cognitive tests of reading ability and long-term memory. For most studies in humans, it has been difficult to demonstrate statistically significant effects of chronic noise, although there is a consistent trend toward increased cardiovascular risk if the daytime exposure level exceeds 65   dB(A) (see XXX for a discussion of the differences between in-air and underwater decibel reference levels and measurements). Within this context, the 12-dB increase in low-frequency noise due to shipping in the past four decades could be a stress factor in addition to its role in masking communications. Another component of chronic noise, at least in the North Atlantic, is the long-range propagation of the sounds from seismic surveys. Autonomous hydrophones located near the mid-Atlantic ridge frequently, particularly in the summer, recorded sounds of seismic surveys taking place over 3000   km from the recording location. The effects of such long-term increases in anthropogenic sound on the stress response of marine mammals have not been determined.

There have been two studies of the short-term effects of noise on the stress response of marine mammals. One detected no change in behavior or catecholamine levels of captive beluga whales exposed to playbacks of the operating noise from a semisubmersible drilling platform at a source level of 153   dB re 1   μPa at 1   m. In contrast, a beluga whale exposed to high-level (>100   kPa) impulsive sounds and high-intensity tones had significantly elevated norepinephrine, epinephrine, and dopamine levels. A bottlenose dolphin similarly exposed did not show elevated catecholamines, but did show an increase in aldosterone and a decrease in absolute monocyte levels after exposure to a seismic water gun.

Among the range of possible mechanisms contributing to the stranding of beaked whales exposed to naval sonar is an acute stress phenomenon, that of hemorrhagic diathesis. A precondition for hemorrhagic diathesis is a depletion or lack of clotting factors or platelet dysfunction. Humans with a hereditary deficiency in clotting factors develop subarachnoid and inner ear hemorrhages similar to those seen in the beaked whales. No studies have been conducted on the clotting ability of beaked whale blood, but in the few cetacean species studied to date, all have shown a lack of certain clotting factors. None of the species studied have stranded in association with naval sonar; so if hemorrhagic diathesis is a contributor to beaked whale strandings, the level of stress and the physiological responses to that stress are different in beaked whales.

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Marine Mammals, Extinctions of

Glenn R. Van Blaricom , ... Robert L. BrownellJr., in Encyclopedia of Biodiversity (Second Edition), 2013

General Features and Habitat Boundaries

Marine mammals are characterized by a number of striking modifications, as compared with terrestrial mammals, in anatomy, physiology, and ecology (Table 2). In some cases, the modifications are sufficiently extreme that phylogenetic linkages to terrestrial ancestry are obscured, difficult to resolve, and matters of controversy among scientists. The degree of modification is correlated with the duration of the evolutionary history of the major marine mammal taxa as marine organisms.

Table 2. Distinguishing characteristics of the major marine mammal taxa

Characteristic Cetacea Sirenia Pinnipedia
Body streamlined X X X
Limbs modified X X X
Rear limbs modified as flippers X
Rear limbs and pelvic girdle absent X X
Propulsion by caudal spine and flukes X X
Loss of pelage X X
Subcutaneous blubber layer X X X
Simplification of dentition X X X
Expansion of anterior skull X
Development of acoustic capability for communication and echolocation X a
Amphibious capability X
a
Echolocation capability is known only for the odontoceti.

Although marine mammals are largely defined by marked departures from the terrestrial mammalian model, it is instructive to consider the major features of terrestrial mammals retained in marine mammals. In the context of extinction processes in general, and anthropogenic extinctions in particular, two retained features are of particular importance. First, although most marine mammals spend most of their lives immersed at sea, they retain a largely terrestrial respiratory architecture and must surface and breathe in order to allow for exchange of respiratory gases. Second, marine mammals are homeothermic, with core body temperatures typically near 38   °C, like their terrestrial relatives. The need to breathe at the surface and the need for major anatomical adjustment to minimize rates of heat loss are constraints that foster vulnerability to unsustainable rates of exploitation and to certain types of pollution. The significance of these constraints is developed in the case studies we present in this article.

The diving capabilities of marine mammals define the three-dimensional nature of their habitats at sea. Nearly all extant marine mammals dive to forage, although the ranges of diving capability and pattern are broad. Most marine mammals also spend significant time submerged while traveling, socializing, or breeding.

Among the cetaceans, sperm whales, beaked whales, and narwhals likely dive deeper and longer than other species on average. Sperm whales can dive to 3000 m, remaining submerged for an hour or more. The diving behavior of beaked whales is poorly known, but there is emerging evidence that beaked whales also may routinely make repetitive dives of long duration to great depth. Narwhals are known to be capable of dives of 1800  m depth and may dive to depths in excess of 1000   m 20–30 times per day. Most baleen whales and many of the smaller cetaceans commonly dive for less than 10   min at a time, and to depths no greater than a few hundred meters.

Among the pinnipeds, the elephant seals (in the family phocidae) have maximum diving capabilities to nearly 2000   m, and are known to make remarkably long sequences of repetitive deep (400–600   m), long (20   min or more) dives with surface intervals of only 2–3   min. These sequences may be maintained day and night for tens of days at a time. Many other phocid seals are thought to have similar capabilities. The sea lions and fur seals (otariidae), in contrast, generally dive for only a few minutes at a time, and generally to maximum depths of a few hundred meters, although many otariids are known to be capable of continuous sequences of repetitive shallow dives of 10–12   h or more.

In contrast to cetaceans and pinnipeds, sirenians are weak divers, generally remaining in shallow water (<20   m) and diving only for 2–3   min when active. Deeper dives (to 70   m) may occur on occasion, and dive duration can be quite long (>1   h) when animals are resting at the bottom. Sea otters are also relatively weak divers, reaching depths of 100   m and remaining submerged for a maximum of approximately 5   min, although most dives are to 30   m or less and last only for 1–2   min. There are no data available on the diving capabilities of the chungungo.

Few field observations of Steller's sea cow were made before extinction, but morphological analysis suggests that sea cows were unable to dive below the sea surface, surviving instead by foraging on kelp forest canopies and other macroalgae floating on the sea surface. Polar bears are able to make shallow dives, but do not typically engage in the extended repetitive dive sequences typical of many marine mammals, and apparently do not forage while diving. Polar bears instead use stealth, quickness, and great strength to capture phocid seals, their primary prey, at seal breathing holes on the ice surface. Thus, the extent to which the at-sea habitat of marine mammals is truly three-dimensional varies widely among the major taxa as well as the individual species. Within species, there is marked ontogenetic variation in diving capability and pattern as well.

The marine mammals are geographically ubiquitous in the world's oceans, seas, and estuaries (e.g., Perrin et al., 2009). Cetaceans occur in marine environments at all latitudes. For example, killer whales may have the largest natural geographic range of the earth's mammals. Most of the mysticetes and some of the larger odontocetes have global ranges or are distributed antitropically. Smaller cetaceans are widely dispersed as well, although individual populations typically concentrate in regions of predictably high local biological productivity. Several species of small cetaceans, including delphinids, phocoenids, and the three extant monotypic families of river dolphins, are found in major river systems in South America and Asia. Pinnipeds occur in all the world's major marine habitats, but most species are concentrated in middle or high latitudes, in close association with regions of high productivity (e.g., Kovacs et al., 2011). In addition, there are several pinniped species or populations confined to isolated large lakes in Europe, Asia, and North America. Most sirenians are confined to tropical or subtropical latitudes, in shallow seas that provide adequate macrophytic food and refuge from predation, and are thermally tolerable. The sea otter is confined to the coastal North Pacific Rim, and the chungungo to the temperate coastal southeastern Pacific. Polar bears occur only in the Arctic and sub-Arctic, rarely traveling south of 60°   N latitude except in the relatively frigid northwestern Atlantic and Hudson Bay.

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Numerical advancements on the analysis of dynamically installed anchors

Yanbing Zhao , ... Yuxia Hu , in Ocean Engineering, 2019

3.3.2.1 Anchor keying without anchor chain effect

Using the RITSS approach, the keying and diving behaviours of OMNI-Max anchors were investigated by Liu et al. (2014a, 2016b). In their analysis, the Tresca soil model was used without considering strain rate and softening effects. The OMNI-Max anchor geometry was somewhat simplified in the 3D space. Two different pullout loading angles at the padeye (θ a = 22.5° and 30°) were investigated by controlling padeye displacement.

Kim and Hossain (2017) carried out 3D analyses using the CEL approach. A modified Tresca soil model was adopted, allowing for the effects of strain rate and strain softening following Einav-Randolph's model (Einav and Randolph, 2005). It should be noted that a load-control, instead of displacement control, at the padeye was adopted to obtain the apparent anchor trajectory. In addition, the simulation was fully integrated, taking into account the disturbed soil conditions during the anchor installation as described in Section 3.3.1. Typical results presented in Fig. 14 show that the anchor pullout capacity increases as the anchor dives deeper. The diving tendency is strongly dependent on the padeye offset ratio (see Fig. 15a) and the loading angle (Tian et al., 2015; Kim and Hossain, 2017). After a parametric study, the relationship between the stabilised anchor travelling angle and the padeye offset ratio is suggested, as shown in Fig. 15b. It shows that the anchor behaviour will change from diving to pulling out, if the loading angle increases or the padeye offset ratio decreases. Kim and Hossain (2017) recommended that the anchor diving will be ensured if the padeye offset ratio is in the range of 0.25–0.53 and the loading angle is θ a < 45°.

Fig. 14

Fig. 14. Pullout behaviour of OMNI-Max anchor (data from Kim and Hossain, 2017): (a) Load-displacement curve; (b) Trajectory at padeye (loading angle of 15°).

Fig. 15

Fig. 15. Diving potential with padeye offset ratio (data from Kim and Hossain, 2017): (a) Definition of padeye offset ratio; (b) Anchor travelling angle with different padeye offset ratios and load inclinations.

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A review of shark satellite tagging studies

N. Hammerschlag , ... D.M. Lazarre , in Journal of Experimental Marine Biology and Ecology, 2011

3.8 So what have we really found out and where can we go from here?

With the increase of shark satellite tagging over the past decade, scientists have been able to describe previously unknown migration patterns, diving behavior as well as depth and temperature preferences for a variety of species. This technology has revealed some unexpected and impressive insights into shark biology; for example, white shark transoceanic migrations of over 10,000  km (Bonfil et al., 2005), whale shark deep dives exceeding 1   km (Rowat et al., 2007; Brunnschweiler et al., 2009) and hibernation in basking sharks (Skomal et al., 2009). However, most satellite tagging studies have only been able to describe the "what" rather than the "why" aspects of shark behavior and ecology. This is best exemplified in the most highly satellite tracked shark species on the planet: the white shark. For example, recent research has revealed that white sharks migrate from the central Californian coast and neighboring areas to Hawaii and back, spending the majority of the year in the middle of the Pacific Ocean, in a localized area between the two coasts, termed the "Shared Offshore Forage Area or SOFA" (Boustany et al., 2002; Dewar et al., 2004; Domeier and Nasby-Lucas, 2008; Weng et al., 2007a,b; Jorgensen et al., 2009). Despite cumulatively deploying an impressive 216 satellite tags (SPOT and PAT tags) on white sharks in the eastern Pacific region, it still remains unknown why exactly the sharks are spending the majority of the year in the SOFA as well as undertaking migrations of over 4000   km to Hawaii. Mating, feeding, and/or parturition have been suggested as possibilities, but empirical evidence is lacking.

In order to get at the important "why" questions, scientists will need to incorporate a variety of tools and develop new satellite tagging technologies. A variety of transmitters and sensors already exist which are useful for measuring subtle changes in shark behavior, such as swimming speed, sound, tail–beat frequency, muscle contraction and acceleration (Sundström and Gruber, 1998; Lowe et al., 1998; Lowe, 2001; Lowe and Goldman, 2001; Meyer et al., 2007; Papastamatiou et al., 2007; Whitney and Crow, 2007). Animal-borne video systems are now available, which can provide continuous video recordings from the shark's view-point (Marshall et al., 2007). In addition to providing location estimates, current satellite tags already have the sensors and capabilities of recording and transmitting measurements of water depth, temperature, and chlorophyll content (Teo et al., 2009). Future work should seek to couple tools, such as accelerometers and video systems, into a single satellite tag. If such a tag were developed, we could use a transmitter to track shark migration, swimming speed and tail beat frequency. If an irregular or abrupt change in swimming speed or tail–beat frequency occurred, a built-in mini camera could record a series of photos and videos. Subsequently these data could be archived and transmitted to orbiting satellites, making them accessible for researchers. With the right tools, time, funding and effort, manufacturers and scientists could work together to develop more advanced tags, affording researchers opportunities to answer some of the critical "why" questions that continue to go unanswered.

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Comparing methods suitable for monitoring marine mammals in low visibility conditions during seismic surveys

Ursula K. Verfuss , ... Len Thomas , in Marine Pollution Bulletin, 2018

4.2.1 Animal dependent factors

As noted above, any monitoring method is affected by cue availability. For methods relying on the animal being available at the water surface (e.g. thermal IR, RADAR and visual observers) the animal's behaviour, its size, surface behaviour, the strength of its exhalation as well as its diving behaviour and school size are factors that will directly affect detection probability. The bigger the animal's body or its exhalation, the larger the school size, the more energetic its surface behaviour, and the more frequently it surfaces, the more likely it is to be detected (e.g. Silber et al., 2009). An animal's colouration may influence visual detection probability, with contrasting colouring being more readily detected. Animals cannot be detected by thermal IR, RADAR or a visual observer while they are submerged. Visual observers, though, may detect animals close to the water surface if the water is clear. The longer an animal dives, the more likely it is that an animal will enter a mitigation zone without being detected by these methods.

For AAM, which detects animals when underwater, sonar target strength (the proportion of energy reflected by the target) is a key determinant of detection probability. This correlates well with body size and may also be influenced by school size. In addition, movement and diving behaviour will also affect detection probability. Animals displaying certain patterns of behaviours, i.e. movements and diving patterns that often take them across the field of detection of the AAM system are more likely to be detected than others, such as deep divers that are below the field of detection. Clutter, created by reflection of the sonar pulses from the water surface, makes animals close to the water surface harder to detect than those in mid water (Urick, 1984).

PAM depends on detecting sounds produced by the animals, and therefore can only detect individuals which are vocalising. Sound production is not obligatory for any species; and for some, acoustic availability bias can be both large and highly variable. The acoustic characteristic of their vocalisation is a key determinant of detectability. The frequency of vocalisations varies enormously between species, from the infrasonic calls of the large baleen whales (e.g. McDonald et al., 2006; Stafford et al., 1998), which can be as low as 10   Hz, to the ultrasonic clicks of dolphins and porpoise at 130   kHz (e.g. Au et al., 1999; Mohl and Andersen, 1973). Similarly, signal duration can vary from >   10   s (blue whale moans, Balaenoptera musculus: Linnaeus, 1758) to <   100   μs (harbour porpoise clicks, Phocoena phocoena: Linnaeus, 1758). Source levels also cover a huge range and include some of the highest values reported for sound producing animals (e.g. Mohl et al., 2003). Some vocalisations, particularly the echolocation clicks of odontocetes, are highly directional, being projected in a narrow forward facing beam (Au, 2012), while others are less directional (e.g. baleen whale calls, Sirovic et al., 2007). The effect of cue directionality on detection can be complicated. If the acoustic energy of a signal is emitted in a narrow beam, detection range is enhanced for a sensor that happens to be within the beam but diminished for sensors outside it. The probability of detecting sufficient sounds for a mitigation decision may also be very dependent on animal movement, e.g. how often the animal orientates itself towards the receiver. Some species show seasonal or diurnal variability in vocal behaviours, which may also vary with the animal's gender and age (e.g. Dunlop, 2016). Most aspects of vocal behaviour vary between activities such as foraging, travelling or social behaviour, and may be affected by group size, diving behaviour, and the presence or absence of predators or prey. A further complication is that vocalisation rates may be influenced by the presence of the seismic survey vessels themselves and by the sounds of air gun arrays. For example, Blackwell et al. (2015) showed that bowhead whales (Balaena mysticetus: Linnaeus, 1758) change their vocalisation rates in response to seismic survey activities. All of these extrinsic factors have a direct influence on the detection performance of a PAM system. Knowledge on the vocalisation characteristics of the target species is needed to understand the likelihood of detecting those animals and hence the effectiveness of these systems for mitigation purposes.

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Black guillemot ecology in relation to tidal stream energy generation: An evaluation of current knowledge and information gaps

Daniel T. Johnston , ... Elizabeth A. Masden , in Marine Environmental Research, 2018

3.1 Diving behaviour: current knowledge

A wide range of diving depths have been reported for black guillemots, although they have not been recorded to exceed 50m (Ewins, 1986; Masden et al., 2013; Nol and Gaskin, 1987; Piatt and Nettleship, 1985; Shoji et al., 2015) (Table 2).

Table 2. Summary of breeding season dive depth studies for black guillemots.

Reference Method n Seasons sampled Mean (m) SD Median (m) Maximum (m) Location
Ewins, 1986 Visual observation 73 1 30–50 – – 50 Shetland, Scotland
Piatt and Nettleship, 1985 Fisheries by-catch 36 2 13.6 – – 50 Newfoundland, Canada
Nol and Gaskin, 1987 Visual observation 799 2 26.4 14.9 – 50 Bay of Fundy, Canada
Cairns, 1992 Visual observation 130 1 21.1 10.5 22.5 48 Northeast Hudson Bay, Canada
Masden et al., 2013 TDR tag 2 1 – – 32 43 Stroma, Caithness, Scotland
Shoji et al., 2015 TDR tag 4 1 9.3 2.8 – 18.7 Bangor Harbour and Lighthouse Island, Copeland, Northern Ireland

When foraging in the water column black guillemots have been shown to undertake W-shaped, V-shaped and U-shaped dives (Shoji et al., 2015). V-shaped and W-shaped dives exploit the midwater, while U-shaped dives are indicative of foraging lower in the water column, suggesting that black guillemot diving behaviour is often related to the seafloor (Shoji et al., 2015).

Tidal-conveyor foraging entails an individual drifting downstream with the force of a tidal current before flying upstream, landing and then diving (Robbins, 2017). Fraenkel (2006) has suggested that in relation to tidal turbines, diving birds will likely be swept between blades within the current flow (through 'entrainment'), or, possess the manoeuvrability to avoid contact with blades. Langton et al. (2011) countered this assumption, indicating that for diving birds to manoeuvre around turbines they must first be able to see, recognise and predict blade movements. Wade (2015) suggests that seabirds diving within tidal streams actively face into the direction of the current; this was concluded from surface observations of locations where individuals submerged and resurfaced, and assumptions that fusiform body shapes are more suited to swimming against the current, minimizing drag and energy expenditure (Lovvorn et al., 2001). The fact that diving birds may actively swim into the direction of the current may increase the likelihood of collision, as turbines approached from upstream may be outside their field of vision (Wade, 2015).

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