Shark Attacks

Myths, Misunderstandings and Human Fear

Blake Chapman

• Publisher: CSIRO PUBLISHING
• Year: 2017
• ISBN:
  9781486307371
• Topic: Biowissenschaften
• Subtopic: Zoologie

Humans spend more time in or on the water than ever before. We love the beach. But for many people, getting in the water provokes a moment's hesitation. Shark attacks are big news events and although the risk of shark attack on humans is incredibly low, the fact remains that human lives are lost to sharks every year.

Shark Attacks explores the tension between risk to humans and the need to conserve sharks and protect the important ecological roles they play in our marine environments. Marine biologist Blake Chapman presents scientific information about shark biology, movement patterns and feeding behaviour. She discusses the role of fear in the way we think about sharks and the influence of the media on public perceptions. Moving first-hand accounts describe the deep and polarising psychological impacts of shark attacks from a range of perspectives.

This book is an education in thinking through these emotive events and will help readers to navigate the controversial issues around mitigating shark attacks while conserving the sharks themselves.

1

Introduction to sharks

The word ‘shark’ has the potential to invoke fear in a way that few other things can. Many people think that sharks are large-bodied, man-eating predators that patrol the beaches waiting for their next meal. However, in addition to being blatantly incorrect, this definition grossly oversimplifies the diversity and complexity of animals that we identify as sharks. In fact, the most recent, comprehensive assessment tallied the currently described number of sharks at 509 species, covering nine orders, 34 families and 105 genera (Weigmann 2016). Sharks may be small, very large or anywhere in between, they range from planktivores to apex predators and they occupy a suite of environments including freshwater rivers, shallow tropical coastal waters, temperate deep water and arctic oceans.

Sharks and their relatives (rays, skates and chimaeras) comprise the Class Chondrichthyes, or cartilaginous fish, as their skeletons are made of cartilage not bone. Members of this Class have occupied the planet for longer than most other groups of animals in existence today. They are the second-oldest extant class of vertebrates, pre-dated only by the jawless fish (living examples of these are hagfish and lampreys), and the oldest extant clade of jawed vertebrates. Fossil records date the earliest Chondrichthyans back 400–450 million years. They quickly radiated out to populate the upper tiers of aquatic food webs and, based on evolutionary survivability, Chondrichthyans could be considered the most successful of all fish (Grogan and Lund 2004; Dulvy et al. 2014). Relatives of modern sharks were occupying the oceans for hundreds of million years before pterosaurs patrolled the skies and Tyrannosaurus rex scavenged the plains. And sharks survived when those species did not. Sharks most certainly pre-date all species of mammals, with an approximate 385 million-year jump on the evolution of higher primates (give or take 25 million years). While the evolutionarily oldest Chondrichthyan species are no longer in existence, their close relatives are. The extinct megatooth shark (Carcharocles/Carcharodon megalodon) is thought to be the largest shark (and predatory fish) ever recorded, with a body length of up to 18 m, and it potentially had a substantial impact on the evolutionary history of marine mammals (Lindberg and Pyenson 2006; Ferretti et al. 2010; Pimiento and Balk 2015). Today, Chondrichthyans represent one of the most species-rich lineages of predators on Earth and they continue to play important functional roles in coastal and oceanic ecosystem structure and function.

Love them or hate them, sharks are undeniably fascinating. Sharks include some of the latest to mature and slowest to reproduce vertebrates, have the longest gestation periods and are among the most dedicated in terms of gestational maternal investment (Cortés 2000). Whale sharks (Rhincodon typus) are the largest fish in the sea, capable of attaining a total length of more than 12 m and a weight of over 21 tonnes. The Greenland shark (Somniosus microcephalus), which can attain lengths of up to 5 m, is the longest-lived vertebrate known, with recent studies estimating a lifespan of about 390 years, give or take 120 years (Nielsen et al. 2016). The breadth and intricacies of shark reproduction are fascinating; some species give birth to live young, whereas others lay eggs. And of course there are the teeth. Sharks serially produce and shed their infamous teeth, with some species cycling through as many as 6000 teeth per year and many tens of thousands over their lifetime. Ultimately, the sheer diversity among shark species – their habitat range, variation in size and shape, biology and physiology, predation and social behaviours – makes this group of animals incredibly complex and captivating.

Our evolving interest in sharks

Although sharks represent one of the oldest vertebrate lineages in existence, we still have a lot to learn about them. Sharks were revered as gods (both good and devil-gods) in various ancient cultures around the world. However, the incredible interest that many people have in sharks today is a relatively recent development. Sharks didn’t stand in the way of cultural expansion, exploration or industrialisation. Relative to terrestrial predators, they were not commonly encountered and (with some minor exceptions) did not comprise a commercially valuable food or consumable resource, as did whales, marine turtles, sea otters and certain bony fish species (Martin 2016). Although shark attacks occurred, and were of occasional regional interest in a few places during the first half of the 20th century, these were generally very isolated events and were rarely afforded any thought beyond the admission that an accident had taken place. The first major turning point in modern concern about sharks was the catastrophic sinking of the US naval warship, the USS Indianapolis, by a Japanese submarine in 1945. The ship had just delivered the crucial components of the first atomic bomb that would level Hiroshima days later. On 30 July, it was hit by two torpedoes and sank in 12 minutes. The 900 men who survived the sinking were left floating in the Pacific Ocean for four days, subject to exposure, thirst and sharks. The main species of sharks implicated was the oceanic whitetip (Carcharhinus longimanus). It is written that sharks consumed dozens to hundreds of bodies, but we don’t know what proportion of those people were already dead before their consumption and how many were actually killed by the sharks. Either way, the fear felt by the sailors would have been incontrovertible. However, much of the detail surrounding this occurrence was classified at the time, and thus not as wide-reaching as it could have been. The resulting interest in sharks from the US Navy, however, prompted a scientific symposium on sharks in 1958, where 34 of the world’s pre-eminent shark scientists came together to address the ‘shark hazard problem’ (Martin 2016). The report from that meeting stressed the overwhelming lack of knowledge in the field and the basic taxonomic and technical problems, such as the lack of standardised common names that would be needed for collaborative data collection (Aronson and Gilbert 1958).

The 1970s marked the start of significant and rapid change in shark interest and accompanying changes in human perceptions of sharks. This has taken us from ignorance, to fear and hatred and now to celebration and concern (although still with remnants of misunderstanding, fear and hatred). In a few short decades, sharks have gone from an ‘out of sight, out of mind’ inconsequential ocean resident to predatory monsters to an iconic symbol of troubled aquatic ecosystems and one of the most complex challenges to conservation, environmental management and regional human recreational safety.

The 1975 release of the movie Jaws was probably the single most influential event in shark–human history. The film managed to play on our innate fear of the unknown and uncontrollable, and simplified an entire, evolutionarily strong, biologically diverse and overwhelmingly complex group of animals into a single terrifying body part. It led to the widespread and non-specific destruction of huge numbers of sharks, whether they were potentially dangerous or not. Simultaneously, though, the movie also stimulated interest, curiosity and a whole new realm of research, as it brought sharks into the minds and living-rooms of the ‘Jaws Generation’ in a big way.

Our more recent interest in sharks has shifted towards gaining a better understanding of these animals overall, their role in aquatic ecosystems and the conservation of threatened populations. The ecological, biological and physiological diversity of sharks leaves them susceptible to threats that are similarly wide-ranging and extinction risks to sharks are substantially higher than for most other vertebrate groups. Only one-third of shark species are considered to be safe from human exploitation and action (Dulvy et al. 2014). While the full degree to which sharks have been affected by human activity is not clear, and is often highly contentious among researchers, the most pressing current threats include the fin trade, habitat destruction and pollution, overfishing and climate change. Overfishing and habitat destruction have contributed to some of the most profound impacts, with 96.1% of sharks threatened by fishing. This includes direct commercial efforts (31.7%), bycatch (57.9%), recreational (0.7%) and artisanal fishing (5.8%). While not as significant, a further 2.9% are considered to be affected by habitat destruction and 0.4% by pollution (http://www.redlist.org).

Population trends

Studies from the 1970s to 2000s showed a generally grim outlook on shark population trends. During this time, fisheries logbooks and long-term shark mitigation beach mesh netting programs were most commonly used to study catch rates, which were then used to assess population trends. Results from netting programs in South Africa and Australia showed declines in bull (Carcharhinus leucas), blacktip (Carcharhinus limbatus), scalloped hammerhead (Sphyrna lewini), grey nurse (Carcharhias taurus) and white shark (Carcharodon carcharias) populations of 27% to more than 99% (Holden 1977; Paterson 1990). Early data from pelagic fisheries suggested that one shark was caught as bycatch for every two tuna, and two to three sharks for every swordfish (Brodie and Beck 1983; Baum and Myers 2004). Reductions in catch rates of 49% to more than 99% were recorded over less than 15 years for 20 species of coastal and pelagic sharks in the north-west Atlantic. Fourteen species of sharks disappeared completely from Brazilian fisheries between 1977 and 1994 (Amorim et al. 1998; Baum et al. 2003).

While population assessments based on net and fisheries catch data are subject to various biases and cannot be used to definitively determine population sizes, such significant figures clearly suggest a declining trend in many species. However, an interesting concomitant increase of shortfin mako (Isurus oxyrinchus) and blue sharks (Prionace glauca) was observed in Brazilian waters (Amorim et al. 1998), as well as blue shark biomass in the north Pacific, which was estimated to have increased by 20% relative to the 1970 numbers (Sibert et al. 2006). As large coastal species declined, catch rates of more wide-ranging species, such as tiger (Galeocerdo cuvier) and hammerhead sharks (Sphyrna spp.) increased, at least temporarily, as reported from the shark nets in New South Wales and Queensland (Australia) and South Africa (Paterson 1990; Reid and Krogh 1992; Dudley and Simpfendorfer 2006).

The longest dedicated population survey of sharks in the US has been running off the North Carolina coast since 1972. This survey has shown declines of up to 99% in a range of species, including bull, dusky (Carcharhinus obscurus) and tiger sharks (Myers et al. 2007). It also found that the average length of sharks decreased by 17–47%, functionally removing a large percentage of mature individuals from the populations.

Not only have indirect fishing pressures (through bycatch) increased, but directed shark fishing efforts have also increased, as more typically targeted bony fish species have become less accessible due to reduced stock or increased management restrictions. The value of shark products, including fins, liver, meat and gills, has also increased, due to greater demand, tougher legislation and decreased stock (Clarke et al. 2006; Lack and Sant 2009). Sharks are caught in at least 126 countries around the world, although the top 20 countries are responsible for 80% of the global annual catch (Lack and Sant 2011; Davidson et al. 2016). In total, the regulated legal global annual trade of shark products was estimated to be around US$1 billion in 2010 (FAO FAD 2010). Instead of reducing directed shark catch rates, initiatives to slow the shark fin trade, such as requiring the entire animal to be landed (i.e. brought on board the ship), encouraged greater utilisation of the whole animal, turning sharks into an even greater commercial resource. Estimates of global reported and unreported shark catches, including landings, discards and finning, were 1.44 million tonnes in 2000 and 1.41 million tonnes in 2010 (Worm et al. 2013). This equated to mortality estimates of 97–100 million sharks per year. Just to reiterate, that figure is 97–100 million sharks killed. Per year. Those numbers astound me.

The increase in targeted commercial shark fishing and the wider and more profitable use of shark products has resulted in the targeting of previously unexploited and untouched populations at greater distances to consumer markets (Dent and Clarke 2015). Many current levels of shark catches are unsustainable, and will need to change if we expect to keep these animals in existence. Fishing exploitation rates currently sit at 6.4–7.9% of shark populations annually, while the average rebound rate for sharks (as a whole, derived from assessment of the life history characteristics of 62 species) is 4.9% per year (Worm et al. 2013). Thus, most species affected by targeted and non-targeted fisheries should be expected to show continual declines. The most recent assessment by the International Union for Conservation of Nature (IUCN) suggests that of the 465 shark species investigated, 74 (15.9%) were considered threatened, including 11 (2.4%) categorised as Critically Endangered, 15 (3.2%) as Endangered and 48 (10.3%) as Vulnerable. A further 67 (14.4%) were assessed as Near Threatened and 209 (44.9%) as Data Deficient. Only 115 (24.7%) were considered of Least Concern (Dulvy et al. 2014).

Counting sharks

Despite their seemingly ever-present status along highly populated beaches, the reality is that sharks are extremely difficult to find, let alone count. The two most common methods used for population assessments are tagging and tracking and photo identification (typically based on fin contours or spot patterns, which are both as unique as our fingerprints), in conjunction with statistical modelling. However, one of the largest challenges facing population studies is the fact that it is simply beyond our current understanding to know exactly what percentage of a population may be present in a certain location (and thus subject to sampling) at a particular time. Sharks can be highly mobile and widely dispersed, unpredictable in their movements and residency patterns, and cryptic. Even population studies conducted over multiple years cannot guarantee that entire populations have been sampled. As a result, population assessments can be unreliable and are often highly contended among researchers. While some studies have found increasing trends in certain shark populations (e.g. tiger sharks off KwaZulu-Natal, South Africa, and white sharks off south-east Australia: Dudley and Simpfendorfer 2006; Reid et al. 2011), most studies indicate that shark populations are decreasing or stagnant. Recent studies have shown a decreasing trend in maximum body size of white, tiger and bull sharks in the last century (Kock and Johnson 2006; Reid et al. 2011; Powers et al. 2013). As these predators (most notably, white sharks) show age-related changes in prey preference, a shift towards smaller individuals could lead to different behavioural patterns of shark populations in a particular region. For example, more individuals may frequent in-shore waters to chase smaller prey, as opposed to focusing on larger marine mammals off-shore, thus increasing the overlap in in-shore water usage between sharks and humans, and the potential for negative interaction.

Why sharks are important

When faced with the conundrum of human–wildlife conflict, especially the dilemma of protecting human life at the expense of another animal, there is always the question of why should we even bother trying to preserve the life of the other animal. The valuation of one animal’s life over that of another is clearly a subjective matter, and everyone will have a different opinion based on their own cultural and personal beliefs. However, leaving broad conservational and ethical reasoning aside, there are several reasons why we really should reconsider killing sharks.

The role of sharks in successful ecosystems

Sharks have an important role in the ecosystems they inhabit. However, the exact role they play varies by species, region, age/size, behaviour and/or sex. The common thought that sharks, as a whole, are apex predators is incorrect. In fact, very few are. Apex predators occupy the highest trophic level and have substantial top-down effects on prey demography and the structure and productivity of their ecosystem through direct (consumptive) regulation of prey dynamics and indirect (non-consumptive) modification of prey behaviour. Direct predation results in the prevention of drastically increased intermediate predator numbers (mesopredatory release). Indirectly, the presence of predators in an ecosystem enhances biodiversity through modification of prey species’ foraging behaviours, spatial use and interactions. Some of the larger shark species (sometimes referred to as the ‘great sharks’) would certainly be classified as apex, or top-order, predators but, when considering diet and tropic ecology, most sharks are more accurately mesopredators. Overall, the breadth and diversity of the functions that sharks play in their respective ecosystems, and at varying levels of the food chain, would be enormous. The removal of apex predators from an environment would have the most obvious and significant effects on the ecosystem because ecologically redundant species would not be as available to mitigate and lessen the impact. Resulting tropic cascades would be likely, with effects propagated throughout the ecosystem. Although not as recognisable, the removal of mesopredators would still be detrimental to an ecosystem, as these species also play a variety of important roles in the environment, including maintaining the structure, function and reliability of the ecosystem.

Indirect effects of predator presence are broad-reaching, and affect everything from flora and fauna composition, productivity, nutrient cycling and (transient...