Overview by: Dr. James Lea
Our oceans. Vast, deep, almost unfathomable. They sustain us. They regulate our climate, provide food, and have an enigmatic beauty that enchants people the world over.
Marine ecosystems exist and provide these functions through a delicate balance that has arisen through hundreds of millions of years of evolution.
One constant through over 400 million years of this history is sharks. Sharks play a crucial role as predators in nearly all marine environments. Not only do predators regulate prey populations through direct consumption, but there are strong indirect effects of prey changing their movement patterns to avoid risky confrontations with predators like sharks. Sharks can have a particularly strong influence as their piercing, sawing teeth and gaping jaws often allows them to consume much larger prey than other fish can – dolphins, dugongs, turtles and other sharks all feature on their menu.
But this balance is in jeopardy. Despite their historical presence as wardens of the deep, sharks are in danger. There seems to be an ever-growing demand for shark products, including their fins, meant and oil. So much so that as many as 273 million sharks may be killed every year. To put that in perspective, that’s only a few million shy of the population of the USA, and over a third of the population of Europe.
It’s an absolutely staggering figure, and comes as no surprise that most shark fisheries are considered highly unsustainable. We’re losing sharks, and fast – many populations have declined by over 90%. Losing predators like sharks means our oceans, and all the valuable services they provide, are also threatened, as the delicate ecosystem balance they help maintain collapses. A future without sharks is a dark one, and we can’t be sure just how severe the impacts will be.
But we can, and will, make a difference. It’s our responsibility to take stewardship of our natural world and ensure all its resources can be sustained to benefit future generations. There is increasing recognition globally of the need to conserve shark populations, with many countries imposing strict quotes or complete fishing bans, while others divert to the rapidly growing, far more sustainable tourism sector. But it’s still an uphill struggle, with poor public perception of sharks and rife poaching of sharks in the high seas – Save Our Sharks has been established to help combat these misconceptions and promote shark conservation.
Why are sharks important?
Ecosystem stability is important as human welfare is dependent on the services ecosystems render, the majority of which are provided by marine ecosystems (Costanza et al., 1997). In both terrestrial and marine ecosystems predators can exert strong top-down forces that shape communities over large spatio-temporal scales and promote long term stability (Estes et al., 1998; Heithaus et al., 2008; Beschta & Ripple, 2009; Ferretti et al., 2010). Sharks occupy high trophic levels in most coastal, demersal and pelagic food webs (Cortés, 1999; Compagno, 2001), and are typically well connected as many species display cosmopolitan diets and wide ranging movements (Cortés, 1999; Bascompte et al., 2005; Sims, 2010). Certain shark species may impose greater influence than other marine predators of equivalent size as the extendable gape and sawing action of many species’ jaws allows consumption of comparatively larger prey (Wilga et al., 2007). Consequently many megafauna species (e.g. dolphins, turtles, pinnipeds) have sharks as their primary, or only, predators (Wilga et al., 2007; Heithaus et al., 2009).
Predators not only influence prey demographics via direct consumption, but can also elicit strong avoidance behaviours in prey through imposition of predation risk (Ripple & Beschta, 2007; Heithaus et al., 2009). Increasingly prey species have been shown to modify habitat use according to relative predation risk, which can in turn alter their trophic interactions (Ripple et al., 2001; Heithaus et al., 2009; Ferretti et al., 2010; Guttridge et al., 2012). For instance, seasonal presence of tiger sharks Galeocerdo cuvier in Shark Bay, Western Australia, causes several prey species (e.g. turtles, dugongs) to forgo foraging opportunities to enhance safety, even if only consumed infrequently (Heithaus et al., 2009). Subsequent alteration in prey grazing patterns can then cascade to affect sea grass species composition and nutrient structure (Heithaus et al., 2008). Complicating such interactions, prey fitness can influence the degree of avoidance behaviour; green turtles Chelonia mydas of poor body condition will favour more productive grazing areas despite the associated higher predation risk (Heithaus et al., 2008).
Due to the complex and context dependent nature of these trophic interactions it can be difficult to predict the degree of predator influence within a particular ecosystem (Ferretti et al., 2010). Whilst the use of models, such as Ecopath and derivatives thereof, can help gauge the magnitude of cascading predator influence, the required parameters are often unavailable due to data deficiency (Stevens et al., 2000; Okey et al., 2004). In actuality it has been through the removal of predators from various ecosystems that the full extent of their influence has been realised. For instance poaching of wolves Canis lupus from Yellowstone National Park, USA, facilitated elk Cervus elaphus proliferation and dramatically reduced vegetation and habitat in riparian areas that elk previously avoided due to risk (Ripple et al., 2001). Subsequent reintroduction of wolves has seen riparian vegetation re-established as elk resume risk avoidance behaviours (Ripple & Beschta, 2007).
Due to the concealing nature of the marine environment and lack of historical data on commercially unimportant species there are very few well documented marine trophic cascades (Estes et al., 2011), despite widespread reports of predator decline (Baum & Blanchard, 2010; Tremblay-Boyer et al., 2011). An example from the Aleution Islands shows that the switching of orcas Orcinus orca to prey on sea otters Enhydra lutris instead of pinnipeds released sea urchins from predation effects and increased grazing pressure on kelp, causing loss of productive kelp forest habitat (Estes et al., 1998). In some pelagic systems it is thought that other predators with higher turnover rates (e.g. tuna and billfish) may be able to substitute sharks with minimal influence on trophic dynamics (Kitchell et al., 2002). But declines in pelagic predators are rarely limited to sharks, which are typically caught as bycatch in other fisheries (Dulvy et al., 2008). For example 10-fold declines in Pacific Ocean longline catches have been reported for 12 pelagic predators (including tuna, billfish and sharks) from 1950-2000, coinciding with 10-100-fold increases in various mesoconsumers (Ward & Myers, 2005).
Consequently predators, in particular sharks, are important for maintaining ecosystem stability and biodiversity, supporting the services rendered by these ecosystems, including commercial fisheries.
In addition to promoting ecosystem stability, sharks support global economies through both fisheries and tourism. Sharks are fished for a variety of products, such as squalene for vaccines and cosmetics, but predominantly for their fins, which are primarily sought after as a delicacy in the Far East for shark fin soup (Clarke et al., 2007; Lippi et al., 2011). Accelerated development of Asian economies, corresponding availability of disposable income and rapid population growth have seen demand for shark fin soup rise significantly over recent decades (Clarke et al., 2007). Consequently there is substantial fishing effort to meet demand, and the value of the trade in shark fins has been estimated at a minimum of USD 400-550 million/year (mpy) (Clarke et al., 2007). But in direct conflict with the consumptive fin trade, sharks are increasingly valuable to tourism industries in many countries (Gallagher & Hammerschlag, 2011). The expanding market for shark watching operations, as tourist values shift from ‘adventure-seeking hunters’ to ‘nature-appreciating observers’ (Whatmough et al., 2011), has prompted studies to evaluate the comparative revenue generated. Most recently reef sharks in Palau have been estimated to generate USD 18 mpy as 21% of tourists visit specifically to encounter sharks, accounting for 8% of the gross domestic product (Vianna et al., 2012). Moreover this revenue is based on an estimated resident population of 100 sharks at the popular dive sites, worth at most USD 10,800 if harvested for their fins, constituting a mere 0.006% of the revenue these sharks would generate through tourism over their lifespan (Vianna et al., 2012). Such discrepancy can provide strong conservation incentive and has been recognised through a ban on all shark fishing in Palau (Vianna et al., 2012). Similarly shark fishing was banned in the Maldives after the realisation that shark tourism generated more than double the revenue of shark fisheries (Anderson & Ahmed, 1993; Martin & Hakeem, 2006; MRC, 2009).
The considerable value of sharks through tourism has been reported for several other locations, including: the Bahamas (USD 78 mpy; Cline, 2008), Canary Islands (USD 22.8 mpy; De la Cruz Modino et al., 2010), Ningaloo Marine Reserve Australia (USD 5.9 mpy; Davis et al., 1997), French Polynesia (USD 5.4 mpy; Clua et al., 2011), Seychelles (USD 4.88 mpy; Rowat & Engelhardt, 2007), Gansbaai South Africa (USD 4.4 mpy; Hara et al., 2003), and Aliwal Shoal South Africa (USD 1.8 mpy; Dicken & Hosking, 2009). Despite differing criteria for revenue estimation (e.g. whether or not indirect revenue was considered from hotels, restaurants etc. through tourists visiting specifically to encounter sharks), these studies consistently illustrate the substantial, in principal renewable, value of shark tourism. Yet they represent a small fraction of the numerous shark tourism operations worldwide, most of which remain unevaluated for revenue contribution (Gallagher & Hammerschlag, 2011). If potential can in part be gauged by the estimated global revenue from whale watching (USD 2.1 billion/year; O’Connor et al., 2009), it remains conceivable that the global value of sharks through tourism could not only exceed their value to fisheries, but provide an alternative, non-extractive source of exploitation and employment that simultaneously maintains their ecosystem role as predators.
However, exploitation through tourism must be developed with caution to ensure its impacts are sustainable and that the ecosystem functions of sharks are indeed maintained. Many shark tourism operations involve SCUBA diving on coral reefs (Gallagher & Hammerschlag, 2011), potentially decreasing reef health through diver damage (Guzner et al., 2010; Poonian et al., 2010), although this can in part be mitigated through more detailed dive briefings (Medio et al., 1997). Another consideration is the degree of provisioning used; over 40% of operations use food to attract sharks for more reliable encounters (Carwardine & Watterson, 2002). Yet there remain concerns that provisioning with a regular food source may alter predator behaviour, condition and community interaction, whilst compromising human safety (Newsome & Rodgers, 2008; Clua et al., 2010). Despite such concerns there are limited empirical studies, which provide varied conclusions: whilst some purport behavioural changes that may impede fitness (e.g. elevated aggression in sicklefin lemon sharks Negaprion acutidens, Clua et al., 2010; increased activity and energetic costs in whitetip reef sharks Triaenodon obesus; Fitzpatrick et al., 2011), others argue impacts may be negligible and outweighed by the economic and protection benefits afforded through tourism over fishing (Laroche et al., 2007; Maljkovic & Côté, 2010; Hammerschlag et al., 2012).
For instance, white sharks Carcharodon carcharias subject to limited food rewards in South Africa responded less to tourism boats over time (Laroche et al., 2007), and in the Bahamas both Caribbean reef sharks Carcharhinus perezi that did and didn’t receive food rewards showed similar residency, with no evidence for a shift in behaviour (Maljkovic & Côté, 2010). Similarly long-term, large scale movements of tiger sharks in the northwest Atlantic and seasonal patterns of bull shark C. leucas abundance in Fiji have since been shown to persist despite regular provisioning (Brunnschweiler and Baensch, 2011; Hammerschlag et al., 2012). Meyer et al., (2009) also reported that several shark species attending provisioning sites in Hawaii maintained seasonal patterns in abundance, and concluded the operations posed minimal safety risk. One potential source of disparity between studies is the differing level of food rewards provided to sharks in attendance (e.g. whether or not the sharks are actively fed or simply chummed), which may affect the strength of associative learning and potential for behavioural changes (Guttridge et al., 2009).
There has also been little evaluation of how provisioning may effect community interaction and structure. Although authors that have considered this issue include Maljkovic & Côté (2010), who reported no evidence of changes in ecological impact, and Hammerschlag et al., (2012) speculate that provisioning influence on tiger shark trophic interactions in the Bahamas may be minimal, assuming their daily ration to be relatively high compared to the quantity and frequency of food obtained during provisioning. In contrast Brunnschweiler and Baensch (2011) postulate that the increasing number of bull sharks visiting a provisioning site in Fiji could alter trophic interactions in the region by redistributing predator influence. Another concern that seems to have gone broadly unconsidered in present literature and requires further investigation is the sustainability of fisheries that supply the bait for provisioning: extractive use of one species to support the non-extractive use of another could provide unforeseen complications, the sustainability of which must be assessed carefully before shark tourism is actively incorporated into ecosystem management programmes.
Together the lack of studies and clear understanding of provisioning impacts emphasise the particular need for comprehensive studies that address the potential issues with provisioning directly. Nonetheless shark tourism operations should still be considered as alternative, sustainable management options, but must be evaluated carefully in context prior to implementation, since the impacts of provisioning may well vary depending on the species, location, season and level of food reward provided.
How are sharks threatened?
Despite their importance, many shark species are experiencing severe population declines that threaten their roles (Dulvy et al., 2008; Ferretti et al., 2010). This is predominantly due overfishing, which has been estimated to account for 96.1% of threats to shark populations, followed by habitat destruction (2.9%) and pollution (0.4%; Ferretti et al., 2010). Due to the aforementioned unsustainable demand for their fins, it has been estimated that 63-273 million sharks are caught annually for the fin trade alone (Worm et al., 2013). In general sharks used to be commercially unimportant, typically caught as bycatch and reported with little accuracy: it has been estimated that only 15% of catches reported to the Food and Agriculture Organization of the United Nations are to species level (Lack & Sant, 2006). Moreover 73% of global shark catch is thought to be illegal and unreported (Clarke et al., 2006). Although the true magnitude of declines remain uncertain due to this lack of baseline data and underreporting of catch, and considerable lack of fisheries-independent survey data (Lack & Sant, 2006; Ferretti et al., 2010), it has been estimated that some populations have been reduced to less than 10% of pre-exploitation levels (Dulvy et al., 2008; Ferretti et al., 2010), with 52% of pelagic shark species considered threatened with extinction (Dulvy et al., 2008). Typically, sharks are more susceptible to overfishing than most commercial teleost species due to life history traits that limit recruitment rates (late maturity, few young infrequently; Musick, 1999; Frisk et al., 2005). Consequently sharks can only withstand very limited fishing pressure and are prone to more rapid collapse versus most teleost populations (Frisk et al., 2005; Ferretti et al., 2010). Accordingly modelling data suggest reductions in mortality of 40-80% would be required in north Atlantic fisheries to ensure the survival of shark populations (Myers & Worm, 2005).
Many of the reported declines are from Atlantic fisheries, in part due to its longer commercial fishing history and more comprehensive datasets (Baum & Blanchard, 2010; Ferretti et al. 2010). For instance, silky sharks C. falciformis and oceanic whitetip sharks C. longimanus are estimated to have declined by over 90% and 99% respectively in Gulf of Mexico longline fisheries since the 1950s (Baum & Myers, 2004). Similarly in the Mediterranean the only shark species with sufficient data to be assessed were found to have declined between 96% and 99.9% (Ferretti et al., 2008). Ten-fold declines in 12 pelagic predators in the Pacific since 1950 have also been reported (Ward & Myers, 2005). Consistent with these figures it has been estimated that general predator biomass has declined by over 90% in half of north Atlantic and Pacific coastal areas compared with unexploited levels (Tremblay-Boyer et al., 2011). Whilst similar levels of data resolution to the Atlantic and Pacific are not available for the Indian Ocean, several studies from the region indicate shark populations are experiencing declines similar to elsewhere. Fisheries-independent visual surveys of reefs in the Chagos Archipelago, recently designated a Marine Protected Area (MPA), suggest that reef sharks may have declined by over 90% since 1975 (Graham et al. 2010; Sheppard et al., 2012). Reports prior to the ban of shark fishing in the Maldives indicate declines there may also be severe (Anderson & Ahmed, 1993; Martin & Hakeem, 2006), with notable declines in large sharks also suggested for the Seychelles (Nevill et al., 2007). Although declines in predator biomass in the Indian Ocean may presently be lower than the Atlantic, modelling data suggest they are on a similar trajectory and may simply be lagged due to later industrialisation of commercial fisheries (Tremblay-Boyer et al., 2011). The decline of sharks even in remote locations such as Chagos is of particular concern as it highlights the increasing expansion of shark fisheries into the high seas, leaving few, if any, remote sanctuaries.
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