Wednesday, 24 October 2012

For the Greater Good: A Call To Remove Unnecessary Shark Control Methods

By Ryan Pearson 
In a world where man has negatively impacted so many facets of the natural environment just to feed the planets vast human population, I pose the question “How is it justifiable to decimate entire populations of marine species to protect our leisurely pursuits?” Looking into the future, the rapidly expanding human population is likely to have more and more seemingly unavoidable impacts on natural systems through things like climate change, over-fishing, eutrophication, and the input of anthropogenic pollutants (Ricel and Garcia, 2011). It is clear that many necessary human endeavours will have some significant negative impacts, with potentially dire repercussions for the state of the entire planet. Research indicates that by 2050, a 50% increase in fishery production from 2011 levels will be required just to feed the human population (Ricel and Garcia, 2011).  This is a figure that many agree the oceans simply can’t hope to support. Despite a general awareness of these impacts, humans often campaign for and implement further destructive mechanisms that are not necessary for the survival of our species, but may have dire consequences for others. Shark control programs are one such example.

In order to protect surfers, swimmers and other recreational beach users from the risk of shark attack, many nations and states have implemented indiscriminate control mechanisms such as shark nets and baited drum lines (Dudley, 1997). These devices usually have a much wider ecological impact than their simple designation to exclude man-eating sharks. 

While on face value, the raw statistics suggest these control methods may be effective in preventing shark attacks, the truth of their impact may be less black and white. Regardless, the amount and type of non-target bycatch caught in shark nets has the potential to be devastating from both an ecological, and an economic standpoint. 

As for shark attacks themselves, the number of fatal attacks has increased over the past 20 years, and yet in Australia still averages only 1.1 fatalities annually (West, 2011). So, when worldwide shark attack occurrences number in their tens annually, by what rights do we have the ability to indiscriminately destroy thousands of marine animals just to offer humans a chance to feel safer while swimming? This review aims to investigate the impacts and perceived benefits of shark control programs, and determine if these benefits outweigh their ecological impacts. So, let’s take a closer look…

Shark Control Methods

The goal of shark control methods is to protect people using coastal areas from the risk of shark attack by decreasing the number of large, potentially dangerous sharks in the area. Along with this comes the peace of mind of tourists, often with the pretence that shark nets completely exclude sharks from their chosen swimming beach. As you will see shortly, this is not the case, and in fact, some of these shark control methods actually attract sharks to the area through direct baiting, or the attractant effect of dead animals caught in shark nets. The main methods that are used commonly are shark nets (or meshing), and baited drum lines. I will also explore potential new methods utilising magnetic fields and electric currents to deter sharks.

Shark Nets
The first and most infamous example of shark control methods are shark nets. These nets are hanging meshing (usually 150- 200m long) suspended from a series of surface buoys which are placed at popular beaches approximately 200m from shore (Figure 1). They generally extend from the surface down to about 3-6m depth (Figure 1), with the seafloor depth at low tide ranging between 6-12m (Taylor et al., 2011). The key feature of these nets is the large (often 500-600mm) mesh size, which is designed to allow small organisms to pass right through while entangling larger individuals (Reid and Krogh, 1992). As you can see, shark nets do not enclose beaches completely excluding sharks from swimming areas. Shark meshing simply forms a partial physical barrier and as a result a large proportion of animals caught (~35%) in shark meshing programs are found on the inner (shoreward) side of the net (Dudley, 1997).
Figure 1. General shark net design (ABC, 2012).

Let’s have a look at effectiveness of shark meshing in the goal of reducing the number of large, potentially dangerous sharks in the area. One study, by Dudley and Simpfendorfer (2006), looking at total shark net catch from 1978-2003 in South Africa suggests that the catch rate of the three shark species considered most dangerous  (tiger sharks (Carcharhinus leucas), bull sharks (Galeocerdo cuvier), and great white sharks (Carcharadon carcharias)) accounted for only 11.1% of the total shark catch (Table 1). On the other hand, the Grey Nurse Shark (Carcharias taurus) accounts for 16.7% on its own (Table 1). This is a shark species which is known to be harmless and is listed as vulnerable on the IUCN red list (IUCN, 2012).

Table 1. Mean annual catch of shark species in shark nets off the KwaZulu-Natal coast between 1978-2003 (Dudley and Simpfendorfer, 2006).
Annual Catch 1978-2003
Mean # of individuals
Carcharhinus ambionensis
Carcharhinus brachyurus
Carcharhinus brevipinna
Carcharhinus leucas*
Carcharhinus limbatus
Carcharhinus obscurus
Carcharhinus plumbeus
Carcharhinus taurus
Carcharodon carcharias*
Galeocerdo cuvier*
Isurus oxyrinchus
Sphyrna lewini
Sphyrna mokarran
Sphyrna zygaena
* 'dangerous' species

Baited Drum Lines
Baited drum lines are another key tool in shark control programs. The design of these structures includes a surface float which is anchored to the bottom, and has another (~2m) rope/chain extending from the float to a large baited hook (Figure 2). 

Figure 2. Baited drum line design for use in Kwazulu-Natal, South Africa (Dudley et al., 1998).

Capture statistics suggest that the structure of animal assemblages caught on baited hooks is considerably different to those caught in the shark nets (Dudley et al., 1998). The non-shark bycatch is generally greatly reduced with baited hooks, though the size class of the sharks caught is much less discriminating (Dudley et al., 1998). In one study by Dudley et al. (1998) in La Mercy, South Africa, the most caught animal on drum lines were juvenile dusky sharks (Carcharhinus obscurus) with a median pre-caudal  length of 75.4cm (Dudley et al., 1998). These sharks accounted for 87% of the catch (Dudley et al., 1998), would likely have been too small to become entangled in shark nets, and were clearly not big enough to be man-eaters. When looking at the more ‘dangerous’ species, the results remain hazy as to how effective drum lines are at achieving their goal compared to shark nets. Only the tiger sharks, which accounted for 4.6% of the catch, were larger than those caught in the nets, while the average size of the bull sharks (3.4% of catch) were considerably smaller (Dudley et al., 1998).

Other Control Methods
Other more novel Shark control methods have been also suggested and tested, including the use of magnetic (O'Connell et al., 2011) and electric shark barriers (Smith, 1991). Both of these methods are non-lethal options for shark control and have shown some promise in testing, with both triggering avoidance behaviour in elasmobranchs, but are yet to be widely utilised.

Effectiveness in reducing shark attacks

If we look purely at the shark attack numbers, the shark control programs in QLD, NSW, and South Africa all seem to be resounding successes. In NSW, shark meshing was introduced in Newcastle in 1949. This resulted in total attack rates dropping from approximately 0.35 per year without netting, to 0.04 per year following implementation, until 1997 (Dudley, 1997). In Sydney there is a similar story with total attacks dropping from 0.46 per year, to 0.03 following implementation. Looking purely at fatal attacks in QLD, in the 28 years between the 1962 implementation and the study by Paterson (1990),  there was only one reported shark fatality (0.036 p.a.), which tells a similar story to NSW when compared to the pre-implementation numbers. In the 44 years from 1918 to implementation, there were 26 fatal attacks (0.591 p.a.) in the same waters (Paterson, 1990).  

However, these QLD numbers are based purely on fatal attacks, which is arguably a biased parameter when considering our advances in medicine. Ultimately, due simply to medical and technological advancements, victims are more likely to survive now when compared to 1919. Therefore, I propose that fatal shark attacks may not be the best measure of effectiveness and total attacks offer a much better representation.  Furthermore, the actual numbers of attacks appear to be in contention and when looking at total attacks in QLD, the results are far less impressive. Dudley (1997) reports 27 fatal from 49 total attacks between 1919 and 1962 (1.14 p.a.), and 9 fatal from 39 total attacks in the 35 years following implementation (1.11 p.a.). Clearly these numbers suggest there has been minimal effect by the shark control program in QLD. 

Another factor to consider is the presence or absence of potential shark attractants to an area. For example, when looking at the QLD numbers, pre-implementation a local abattoir was discharging animal waste into the water at Townsville, a place where 9 fatal attacks occurred from 1919 to implementation in 1962 (Dudley, 1997). This potential food source and shark attractant has not existed since shark control programs began in 1962 (Dudley, 1997). Similarly, pre-implementation, whaling was still occurring on the Gold Coast, which may have had an effect on the feeding patterns of sharks (Dudley, 1997).

When considering the effectiveness of reducing shark attacks, it is also important to take population comparisons. Over the past century the human population has increased dramatically, while shark numbers have been subject to enormous declines in the last 30 years alone. These changing populations are sure to have some impact on the base number of encounters between people and sharks, which is really the only way an attack can occur. But how has it changed this dynamic? Some would argue that an increased human population and with it, increased swimmers and surfers in the water increases the chance of attack. However, one study looking at the effect of humans in the water with sharks, may suggest otherwise. 

Ritter and Amin (2012), investigated the presence of human swimmers and their effect on the behaviour of large bull sharks – a species believed responsible for a number of shark attacks. It was found that when a human entered the water, large bull sharks left the area almost immediately (Ritter and Amin, 2012). Furthermore, it was suggested that body position in the water was correlated to how close the sharks were willing to come to a swimmer (Ritter and Amin, 2012). It was found that when presented with a vertical body position (with the persons head out of the water), large bull sharks were much less likely to approach than when the swimmer was laying on the bottom (Ritter and Amin, 2012).  

Now, based on these findings, it could be hypothesised that simply by virtue of being popular, tourist beaches would be likely drive sharks away due to the presence of many humans in upright positions in the water. Another study suggests that a recent rise in shark attacks, may be due to “people accessing previously isolated coastal areas” (West, 2011). Anecdotal evidence suggests that many shark attacks in recent years have occurred in remote locations, usually at dusk or dawn - times with the fewest people in the water. Taking this a step further, it seems to me that the beaches that are protected by shark nets (popular tourist beaches), are the very beaches that probably need the protection the least. 

Other factors to consider are the shark populations themselves. Excluding the mandate of shark control programs to reduce the number of large, potentially dangerous sharks near swimming beaches, shark populations are under enormous threat from all directions. Fishing (and associated bycatch), pollution, habitat degradation, and climate change are among the most notable threats (Simpfendorfer et al., 2011). It has further been suggested that shark populations subject to commercial fishing exploitation generally experience a decline in the abundance of large size classes (Stevens et al., 2000), a phenomenon which appears to be achieving the shark control programs goals incidentally. 

In large part, due to these other threats, shark numbers in the last 30 years are estimated to have reduced by 70-90% (Stevens et al., 2000). Declining shark populations can be observed by comparing the catch per unit effort (CPUE) of many fisheries throughout the world. An example of this can be seen in the declining shark catch rates in the NSW shark meshing program despite constant CPUE over the past 20 years (Figure 3).

Figure 3. NSW Shark meshing program statistics showing (a) number of meshing days per year, and; (b) total sharks caught per 100 meshing days (Reid et al., 2011).

Bycatch in Shark Nets

When looking at this issue, there are really two types of bycatch – non-shark bycatch, and bycatch of sharks that don’t pose a threat to human safety. It is a common sight on news programs in Australia to see whales entangled in shark nets. When this occurs there are generally two outcomes, the whales drown or humans risk their lives and considerable expenditure to go out there and save the whale. These are high profile incidents that help to raise awareness as to the negative impact shark nets can have on non-target species, but there are many more species that regularly get caught up and drown in these same nets without the same level of coverage. Like the humpback whales seen on the news, many of these other bycatch species are protected or endangered.  In QLD  and NSW shark nets, bycatch includes whales, dolphins, dugongs, turtles (green, loggerhead, leatherback), and teleosts, along with rays (bluespotted, shovelnose, manta, eagle, cownose) which are the most represented group in the nets (Gribble et al., 1998, Dudley, 1997, Krogh and Reid, 1996). Looking at the Gold Coast Shark Nets between 2000-2008, the numbers suggest bycatch accounts for a significant portion of total catch. For example, 445 non-shark individuals were recorded in the nets (Table 2), compared to 426 shark (OESR, 2012). Using these numbers, we have an estimated bycatch rate of 51.1%, but if we take into consideration the number of sharks that were caught that were over 2m in length (and therefore, potentially dangerous), the total comes to only 162, making the bycatch rate 81.4% (OESR, 2012).

Table 2. Breakdown of non-shark species caught in shark nets on the Gold Coast 2000-2008 (RSN, 2008)
Individuals Caught
Bottlenose Dolphin
Common Dolphin
Cownose Ray
Dolphin (unknown)
Eagle Ray
Eastern Shovelnose
Green Turtle
Humpback Whale
Leatherback Turtle
Loggerhead Turtle
Manta Ray
Olive Ridley Turtle
Ray (unknown)
Shovelnose Ray
Spinner Dolphin
Turtle (unknown)


In conclusion, there is no doubt that shark control methods are quite effective at catching and killing sharks, but, the evidence that suggests they are effective in preventing shark attacks is questionable at best. To the casual observer, it may appear that shark nets and baited drum lines are the perfect attack prevention solution, but when digging deeper it becomes clear that the issue is a difficult one. Many factors are unknown, or have not been explored fully, such as the impact of terrestrial and marine based shark attractants, overfishing, and human population growth. But regardless of the effectiveness, the debate still stands. How many marine animals is it ok for us to kill to help us feel safe and protect our lifestyle?
Bycatch in shark nets can have devastating impacts on the ecology of numerous harmless and threatened species. Furthermore, the devastating impact we already have on shark populations means they are already less effective in their role as apex predators, and we may be on the way to removing their existence completely. If that were to happen, the possible impacts on human and environmental health are highly debated, but most agree the consequences could be dire. 

The number of shark attacks worldwide pales in comparison to the damage caused by many other activities that are deemed ‘safe’ in human eyes.  It seems that in order to protect a handful of human lives a year, we are further risking the greater good, and potentially the lives of many more humans and non-humans should ecosystems collapse. So, once again I pose the question “How is it justifiable to decimate entire populations of marine species to protect our leisurely pursuits?” My answer: It isn’t. What’s yours?

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