Shark populations on the Australian East Coast—what are they doing and how do we know?
You don’t need to spend a lot of time scrolling through fishing pages and other outdoor recreation pages to see photos of fish being reeled in with shark bites, or simply photos and videos of sharks swimming around boats, creeks and other areas. Along with the photos, there are the inevitable comments about (apparently) increasing shark populations and the ‘need’ to allow more shark fishing, increase commercial limits or other methods to cull sharks, without using the controversial word ‘cull’. The truth is that Queensland (QLD) and New South Wales (NSW) government shark control programs already cull sharks at popular beaches without actually saying they do. Some of the fishing community want this expanded beyond just beaches to take even more sharks out of the ecosystem.
So what exactly is happening here, and what does science tell us? Just as importantly, why should I believe the available science over my friend’s uncle Bob, who has spent the last 50 years fishing and insists that shark populations are exploding/out of control/too high? I’ll explore some of these questions here and hopefully explain why there are often such contrasting and sometimes conflicting perspectives depending on who you ask.
Firstly, let’s get the main point out of the way; most shark species commonly seen on the Australian east coast by the public and studied by scientists are, unfortunately, showing decreasing trends in their populations. This isn’t something new invented by scientists, ‘greenies’ or other groups, but has been the case for decades. The source of the data in many studies is surprisingly ironic in that it’s the drumlines used for the culling themselves.
Over the last few decades, as much as we disagree with their existence, drumlines and other lethal shark control devices like nets, have inadvertently provided us with a relatively accurate and curated sampling method which is free to access; and samples in several locations along the coast (Roff et al., 2018). Using the catch data from these shark culling devices in QLD, we see some alarming trends for every one of the four groups represented with hammerheads and great whites showing a 92% decline since 1962 (Roff, 2018 #751). Even species we complain about and often define as having larger populations, such as tiger sharks and carcharhinids (a grouping known more commonly as whalers) have declined by 74% and 82%, respectively (Roff, 2018 #751).
...most shark species commonly seen on the Australian east coast by the public and studied by scientists are, unfortunately, showing decreasing trends in their populations.
Similarly, in NSW, survey methods conducted by scientists also highlight declining populations in all species assessed (Reid, 2011 #810). This is highly concerning to marine scientists when we consider the important role sharks play in the ecosystems off our coasts. The irony is that healthy fisheries depend on shark populations, as do healthy reefs, oceans and many other parameters that draw people to the ocean, including tourists.
Globally the picture is sadly no better, with recent studies covering 126 countries also highlighting significant declines to shark and ray populations in half the countries where data was sourced (Davidson et al., 2016). Overfishing is identified as the primary cause of this in every study listed above, and is highlighted in larger studies exploring the issue and potential solutions (Davidson, 2015) and is a bigger issue for species that are migratory and/or pelagic in their ecology (Dulvy et al., 2008, Dulvy et al., 2014). Therefore, it is abundantly clear that even in Australia, where we can be proud of many of our sustainable fishing practices, we have a long way to go to give protection to ecosystems on broader scales, particularly when they involve animals with significant public relations issues.
Secondly, we need to look at shark life histories and realise that for most species, a population explosion is simply not possible because of how infrequently they reproduce and how few young the majority of species have (Simpfendorfer et al., 2011). As scientists, we refer to this as a ‘K-selected’ life history, and it sits in stark contrast to most other species of fish which may reproduce several times a year and have hundreds to thousands or tens of thousands of offspring each time. Therefore, most sharks species simply cannot increase their population rapidly enough to cause a supposed ‘explosion’ nor can they sustain significant fishing pressure in the same way we fish for other species (Dulvy, 2008 #348).
Finally, we come to the question of why fishermen are losing their catches more frequently if shark populations have indeed declined…many of them do indeed spend significant time on the water, around the ocean. They are not unknowledgeable about what happens under their boat. But the issue here isn’t so much knowledge or observation. Instead, it comes down to basic sampling techniques and shark ecology as well as some human characteristics.
This forms the most scientifically accurate picture possible and eliminates the bias from only using baited sources, which we know can draw sharks from long distances.
As scientists, sampling technique is key to good experimental design—part of this involves eliminating sampling bias as much as possible and randomising the location we sample from to get the best possible picture of what we’re measuring. Since we can’t count every shark in a specific area, we need to accept that counts from random areas indeed represent the entire area we refer to. Scientists use a variety of different methods to collect this data ranging from numbers caught in shark control programs (Roff, 2018 #751), to records by divers (Rizzari et al., 2014), trained reef surveyors, baited and non-baited cameras, fisheries catches, and even the reports from fishermen themselves. This forms the most scientifically accurate picture possible and eliminates the bias from only using baited sources, which we know can draw sharks from long distances.
Unfortunately, most fishermen aren’t collecting data and therefore aren’t attempting to randomise sample locations, reduce bias or ensure they cover enough areas for the claims to be considered robust data—this isn’t their fault. They’re simply out on the water to enjoy a day of fishing. Finally we know that people tend to frequent the same popular fishing spots close to shore and easily accessible from major population centres. With increasing populations, more people will be fishing in the same spots, and therefore, more incidents of sharks taking catches will be reported—with social media and easy access to communication, more people report this than in the past, when it was ignored and just considered bad luck.
The flipside of this is that as apex predators, sharks are not above an easy meal and are fully capable of learning, associating and taking advantage of opportunity (Guttridge et al., 2009). Sharks inhabiting a reef learn to associate the sound of boats with easy meals, an easy buffet of already-hooked and easy-to-catch meals, served every weekend of the year (Mitchell et al., 2018). It’s therefore highly plausible that this combination of factors would make it seem like there are more sharks stealing catches, when in fact there are less. Sharks have simply learned where and when to go for an easy meal, and numbers at these particular reefs are not representative of the overall population.
...sharks are not above an easy meal and are fully capable of learning, associating and taking advantage of opportunity.
Therefore, in concluding this article, we can see that all of the available science points to decreasing shark populations off our coasts. While many don’t see sharks for the benefits they provide, it’s important to understand that shark culling is not the answer and that along with an ethical right to simply exist, sharks are incredibly important to the ecosystems they rule over and many of those benefits actually contribute to activities such as fishing, snorkelling on a coral reef, diving and many others. We need healthy shark populations for many reasons outside the scope of this article, but hopefully, scientifically approaching things can help clear up the truth about shark populations and ultimately aid in dispelling many of the myths that surround them.
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