It is difficult to find any scientific evidence demonstrating that shark culling has any effect on the number of shark attacks. Whilst it may not prevent shark attacks, the long term shark cull program in Queensland does catalogue (and possibly contribute to) the decline in shark populations.
Calls for the establishment or expansion of shark cull programs hits the media every time there is a shark attack (most recently in Western Australia and Northern New South Wales). This keenness to do something to avoid further attacks is completely understandable. But does shark culling reduce shark attacks?
The Queensland shark culling program (in place since 1962) was reviewed by Jessica Meeuwig . Meeuwig concludes that the effectiveness of the Queensland drum lines is hard to evaluate because the occurrence of fatal shark attacks is such a rare event and 83% of the Queensland drum lines are deployed at locations where a fatal attack has never occurred – hence how does one demonstrate an impact. Shark attacks have indeed reduced since the drum lines were introduced, but not just in the areas where the drum lines are deployed, but across the entire Queensland coast. Hence the difficulty of trying to quantify the impact of an intervention strategy on a very rare event (see the Simpsons bear patrol scene).
Figure 1: Annual shark cull by key species since 1991 for Qld (source: data.qld.gov).
Even if the benefit to humans is difficult to demonstrate, the change to shark populations looks clear (Figure 1). There has been an average of 766 sharks caught per year (1986-2016) across both drum lines and netting (Figure 1). Meeuwig has looked closely at the period 2001-2013 and concludes that only 3% of the sharks killed on Queensland drum lines are not at conservation risk. Of those 97% at conservation risk, 5.2% of the catch is of “endangered” species, 9.6% of the catch are “vulnerable” and 80.6% are “near threatened”.
Shark cull catalogues the decline in shark numbers
The Queensland shark cull program consists of hundreds of hooked drum lines and tens of shark nets. We don’t have data on how the number of nets or drumlines has changed through time (the catch effort). But if we assume the catch effort hasn’t changed that much, then the trend in catch numbers through time should broadly reflect the shark population. If all is well with the shark population then we would expect to be catching about the same number of sharks every year. Conversely, if the shark numbers are declining through time then this is bad news for the sharks. A simple series of trend analysis tests using Truii (Linear regression, Mann Kendall and Spearman’s Rho) all show very strong evidence of a declining trend in shark catch (0.01 significance level – i.e. a 99% confidence that this decline is not due to chance). For those of you unconvinced by statistics – simply look at the slope of the lines in Figure 1 – decreasing through time.
This analysis is pretty simplistic. For less mobile species, this decline may simply be an impact on a very local population (the whole purpose of the culling program). However, for more mobile species such as tiger sharks we would not expect to see a declining trend as new sharks will move into the area as others are caught – effectively maintaining the catch rate through time. However we see a similar decline in tiger shark numbers as for other less mobile species, indicating that the culling program may be cataloguing a more broad scale decline in shark numbers.
The decline in catch may be due to the culling program directly, other environmental factors or most likely; fishing activities . Regardless of the reason for the decline, we are now catching less sharks. This result is not the same across all species, the Whaler sharks show no real decline through time, however the Hammerheads and the collective ‘other species’ show significant decline. Comparing the periods 1986-2001 with 2002-2014 we see the average number of Hammerheads caught is down from 133 to 61 (54%) and the collective ‘other species’ is down from 416 to 263 (36%) (Figure 1).
For a more detailed look at where every shark was caught from 1986-2014 see Figure 2.
Figure 2: Total annual shark cull by region and year for Qld – click on a region to see the underlying data (source: data.qld.gov).
What about by-catch?
The shark cull program is designed to target sharks, however nobody told the dolphins. The by-catch data shows that 2,576 non-target species where caught (2001-2016). Fortunately 1,484 where released alive. Of the remaining 1,092 non-target species that where killed by the shark cull program;
- 420 where Rays,
- 358 where fish (mostly Tuna),
- 72 where Turtles (59 where endangered Green and Loggerhead turtles, 3 where critically endangered Hawksbill turtles).
- 238 where Dolphins (various species)
- 4 were Whales ( 3 Humpbacks and an Antarctic Minke Wale – all caught off the Gold Coast)
The shark culling program may do little in terms of swimmer safety but it is a potentially a useful sampling program to catalogue the decline in shark numbers, it also seems pretty effective at killing the other marine creatures that we hold dear.
To their credit, the Queensland government is engaged with ongoing research and development around the shark control program.
As a final point to add a little perspective. While any death is a tragedy, your risk of dying while driving over the next 12 months is around 50,000 to one compared to 50-60 million to one of being killed by a shark.
About the data visualisations
The data visualisations are contained within their own Dashboard. You can link to the Dashboard, or use the shortcode from the visualisations to embed them in another web page. This data is from data.qld.gov, a wrangled version is available on Truii for your own analysis.
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