A cull of logic and math

September 18, 2014 0 Comments

A cull of logic and math

In case you missed it.  In a remarkable win for common sense, the proposed full implementation of the WA shark cull has been shelved by the EPA (Environmental Protection Authority).  Their stance can be seen in the following statement by the regulators chairman Paul Vogel .

At this stage, the available information and evidence does not provide the EPA with a high level of confidence. In view of these uncertainties, the EPA has adopted a cautious approach by recommending against the proposal

So what could possibly make the EPA that uncomfortable to recommend against the WA government?

To understand, we need to look back at the trial cull.

In January the WA government controversially introduced a 13 week trial period where baited drum lines were set off Perth and South West beaches. This was in direct response to an increasing trend of shark attacks over the last five years culminating in the death of Chris Boyd off Gracetown. All of these attacks have been attributed to the Great White Shark.

For the trial, 72 baited drum lines were deployed in two zones, one off Perth and the other in the states south.  Bull, tiger and great white shark species measuring over 3m would then be destroyed.

The logic being, less dangerous sharks in the water would equate to less attacks.

The cull by numbers

  • 180 Animals were caught
  • 172 sharks caught (96% of total catch)
  • 165 considered a dangerous species (92% of total catch were Bull, Tiger or Great White)
  • 164 Tiger Sharks caught (91% of total catch)
  • 112 Animals released alive
  • 101 days of trialing the cull
  • 72 baited drumlines
  • 68 sharks killed (38% of total catch)
  • 50 sharks greater than 3m caught and killed (28% of total catch)
  • 20 Sharks found dead on the line (11% of total catch)
  • 18 undersized sharks killed (10% of total catch)
  • 7 Stingrays caught (4% of total catch)
  • 5 Sharks able to release themselves (3% of total catch)
  • 5 Makos caught (3% of total catch)
  • 4 Sharks unable to be released alive (2% of total catch)
  • 2 culling zones (Perth and South West)
  • 1 Spinner Shark caught  (0.5% of total catch)
  • 1 Dusky whaler caught (0.5% of total catch)
  • 1 Bull Shark caught (0.5% of total catch)
  • 1 Northern Blowfish caught (0.5% of total catch)
  • 0 Number of attacks by Tiger sharks in last 10 years
  • 0 Great whites caught (0% of total catch)
  • 0 Numbers of attacks during the trial

The WA government called this a successful trial and applied to extend the cull for a further 3 years.  Was it really a success?

From a catch point of view, the effectiveness of the cull was 28% with a collateral damage of 10%. Meaning 1 in 3.5 animals caught and destroyed were meant to be, while 1 in 10 were not. Numbers also don’t take into account how many “released” animals actually survived the injuries sustained in the process.

shark cull

A 2.7m Tiger Shark sinks to the bottom after being released "Alive". Photo A. Corbe

From the view of human lives saved, it was a raging success. In a Dilbert world pie chart, 100% humans saved, 0% not saved.

shark cull dilbert 

So 28% success rate in catching = 100% human lives saved. Looks very successful. Imagine if we could increase the catch effectiveness – we could save even more human lives!

But hang on, wasn’t it less dangerous sharks in the water would equate to less attacks? And wasn’t it the Great White responsible for the attacks?

Except none were caught..

So somehow 0% caught=100% saved but … doesn’t that mean not catching these sharks is as effective as catching them?

You can now understand how the EPA would be nervous endorsing this sort of reasoning.

It simply didn’t add up.



Russel P
Has spent over 30 years of his middle aged life trying to spend more time in the ocean. Likes to surf, bodysurf, free dive and pretend he enjoys chasing big waves.

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