Nick Carroll: A Link Between Sharks and La Nina? Could It Be?

5 Nov 2020 23 Share

Nick Carroll

Senior Writer

COASTALWATCH | NICK CARROLL

There’s More to This Study Than Meets the Eye

If you live along Australia’s east coast, spring’s the time when you get used to wacky water temps.

Up one day, down the next! No wonder they called it a “spring suit”.

And with La Niña upon us, 2020 feels even more wayward than usual.

Now, with news of yet another shark attack in Port Macquarie this week, it only seems appropriate to wonder: could changes in the watery climate affect the behaviour and movement of sharks?

And lo! Just as you think of it, up pops a study on that very subject.

The study, funded by the NSW DPI fisheries division through the Shark Management Strategy, takes a look at whether or not environmental factors like water temps, wind, wave and tide action influence the activities of white sharks off eastern Australian coastlines over time.

Researchers included some big hitters, including three scientists aligned with the DPI: Craig Brand, Chris Gallen and Paul Butcher.

And their conclusion? Nah, not really.

They studied data from 444 white sharks tagged in the drumline program off NSW, over four years, from late 2015 to late 2019. All the sharks were fitted with transmitters capable of being picked up by the Smart Buoy array off the NSW coast at distances of up to 500m. 87% of the shark dataset was made up of juvenile sharks, with some sub-adults and near-babies in the mix.

Data was examined according to time of day and month, water temps, swell height, tide, and lunar phase.

By far the most individual shark detections were off Forster. A fair few were off South West Rocks and Crescent Head. Lots of hits off Hawks Nest, but these were mainly from just five sharks who hit the receiver numerous times.

Sharks were more active during the day, peaking around 11am, and less active during surf two metres or bigger. Less during tidal peaks and troughs; more during full moons. Activity peaked when the water temp was between 18-24 degrees C — in other words, most of the time off this coast.

But generally it seems as if fluctuations like these have little effect on shark numbers or behaviour over time. That is aside from time of year, for which the authors note a “strong seasonal variation”. Numbers peaked in September, with a decline between October and April.

Town Beach at Port Macquarie was closed on Monday after a shark bit a 13-year-old surfer that morning.

Town Beach at Port Macquarie was closed on Monday after a shark bit a 13-year-old surfer that morning.

Says the report: “The bulk of the total variation in detection data (~79%) remained unexplained by our model.

“We found that the deviance explained by temperature was only 17% of the deviance explained by the temporal factor ‘month’, suggesting that other environmental factors, not accounted for in this study, are driving seasonal variation.”

Yeah, like, err, food? The whale migration perhaps? Worth a look?

Maybe in another study.

Yet as almost always with these research papers, it’s the stuff around the edges that gets me. Usually because they include information the DPI has never made publicly available.

For instance: of the 444 white sharks whose SMART drumline tag data is part of the study, 75 of ‘em had been caught twice, seven had been caught three times, and three had hit a drumline a total of four times. 

That means around 20% of white sharks are not sufficiently deterred by the drumline experience to stay away from them in future — and a very small minority appear not to give any kind of a shit whatsoever.

Also: the sharks, once tagged, can be individually identified by the receiver buoy. Yet this information — shark size, type, gender, etc — is nowhere to be seen on the shark app alerts you might be getting on your phone every day. You could know if it’s a 1.7-metre semi baby white shark, or a three metre wannabe super-predator like the ones who’ve been taking people out this year. But you don’t. Why not?

Also: the receivers themselves are a bit questionable. The study takes this into account, explaining in part that “receiver performance showed an optimum between 17 degrees C and 20 degrees C, followed by a drastic decrease in detection efficiency”. It follows this bolt from the blue by saying shark activity in waters above 20 are “likely much higher” than the study itself indicates.

The SMART buoys aren’t reliable in waters over 20 degrees?? That will be exciting news for anyone who surfs anywhere north of Ulladulla, and relies on the app alerts for knowledge. Specially in a La Niña year.

Also: this research was performed in the context of the Australian Environment Protection and Biodiversity Conservation Act of 1999. White sharks are listed as “threatened” under the legislation.  

Check out the research paper yourself, if you’re in the mood.

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