Chum! Yum!

We need to talk about chumming

On Thursday a young man in his youthful prime was bitten by a shark in False Bay, and lost his life. Coincidentally, in addition to the usual (limited – three operators only) cage diving activities at Seal Island, an American research vessel had been operating in the bay in the days prior to the attack, using chum to draw sharks closer to the vessel in order to tag them. Naturally, many members of the public – who until last week had been given very little information aside from rumours and gossip on the subject of this highly-funded research cruise – are pointing fingers at the practice of chumming, claiming that it has to be changing the sharks’ behaviour and leading to increased risk to water users in False Bay.

Actual scientific research on the subject

I have read two papers on the subject of chumming (Australians call it berleying) and its effect on shark behaviour, as well as a brief discussion in Thomas Peschak’s excellent book South Africa’s Great White Shark.

The effects of berleying on the distribution and behaviour of white sharks, Carcharodon carcharias, at the Neptune Islands, South AustraliaBruce & Bradford (CSIRO study)

This paper is based on studies in Australian waters on white sharks (summarised here, full paper here), at a seal colony (North Neptune Island in South Australia, 60-70 miles offshore) where shark ecotourism takes place. Over the years the number of days on which the water is chummed, and the number of operators doing so, has increased, and a previous study done in 2001-2003 provided a useful baseline from which to work. A study comparing the movements of sharks tagged in 2010 with sharks tagged in 2001 found that, while no sharks took up residency at North Neptune Island (white sharks are generally highly migratory and only spend short periods in one place), the average amount of time that sharks spend at the island has almost doubled (to 21 days) over the last 10 years.

The study also found that the daily average number of sharks seen by the operators has increased, but rather than indicating increased shark numbers, this reflects the fact that sharks are staying longer at the island. The sharks’ movements around the island also more closely match those of the cage diving operators, as they arrive and depart at around the same time as the boats do each day (regardless of whether there are operators present).

It’s important to note that the Neptune Islands are far offshore, and not much else seems to happen there except for shark viewing.

Don’t bite the hand that feeds: assessing ecological impacts of provisioning ecotourism on an apex marine predatorHammerschlag, Gallagher, Wester et al (Functional Ecology, March 2012)

The second paper (summary here, full text here) studied two separate populations of tiger sharks, one in Florida, where chumming is banned, and another in the Bahamas where there is a thriving shark diving industry that feeds the sharks to draw them closer to divers. The hypothesis was that if the Bahamas sharks were responding to being fed, their distribution and movements would concentrate far more closely around the feeding areas than the Florida population does around its local area.

The conclusions were surprising: not only did the Bahama sharks range freely, but they did so over distances five times greater than their Florida cousins do, suggesting that they are not affected in a measurable way by being fed on baited shark dives.

A note about methodology

You will notice that both these studies had a sort of a “control group” that was unaffected (or less affected) by the factor (chumming) that they wanted to study. In the Australian one, the authors were able to refer back to a previous study done when there were fewer cage diving operators going out for fewer days a year, to see what had changed, knowing that one environmental variable (frequency and amount of chum) had increased. In the American study, two separate populations of sharks could be studied.

This is good science – in complex systems you cannot draw conclusions without attempting to eliminate the other factors which could influence your observations. While it would have been interesting in and of itself to find out how far the tiger sharks of the Bahamas range, the information would not have enabled any conclusions to be drawn about factors specific to that population (being chummed) without the inclusion of a parallel study on the Florida tiger sharks.

The False Bay chumming question

I’m a mathematician. It’s easy to study numbers and equations: they sit still on the page, they don’t interact with other equations unless I make them do so, and the results of my calculations are (hopefully) unambiguous. It’s this purity and dichotomy between truth and falsehood that atttracted me to the subject in the first place. Studying the natural world seems much harder to me. Sure, you can study sharks, but there are a thousand other variables – many that are specific to the location of the shark population in question – that can affect your study.

Thinking locally, seal populations might rise and fall because of external factors or changes in their food source (or, if you’re Namibian or Canadian, culling). Climate change, ocean warming, or fishing pressure (or something else) might adjust the abundance and type of fish that visit False Bay and how long they do so, thus changing sharks’ movements around Seal Island and inshore. Teasing out the effect of a single activity – chumming, in this case – is far from straightforward. It takes time, and lots and lots of money.

We will never, ever put this question to rest until we can do the science, run the numbers, draw the maps, and get enough observations to draw statistically relevant conclusions. The movements of the sharks both within False Bay and in the open ocean needs to be studied. We can make inferences about the effects of chum in False Bay from the research done in other places (such as the two papers I mention above), but the uniqueness of every location, the pressures exerted on it by the populations along the coastlines nearby, and its own peculiar history means that – in this situation, at least – I don’t think we can stand on the shoulders of others in order to draw hard and fast conclusions.

The sooner we start, the better – in the (probable) absence of an earlier local study such as the one used in the Australian research paper, some clever science will be required to get meaningful results. It’s a great pity and a terrible irony that the very research that could have led us to some answers about the effects of chumming has been suspended – because (it seems) of suspected or perceived culpability in the shark bite incident on Thursday. And no one is in a position to say whether the activities of the research cruise did lead to the incident, because we have no prior research from which to draw conclusions. Thinking about this too much is depressing, and gives me a headache. More tomorrow.

Disclaimer

Tony has been on baited shark dives in the open ocean and at Aliwal Shoal, and we both have been shark cage diving in Gansbaai. We found it a life-altering experience to be with the sharks in these settings, knowing that without the use of chum the chance of seeing any sharks would have been vanishingly small. We are both in two minds, and have been for a long time, as to whether using artificial means to draw the sharks to a boat outweighs the wonder of seeing them in the water, and the effect this can have on one’s view of sharks as something to be protected rather than feared. Perhaps our having been on baited shark dives shows that we feel that the ecotourism and mindset-altering benefits of these dives is more significant than the possible additional risk of changing the sharks’ behaviour with chum… I don’t know, and I can assure you the debate occurs at least once a week in the Lindeque household!

Update

A study has been done on the effects of chumming in False Bay – click here to read about it!

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Clare

Lapsed mathematician, creator of order, formulator of hypotheses. Lover of the ocean, being outdoors, the bush, reading, photography, travelling (especially in Africa) and road trips.

6 thoughts on “We need to talk about chumming”

  1. Points to ponder…..
    1. All the fishing vessels cleaning their catch on the way to Kalk Bay, and all the fish cleaned in the harbor – is this any different to chumming?
    2. The seal population growth on Seal Island after years of culling.
    3. The depletion of fish stock, tuna etc of False bay – the natural food of sharks.
    4. The cooling of the water temp of False Bay. In years past the kelp was only to Smitswinkel.

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