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Coastal Management

Does long term SCUBA diving affect shark behaviour?

Paper: Bradley, D., Papastamatiou, YP, Caselle, JE (2017).  No persistent behavioural effects of SCUBA diving on reef sharks.  Marine Ecology Progress Series.  567: 173-184.  doi: https://doi.org/10.3354/meps12053

Image: SCUBA diving with sharks (Photo Credit: Manoel Lemos via Flickr, CC BY-SA)

 

Millions of sharks are caught and traded every year and a quarter of shark species are at risk of extinction.  However, depending on the shark*, they can be worth more alive than dead.  For example, money paid by tourists to see long-lived sharks can add up and surpass the amount of money received for selling the shark for meat and fins.  Today, shark-based tourism contributes millions of dollars every year to local economies and significant efforts are being made to continue the shift away from shark fishing to shark watching.  It’s not all sunshine and flowering seagrass though, there is evidence that high tourist activity negatively affects animal health, foraging time, habitats and ecosystem function.  This is especially worrisome for apex predators like some sharks as changes in their abundance can influence the rest of the marine community.  Previous studies have examined the response of sharks to SCUBA diving in the short term (some sharks are attracted, some sharks are repelled).  However, Bradley and her team wanted to investigate sharks’ long term response to SCUBA diving and inform if diving operations could exist in the long term without negatively affecting the sharks themselves and the habitats they live in.  

Black tip reef shark – one of the species of sharks in Palmyra Atoll (Image credit: NOAA via Wikipedia Commons)

Palmyra Atoll is a historically uninhabited, ring-shaped coral reef island which has been a US Fish and Wildlife Refuge since 2001.  Due to its remote nature, large population of blacktip reef sharks (Carcharhinus melaanopterus) and grey reef sharks (Carcharhinus amblyrhynchos), absence of shark fishing and concentrated scientific diving activity, it was the ideal place to single out the effects of SCUBA diving on sharks.  GoPros on metal and PVC frames were used to monitor 4 sites that were regularly dived (n=25) and 4 sites that were not dived at all (n=25).  At each site, GoPros were placed at different depths, 6-7 different locations within the site and at different times of the day for 120 minutes each.  Each camera was accompanied by a piece of mackerel contained within a plastic canister with holes.  This contraption allowed whiffs of fishy carcasses to entice nearby animals to come into the camera frame, but also ensured that animals were not being fed (mimicking feeding activities).  

Grey Reef Sharks, another common species by Palmyra Atoll (Image Credit: Paula Ayotte, NOAA, CC0)

When analyzing the video recordings, scientists took note of the first time they saw a species, the maximum number of individuals in a single frame and the time at which there were the maximum number of individuals in a single frame.  These are common measurements that give researchers insight into relative abundance.  For example, fish tend to arrive more quickly to bait if there are more of them around and counting the maximum number of individuals in a frame ensures you’re not counting the same individual twice.

In addition, 45 grey reef sharks were tagged and tracked using acoustic tags.  These tags emit sound that is detected by stationary receivers within 300m.  The number of sharks detected were then compared between 2014 when there were over 700 dives to 2015 when there was no diving.

Results showed that there was no significant difference in the presence, abundance and length of time sharks spent between heavily dived sites and undived sites.  This means that well-managed shark diving tourism may indeed be an achievable long term operation without compromising shark populations–at least at these levels (~1400 dives/year) and for these species (blacktips and greys).  While short term impacts were observed, scientists hypothesize that after exposure to diving humans over a decade, sharks learned that diving humans were not a threat or an opportunity, in fact, they were indifferent.  

Of course, impacts are entirely context dependent and shark diving regulations and management should consider codes of conduct in the water (e.g. no touching) as well as managing diving tours over space and time.  Given that 40% of the marine protected coral reef areas that sharks live in are smaller than their activity range, managers need to be thoughtful as the success of shark diving tourism depends largely on divers actually seeing them.  

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** There are over 400 species of sharks that range in shape and size.  Each one of those species has a unique ecological role and life history.  Some sharks are large and long-lived and reproduce every few years, while others have shorter life spans and reproduce much faster.  Likewise, some are apex predators and others are preyed on.

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