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Tracking Sevengill Sharks in San Diego

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Article contributed by Michael Bear

Beginning around 2009, those of us who had been diving in the San Diego area for over a decade began hearing increasing numbers of  reports of encounters between local divers and Broadnose Sevengill sharks.

This seemed a bit odd, since those who had been heavily involved with the diving community in San Diego since 2000 and before that,  did not recall hearing any reports of encounters with this shark before 2008.

Unfortunately, part of the problem, was and is, that no shark researchers from Scripps Institution of Oceanography or Birch Aquarium or anywhere else for that matter, had ever been down to do any baseline studies of sevengill populations–so, we had nothing to compare what we were seeing and hearing to, scientifically speaking. So, just as a matter of personal interest, I set up a website devoted to informally tracking sevengill shark encounters in the San Diego area, not thinking much would ever come of it.

Initially, because I am not a shark researcher, funded or otherwise, it was to be a site for collecting anecdotal evidence for sevengill encounters for my own personal interest, to see what exactly what going on.Shortly after this, the numbers of reported encounters between local divers and sevengills spiked, going from half a dozen or so [reported] encounters per year, from 2008-9, to one nearly every weekend, just this year, in a well-known tourist area you may have heard of, called La Jolla Cove.

Sensing something unusual was happening, but not sure what, since the professionals are loathe to go ‘out on a limb’ and make pronouncements without seeing prolonged baseline studies spanning many years, we set up a section of the website devoted to both still and video documentation of these encounters, as well as an online web data collection form, which divers could fill out, giving both ‘hard’ and ‘soft’ data from their encounters, ie: temperature, depth and time stamp data from their dive computers, as well as, the ‘soft,’ or anecdotal data from their encounters, to supplement any photographic or video they may have taken.

The video and still photo data ranges from poor to excellent quality high definition videos, showing both adult and juvenile,  male and female sevengills, including a couple females showing obvious bite marks that we think are related to mating behavior.

We have asked San Diego dive clubs and lists, with a combined membership of around 1500 divers, to encourage their divers to document their encounters on video as well as try and look for claspers during their encounters, to try and determine how many of these sharks are male–in the beginning, they tended to be mainly female, with males beginning to show up more recently–as evidenced by the photos and video. What I am personally  interested in doing, is getting enough video and photographic data to begin tracking individual sevengills, based on the unique freckling pattern seen on the dorsal side of the species, along the lines of the work being done in South Africa by a graduate student of  Dr.  Alison Kock, a biologist  with Save Our Seas, who has pioneered the use of a pattern recognition software known as SPOT, used initially by Whale Shark researchers, to identify individual whale sharks, to ID individual sevengills in False Bay.

I have been in touch with Alison and she has been encouraging also. She sent me a paper done by one of her grad students, showing that the SPOT algorithm can be used with up to 95% accuracy to ID individual sevengills. We are interested in doing a follow-up study on this. Right now, we’re working on getting a ‘control group’ of photographs and video stills to compare our photographic database to and begin running tests of our shark images through the SPOT software. It may also provide an intriguing, low cost alternative to tagging, which we are obviously not set up to do here. Although we are not researchers, Vallorie Hodges at OCA has kindly agreed to be a Principal Investigator [PI] on this project and have offered to have the results written up for us in an academic paper, should we decide to go that route. Our form asks for ‘hard data’ from each diver’s dive computer, such as depth, time and water temperature and GPS numbers for the dive site, if known, estimated shark size and gender, and then, data about the marine environment they had the encounter in, such as Kelp Forest, Sandy Bottom, etc. I have also recently partnered with Jeffrey Gallant of the Shark Research Institute (SRI) Canada and the Greenland Shark and Elasmobranch Education and Research Group (GEERG)’s Shark Observation Network [www.sharksonline.net] and merged our databases together, meaning: an entry on the Sevengill Shark Sightings site will be automatically entered into the SON database. SON also allows the uploading of photos and video.

As I mentioned above, the problem with trying to determine what kind of population increase [if any] we may be seeing is: 1) no baseline studies of sevengills has ever been done for this area and, 2) no baseline studies of number of local divers entering the water from 2007 onward has ever been done either, so we have no idea how much of this ‘uptick’ of reported encounters is due to an increase in the population of sevengills or an increase in the number of divers entering the water [we suspect it’s a little of both]. San Diego has an active diving community and the fact that reported encounters only began [sporadically] around 2007 and are now almost weekly at La Jolla Cove, tells us something is happening, we just don’t have the long-term data to tell us exactly what. As Vallorie Hodges said the other day, we may be  ‘it’ in terms of any baseline studies being done for the year 2020.

We have noticed the number of females seems to increase beginning in late march, in the Ocean Beach area, which may mean they are coming in to breed or ‘pup.’ But, the question then becomes, ‘from where?’

The increase in encounter reports from the La Jolla Cove area is also right next to an area called the ‘Children’s Pool,’ which has been a city-sanctioned ‘seal sanctuary’ for some time and the local sevengills may have simply discovered an easy food source there–we just don’t know. We can’t know what they are feeding on, until we cut one open, and examine the stomach contents, something not likely in the near future.

So, while we are not shark researchers affiliated with any academic institution, we are endeavoring to allow our data to be examined by those with training in science–the main problem being: we need long-term data, and we only started collecting our data in January of this year…….so, we have a long way to go before any definitive conclusions can be reached using it.

I am hoping, however, that using the SPOT algorithm on our photographic and video database will allow us to determine how many of these sevengill encounters are repeat encounters with the same small group of sharks, or if we have a more wide-ranging population here.

What we are doing here in San Diego is modeled on REEF and Reef Check’s concept of the ‘citizen-scientist,’ who, although not a scientist by training, works under the tutelage of actual scientists, to collect data according to scientific protocol.

For more on this project, please see here: Scientific American: Citizen Science Projects


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