After an apparent increase in reported sightings of sevengill sharks in the San Diego area, scientific diver Michael Bear set up a website to allow divers to log encounters with this species. Although more data is still needed from the diving community, the goal of this long-term population dynamics study is to identify individual sharks that may be returning each year, and to establish possible reasons for this new behavior. Here, he recounts how the passion of an amateur evolved into a scientific study making use of advanced techniques for the study of sharks.
Sevengill sharks, named for having seven gill slits on either side of their bodies, reach lengths of up to 3 m, with an average length of 1.5 m. Sevengills are thought to reach sexual maturity when they reach 1.5 to 2.2 m in length1. They weigh up to 107 kg and are known to live as long as 49 years. According to D.A. Ebert, the sevengill shark (Notorynchus cepedianus), a common coastal species of most temperate waters, has often been over-looked as an important apex marine predator2.
Currently, the California population of sevengill sharks appears to be concentrated in the Humboldt and San Francisco Bays3. These two regions provide nursery areas and safe havens for juveniles. The future of the sevengill shark in this region is highly dependent upon the conservation of these habitats.
Although this shark has a wide range, it is subject to intense fishing pressure as a result of being restricted to inshore waters. Currently the World Conservation Union (IUCN) lists the sevengill shark as "Data Deficient": data is lacking in most regions, making it difficult to determine the overall status of this species. However, it is currently assessed as "Near Threatened" in the eastern Pacific Ocean4.
This Study: Why Now?
In October of 2009, I began hearing reports of encounters between local San Diego divers and sevengill sharks. Having been diving locally since 2000, I thought this unusual, since this was the first I had heard of these encounters in nearly a decade. Between 2000 and 2006, almost no reports were documented. But in 2008, that all changed and they began appearing on the dive lists, one here, two there, until it was obvious that something was happening.
Around this time, I had my own memorable encounter with a sevengill. I was diving off of Point La Jolla when a large seven-footer (2m) glided majestically between me and my dive buddy, who was no more than two meters away. To say we were startled would be an understatement. The reasons for this sudden appearance of sevengill sharks are still unclear. In over nine previous years of diving in the San Diego area, I had never seen one before.
The most common theories to explain these unexpected visitations have been prey migration, changes in deep ocean currents, altered mating/pupping habits, global warming, and El Nino conditions. But not enough evidence exists for a “neat theory” to develop. So, as a matter of personal interest, I set up a website devoted to informally tracking sevengill shark encounters in the San Diego area (http://Sevengillsharksightings.org), not thinking much would ever come of it.
Although I put out the word for submissions on local diving boards, I was unprepared for the spike in responses that I received. In the first year alone, over 20 separate sevengill encounters were logged by local divers and 17 videos were submitted, to say nothing of the database of high quality photographs that evolved over the months. In the period since then, we’ve accumulated a database of over 30 photographs and 36 videos which clearly indicate the presence of cepedianus in the area.
There was no question that something was happening in San Diego in regards to sevengills. It was now a matter of figuring out what, exactly, was happening, and why.
Questions to Ask
There are two factors that must be considered when attempting to answer the question Are sevengills moving into the area, or does it just seem that way?
1. Have any baseline population studies of this species ever been done locally? If so, this would provide the numbers necessary for comparison and the development of an accurate ratio.
2. Have any studies been done to track the number of divers going into the water over the same period of time? In other words, is this a case of simply more divers entering the water, rather than more sharks being present?
Unfortunately, the answer to both questions above is: no.
To my knowledge, no baseline studies have ever been done on local sevengill shark populations by any of the local marine institutions. Unless the studies have been buried in the archives of the Scripps Institute of Oceanography, or are only available on JSTOR, the proprietary academic database. And no baseline studies have been done locally to survey the number of divers entering the water, or even being certified here in San Diego, in the last two years.
This means, in effect, that I was starting my own baseline study.
Here is how it works: When a San Diego diver encounters what he or she believes is a sevengill shark (local divers are pretty savvy when it comes to recognizing this species, with its distinctive thresher-like caudal fin and pushed back dorsal fin), the diver visits the website and enters hard data about his or her encounter, such as:
- Date and time of the dive
- Water temperature
- GPS of the location. For popular beaches, this is readily available on Google Earth or Google Maps. If on a boat, hopefully, the GPS for the location can be obtained from the skipper or from the on-board GPS device after the dive.
- The maximum depth and time of the encounter
Then, the diver enters the ‘soft’ data, which is weaker, as it relies on memory and perception, both of which can be altered or impaired significantly during an encounter with a large predator:
- Estimated visibility
- Underwater activity during the encounter, such as sightseeing, photographing marine life, spearfishing, etc.
- Estimated size of the shark and its gender. Instructions are provided on the website to look for claspers between the anal and pelvic fins in order to determine whether the shark is a male. Local divers who have had repeat encounters have gotten pretty good at this and even when they have not, claspers often are clearly visible in the video they’ve taken.
After all the relevant information has been logged, the diver is given the opportunity to submit photographs or videos taken during the encounter. In many cases, these videos and images have been surprisingly good, especially in light of the stress of the situation. Some even capture details such as the unique black freckles visible on the head and dorsal areas of sevengills, which would prove important for the next step of the project.
Citizen Science: Using an Astronomical Algorithm
In early 2010, in keeping with the new paradigm of community science, where lay persons seek out professional scientists to mentorthem in gathering data for what are sometimes called “citizen science” projects, I partnered with Dovi Kacev, a PhD candidate in Shark Biology at San Diego State University, to begin subjecting some of the higher resolution photographs of the dark 'freckling' pattern on local sevengills to an open source algorithm developed by astronomers to identify celestial star patterns. This had already been used successfully by Australian marine biologist Brad Norman and information architect Jason Holmberg, both of ECOCEAN to identify similar patterns on whale sharks5.
In a recent, unpublished South African study from the University of Capetown, these freckles have been used to successfully determine the identity of individual sharks with a high degree of accuracy. So far, data analyzed with the algorithm has identified 7 individual sharks in the San Diego area, based on the unique freckling patterns on each animal.
In June of 2011, by mutual agreement, the database of logged encounters from Sevengill Shark Sightings was migrated over to the database of the Shark Observation Network at www.sharksonline.net, run by Jeffrey Gallant, as part of a partnership between the Greenland Shark and Elasmobranch Education and Research Group (GEERG) and the Shark Research Institute (SRI Canada)6.
Although based in Canada, the Shark Observation Network provides a global database for divers to log shark sightings anywhere in the world onto a Google Earth-like map.
Goals of the Study
The goals of this long-term, population dynamics study are two-fold:
- to log individual diver encounters with this species in the Southern California area in the Shark Observation Network, whenever possible with photos and video, documenting the encounter and entering them into a Google Earth database for future geographical reference
- to submit the highest resolution photographs in the database to the SPOT algorithm used by Brad Norman et al. to attempt to determine which, if any, individual sharks are returning to the La Jolla area from year to year—we already have one instance of a shark, identified in an email by David McGuire of Sea Stewards, as possibly one of their tagged sharks from the San Francisco Bay area
I am fortunate to have received sound mentoring and guidance from some very knowledgeable folks in the shark research community, namely Vallorie Hodges, DSO [Diving Safety Officer] for the Oregon Coast Aquarium and David Ebert, Program Manager of the Pacific Shark Research Center at Moss Landing, California. Both Vallorie and David, although extremely busy with their own research work, have been more than generous with their time and suggestions and are following the situation with cepedianus with interest. Not to be overlooked is, of course, Alison Kock, formerly of Save Our Seas Foundation in South Africa, who works extensively with both sevengills and Great White sharks, and who has also shared her knowledge and time selflessly.
Hopefully, with the assistance of knowledgeable divers and scientists and several more years of fruitful data collection, we may have enough information to form an educated understanding of why Notorynchus cepedianus has appeared in San Diego.
About the author:
Michael Bear is a contributor to Marine Science Today and an active AAUS (American Academy of Underwater Sciences) diver for the California Science Center in Los Angeles. He lives and works in San Diego.
Footnotes and Citations:
1. Source: Marinebio.org, Sevengill Sharks, Notorynchus cepedianus at: http://marinebio.org/species.asp?id=1521
2. Ebert, D.A. 1991. Observations on the predatory behaviour of the sevengill shark, Notorynchus cepedianus. South African Journal of Marine Science. 11: 455-465.
3,4. Marinebio.org: Sevengill Sharks, Notorynchus cepedianus, ibid.
6. Scientific American, 2011:http://www.scientificamerican.com/citizen-science/project.cfm?id=shark-observation-network
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