Thursday, June 30, 2011

Shark Research in Action: An Introduction to the Bimini Biological Field Station

Guest Blog
by Tyler Clavelle

Ask any shark enthusiast why they love sharks and you are likely to get a variety of responses, all of which struggle to capture the same elusive feeling of awe one has when a dorsal fin materializes out of the blue. It’s an addictive feeling that both excites with adrenaline and eventually calms with the appreciation of being together with one of nature’s oldest and most highly evolved creations. I am fortunate enough to have experienced the feeling many times before; however, none were quite like what I felt last week as I stared through my mask at 117 lemon sharks swirling around me in tight formation. They may have been small (the largest measuring 101 cm) and enclosed in a holding pen, but the recently tagged cohort of youngsters before me represented the future of this critical shark in The Bahamas in a very big way.

Welcome to the Bimini Biological Field Station (BBFS) in Bimini, Bahamas! BBFS, commonly referred to as the “Sharklab”, has been researching lemon sharks (Negaprion brevirostris) in The Bahamas and Florida since 1990 when it was established by world-renowned shark biologist Professor Samuel “Doc” Gruber. Fed up with the hassle of working from research vessels in the Bahamas since the late 1970’s, Dr. Gruber opened the Sharklab to conduct fieldwork on a more continuous and permanent basis, with the dual goals of elucidating the role of a large coastal shark, and educating future generations of shark biologists in the difficulties of fieldwork. The past twenty years at the Sharklab have seen dozens of PhD and MS students, as well as hundreds of undergraduate and graduate volunteers from all over the world living and working together to further our understanding of these magnificent and critically important apex predators.

Having already spent over twenty-five years researching lemon sharks prior to forming the BBFS, Dr. Gruber knew very well the islands of Bimini, and the important role they serve as a nursery for lemon sharks. Located just fifty miles (85 km) east of Miami, Bimini’s mangrove-fringed lagoons and tidal flats are ideal habitats for neonate (newborn) and juvenile lemon sharks. Every spring, around April and May, pregnant lemon sharks arrive in Bimini and give birth in the shallows to litters averaging 4-18 pups. These new sharks will spend the next decade in the waters around Bimini, starting off in close proximity to the mangroves for the first four years or so before venturing out into the lagoon and flats as they grow larger. Eventually, as they near sexual maturity, they will leave Bimini, with only the mature females likely to return to give birth to their own litters. The males are much more nomadic and rarely return to their birth nursery.

Thanks to the annual PIT Tagging Program, named for the type of tag used (Passive Integrated Transponder), the Sharklab maintains the largest and longest running mark-recapture and genetic database of any shark population in the world. The fishing, which is done with ~180 meter long gillnets made from a mesh of 20-pound test monofilament, is carried out in two major nursery areas, referred to as “The North Sound” and “Sharkland”. Gillnets are illegal in The Bahamas, but the lab maintains a research permit that allows their use (and we do so in a non-lethal manner). Each nursery is fished for six nights, with gillnets set at the same three stations each night. The nets remain in the water for twelve hours, a full tidal cycle, and are checked every fifteen minutes, with all captured sharks removed and transported to a fourth boat that serves as a tagging station.

Photo credit: Matt Potenski
At the tagging boat, each shark is “worked up”, a process whereby each shark is sexed, weighed, measured, a genetic sample taken, and, if the shark is a new capture, given a PIT tag. The PIT tag is a small (rice-grain sized) radio-frequency ID tag that usually remains in the shark for the duration of its life and, if recaptured, allows precise and accurate records of that shark’s capture and work-up history.

The Bimini Sharklab could not continue if it were not for our hardworking crew of dedicated volunteers.
June 2011 marked the 17th annual PIT Project and, as with the sixteen years prior, it was an exciting but strenuous shark-filled month. The 2011 PIT crew consisted of twenty-two staff and volunteers from the U.S., England, and Switzerland, including Dr. Gruber, Dr. Bryan Franks, and Dr. Steven Kessel. Additionally, the Sharklab was thrilled to welcome our benefactor, Sundance Boat CEO Wally Bell and his daughter Alexis, whose generous boat, trailer, and motor donations make our work possible. The weather cooperated well, with a constant breeze keeping the mosquitoes at bay; and 2011 was the first in all seventeen years without a rainstorm! After twelve long and sleepless nights the totals are in, and they show some interesting trends.

Sharkland, the first nursery to be fished, traditionally yields the most sharks of the two nurseries and is particularly hectic on the first night. This year’s catch, at 117 total (49 on the first night), is the lowest in recent years. Interestingly enough, neonates (newborn lemon sharks) constituted 56.4% of the total Sharkland catch, compared to last year’s 25.6%.

Not to be outdone by Sharkland, the North Sound had a big year with 62 sharks caught on the first night and a nursery total of 92 sharks. Of the 92 North Sound sharks, 69 were neonates, with the remaining 23 being recaptures. As a result, more neonates were caught in the North Sound than in Sharkland, an unusual and interesting occurrence. The total catch for the 2011 season was 209 lemon sharks, a number about 10% lower than usual but still well within the range of variation.

While the twelve nights of fishing during PIT may seem long and tiring, they are nothing compared to the twelve years, on average, it takes a lemon shark to reach sexual maturity. The slow-growing, late-maturing strategy, combined with relatively low reproductive output (fecundity), is characteristic of all sharks, not just lemons, and makes them highly vulnerable to over-fishing. If we as a planet are to successfully protect these apex predators, it is imperative that research such as that of Dr. Gruber’s Sharklab, continues to shed light on the life cycle of sharks. While the movement to protect sharks is there and the momentum is growing, conservation policies will only be effective if they are based on sound, empirical research.

Tyler Clavelle is the assistant lab manager at Sharklab. If you are interested in volunteering at the lab, please do not hesitate to contact Dr. Gruber at sgruber@rsmas.miami.edu, or our station managers at bbfssharklab@gmail.com. Please also visit our website www.miami.edu/sharklab for details.

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