Sunday, August 14, 2011

Shark Research in Action: The Role of Mangroves as Nursery Habitat for Lemon Sharks

Guest Blog
by Tyler Clavelle

Since 1990, researchers at the Bimini Biological Field Station (Sharklab) on the small islands of Bimini in the Bahamas, have been cruising the shallows in search of their study subjects: lemon sharks. Lots of them. To the untrained eye, the mangrove-fringed lagoon between Bimini’s two main islands would seem an unlikely home for hundreds of sharks, but below the surface, the reasons are clear.

Bimini Bay.  Photo: Kristine Stump.
Mangroves and the nearby seagrass beds provide critical nursery habitat for many species of fish, shrimp, lobster and conch. With their close proximity to coral reefs, Bimini’s mangroves are essential for the growth and development of reef- associated fishes and predators, providing both food and protection for the juveniles. Additionally, the structural complexity and density of adjacent seagrass beds increase biodiversity and capacity for the area. In these nursery areas, the young lemon shark (Negaprion brevirostris), much like its adult counterpart, is king.

Juvenile lemon shark.  Photo: Kristine Stump.
Over the past twenty years, studies at the Sharklab have focused predominantly on the early life history of lemon sharks. The results have overwhelmingly supported the necessary role that Bimini’s mangroves play in a lemon shark’s long quest toward adulthood. Each year, in early spring, pregnant females arrive in Bimini to give birth near the protection of the mangroves. Genetic studies have revealed that these mothers will repeatedly use Bimini, in some cases exclusively, with 55 females producing 644 newborns (neonates) between 1995 and 2000. Juvenile lemon sharks remain in their primary nursery for approximately three years, rarely venturing away from the mass of entwined mangrove roots, which serve as a refuge from larger predators (e.g., barracuda, sub-adult lemon sharks and bull sharks). Furthermore, lemon sharks display an innate homing mechanism, a characteristic that prompts them to return to their natal mangroves when displaced. Loss of these areas can therefore destroy the only habitat these sharks will use as juveniles.

In Bimini, there are several distinct mangrove-fringed nursery areas in which the Sharklab conducts its research. Of these nurseries, the semi-enclosed, 3 square kilometer North Sound has received the majority of the attention. Unfortunately, over the past decade, the North Sound has seen extensive habitat degradation due to the development of the large Bimini Bay Resort and Marina.

Mangrove destruction on Bimini.  Photo: Kristine Stump.
Originally conceived in 1997, the Bimini Bay project has been an ongoing source of controversy for the past decade. Large-scale dredging, filling and mangrove removal have characterized years of inappropriate and ecologically mismanaged construction. At various times during the construction, work was postponed for long periods of time. Despite only partial completion and being located on one of the smallest family islands, Bimini Bay now boasts 350 luxury homes and condos, as well as the largest marina in the Bahamas. Recently, the North Sound has seen the most direct habitat loss, with the near entirety of the Sound’s western coastline stripped of mangroves. According to a recent article, this new stage of development, referred to as Phase II, intends to add “a new beach club restaurant, luxury day spa, oceanfront hotel, small casino, a "Nikki Beach" style oceanfront bar and lounge, 30 or so Maldives-styled 'over-water' bungalows and 91 new up-scale oceanfront homes”(“Bimini Bay Resort Readies for new Era of Development” by Michael Gerrity) all within a mile-long narrow stretch of the North Sound’s west coast.

As unfortunate as events in Bimini are, they provide a natural experiment upon which Sharklab Principal Investigator and University of Miami doctoral student Kristine Stump is focusing her dissertation. Kristine’s project, a Before-After Control-Impact (BACI) study, addresses the impacts of habitat loss in a lemon shark nursery. Using the extensive volume of existing Sharklab data concerning the life history, physiology, feeding and diet, bioenergetics, growth and behavioral ecology of lemon sharks in Bimini as a baseline, Kristine is quantifying the large-scale human-caused impact on the nursery with recent data collected from October 2008 to the present. Using these data, Kristine is creating a computer model addressing the potential impacts of habitat loss by elucidating factors within the nursery that are most influential in determining the sharks’ survival and movements. The ultimate goal is to help educate decision makers in order to mitigate impacts of future developments in Bimini and beyond.

Bimini Marine Protected Area boundaries.
In Bimini, while drastic damage has already occurred, there have been successes in reducing the scope of Bimini Bay, as well as establishing a no-take marine protected area in the remainder of the North Sound and East Bimini. Additionally, Bimini Bay is now operated by Rock Resorts, a Colorado-based company with a more environmentally conscious reputation, a change many are excited about. With the recent landmark legislation declaring the Bahamas a shark sanctuary, there is much to be thankful for in terms of shark conservation. The Bahamian shark population, one of the world’s healthiest, is now safe from the threat of a commercial shark fishing industry. However, in order for Bahamian sharks and others around the world to remain healthy, it is imperative that known mating, birthing and nursery areas are protected and remain viable. The Sharklab is excited for the results of Kristine’s study, and hopes her model will be used to prevent future loss of habitat vital to the sharks we love.

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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