Mangroves tend to be areas where muddy sediments accumulate and characterised by murky waters. But in a few special places like the Bahamas, the crystal clear waters during high tide give perfect conditions for exploring a unique and dynamic underwater environment and its mysterious inhabitants. Bimini’s mangroves have been visited by and inspired greats, such as author Ernest Hemmingway and civil right activist Martin Luther King, to inspire their writings. There’s nowhere quite like it, where you can observe so clearly the links between mangroves as nurseries and our juvenile megafauna. Studies conducted by the Shark Lab over the past 30 years have further uncovered some of the more intricate ways that mangroves are important for juvenile lemon sharks.
The mangroves of the Bahamas provide essential nursery habitat for well over 100 species of fish and marine invertebrates. The distinctive tangled root systems provide protection to small animals by being impassable to larger predators, and in doing so act as important protective hideouts for species of reef fish, sharks, crab and shrimp during the crucial early stages of development before making their way out into the open ocean as adults. Amazingly, recent research (2012, Dr Tristan Guttridge) illustrated that utilisation of mangrove refuge areas was directly related to the amount of danger each shark was in: smaller, more vulnerable sharks used the refuges more often and for longer periods of time, and refuge use was highest during high-tide, when large predators can come closest to the mangrove edges. Therefore, the presence of these inlets around Bimini are likely providing valuable refuge during the most dangerous time of the day, and in doing so allow greater survival of the lemon shark population at a key life stage.
Every year the Shark Lab complete a survey called the PIT project where they capture and PIT tag nearly every juvenile and newborn lemon shark in two nursery areas (watch the PIT Project video here). Through this survey and the samples taken from newborn sharks over the past few decades, they were able to come to the conclusion that lemon sharks are using natal philopatry, meaning when mature (12-14 year old) females are ready to pup, they return to the same mangrove nursery that they were born in and drop their own pups there.
Clemency White first volunteered at the Shark Lab in spring 2017, interested in how animals use their senses to understand their environment and how sensory systems are impacted by our ever-changing world. She’s now conducting a PhD aiming to determine the link between the shark’s sensory system and their mangrove nursery, combining it with the wealth of experience and knowledge on the lemon shark population collected by the Bimini Shark Lab over 30 years.
"The real value of PIT lies in its longevity. The founder of the Bimini Shark Lab, Doc Gruber, began the annual mark-recapture survey of two major juvenile lemon shark mangrove nurseries in 1995, and we have been sampling in exactly the same way and in the same locations every year since. We estimate that each year we catch up to 95% of the juvenile (age 0-5 years) lemon sharks that were locally born. As a result, we have a good understanding of the juvenile populations of these nurseries: lemon sharks are born between 55-65cm in length, weigh around 1kg and will grow 4-6cm/year. Only 35 – 65% of newborn lemon sharks will survive their first year of life, but their chances of survival will increase year on year if they make it past this milestone.
We continue to monitor these populations annually, and have tried to use this data to prevent further proposed development in the area, and establish North Bimini as a legitimate Marine Reserve." - Clemecy White
Despite having a relatively low plant diversity, the biodiversity of the occupants of Bimini’s mangroves is rich. Between the two islands of Bimini, 153 species of fish alone were identified in the mangrove communities. Further, small juveniles were sampled from all of the families present, confirming the importance of Bimini’s mangroves for the life cycle of many species and recruitment of fish onto coral reefs. One of the most common fish found in these surveys was the yellowfin mojarra – the favoured prey of lemon sharks. We know that juvenile lemon shark presence is linked to the safety and the prey abundance in the mangroves and nearby shallow water seagrass beds. So not only are these habitats important for the species within, but also form a crucial part of Bimini’s wider food web.
It is very unique in shark science to have a dataset spanning 30 years, and the long-term understanding of how Bimini’s lemon sharks move, grow and survive is fundamental to our ability to conserve them.
Bimini is also an important site for the wider western Atlantic lemon shark population, as the pups born in Bimini will move out of the lagoon and into wider Caribbean and US territorial waters once they are mature. While lemon sharks are protected from fishing in Bimini by the Bahamian Shark Sanctuary, once they swim out of The Bahamas they are at risk. The species was recently relisted on the IUCN Red List as Vulnerable with population trends declining, and so continuing PIT to monitor a major nursery site of the lemon shark continues to be hugely important for the population as a whole.
The islands of Bimini harbour the only mangrove habitat on the entire western Great Bahama Bank, a fact so significant that Bimini was designated as the highest priority site for a Marine Protected Area by the Bahamas government back in 2000. Yet today, Bimini's mangrove habitat shrinks smaller and smaller as a result of foreign development. In the mid 2000s, more than 30% of North Bimini’s fringing mangrove was removed and 750,000m3 of sandfill was dredged to facilitate the construction of a large hotel and casino.
As the mangroves of Bimini disappear, so too will the natural beauty that has made these islands such an amazingly unique place. Without a foundation to grow from, the ecology of the islands will collapse. Hopefully soon, both visitors and locals will embrace Bimini's natural beauty and all it has to offer, ecologically, economically, and culturally.
"Our best opportunity to protect these sites from further development is through the designation of protected areas, specifically restricting mangrove removal. It’s also vital that we conduct more research on the effects that development and mangrove removal have on sharks, as well as the ecosystem services mangroves provide, in order for us to advise the best protection for them.
As an individual, you can help by supporting mangrove planting initiatives, donating to mangrove and shark conservation charities and by raising awareness for mangrove protection by social media and word of mouth. Finally, if you ever get the chance to visit a mangrove forest yourself – go! Seeing the wonder that these beautiful forests have to offer is all the motivation you will need to want to protect them." - Clemency White
Guttridge, T.L., Gruber, S.H., Franks, B.R., Kessel, S.T., Gledhill, K.S., Uphill, J., Krause, J. and Sims, D.W., 2012. Deep danger: intra-specific predation risk influences habitat use and aggregation formation of juvenile lemon sharks Negaprion brevirostris. Marine Ecology Progress Series, 445, pp.279-291.
Jennings, D.E., DiBattista, J.D., Stump, K.L., Hussey, N.E., Franks, B.R., Grubbs, R.D. and Gruber, S.H., 2012. Assessment of the aquatic biodiversity of a threatened coastal lagoon at Bimini, Bahamas. Journal of Coastal Conservation, 16(3), pp.405-428.
Feldheim, K.A., Gruber, S.H., DiBattista, J.D., Babcock, E.A., Kessel, S.T., Hendry, A.P., Pikitch, E.K., Ashley, M.V. and Chapman, D.D., 2014. Two decades of genetic profiling yields first evidence of natal philopatry and long‐term fidelity to parturition sites in sharks. Molecular Ecology, 23(1), pp.110-117.
Jennings, D.E., Gruber, S.H., Franks, B.R., Kessel, S.T. and Robertson, A.L., 2008. Effects of large-scale anthropogenic development on juvenile lemon shark (Negaprion brevirostris) populations of Bimini, Bahamas. Environmental Biology of Fishes, 83(4), pp.369-377.
Barker, M.J., Gruber, S.H., Newman, S.P. and Schluessel, V., 2005. Spatial and ontogenetic variation in growth of nursery-bound juvenile lemon sharks, Negaprion brevirostris: a comparison of two age-assigning techniques. Environmental Biology of Fishes, 72(3), pp.343-355.
Gruber, S.H., De Marignac, J.R. and Hoenig, J.M., 2001. Survival of juvenile lemon sharks at Bimini, Bahamas, estimated by mark–depletion experiments. Transactions of the American Fisheries Society, 130(3), pp.376-384.
Newman, S.P., Handy, R.D. and Gruber, S.H., 2010. Diet and prey preference of juvenile lemon sharks Negaprion brevirostris. Marine Ecology Progress Series, 398, pp.221-234.