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Why do I care about lemon sharks in The Bahamas?


By Molly Kressler, PhD student, University of Exeter

Links: Marine Predator Ecology & Conservation Profile



Juvenile lemon shark with juvenile mangrove snappers. Credit: Chelle Blais


Molly is a PhD student at the University of Exeter supervised by Richard Sherley, Sasha Dall, Dave Hodgson, and Matthew Smukall (external, Bimini Biological Field Station Foundation). Molly’s research interests are behavioural ecology and marine conservation, and as a result she has worked in both coastal marine ecosystems and aquatic laboratories, stretching from south-eastern Florida to Berlin, Germany, over the past six years.


Why do I care about juvenile lemon sharks (Negaprion brevirostris) on a small island in The Bahamas? To answer this, you need to understand that it’s really because I care about the mangroves.


I am wild about mangroves. And I am wild about conservation of wild places. And I am wild about animal behaviour, but more on this in a bit. As a young child, I was extremely lucky to visit my grandparents in south-eastern Florida twice a year. Situated on the Treasure Coast, on a small barrier island nestled between a brackish river and the North Atlantic, their home was teaming with wildlife buzzing in and out of the mangrove shoreline. Mangroves thrive in coastal salty and brackish environments and host biodiversity we can only begin to imagine. And it is here in Florida’s mangroves where the story of my love for little lemon sharks begins.


A key factor in this story, that I will never underappreciate, is the enthusiasm of my grandfather, Robert, or as we the grandkids call him, ‘Poobah’. He shared – or really planted and fed – my love of the mangroves. Poobah was and still is involved in local grassroots efforts to conserve the remaining mangrove forests of southeast Florida. He introduced me to passionate researchers, naturalists, and organisers in the local area. I don’t know if he knew what he was doing, but his efforts, passion, and thoughtfulness to introduce me to these people, developed a young aspiring woman in science. Through his network, I gained access to opportunities to see what this thing ‘conservation’ was all about. I worked as a naturalist assistant for an environmental outreach centre in southeast Florida. I lead canoe tours of the mangrove nurseries in the Indian River Lagoon. And I collaborated with local stakeholders to survey and catalogue the biodiversity of essential habitat, the salt marshes and mangroves, up and down the area’s coastline. I was hooked, and I knew one day I wanted to dig deep into these mangroves and help protect them.


Then in 2014, I started my undergraduate degree at the University of St Andrews, in Scotland – a far cry from the sunny humid shores of south-eastern Florida, and not a mangrove in sight. While here, I was able to explore and nurture my other interest: behavioural ecology. By the time I graduated, I had learned so much from pure theoretical studies of behaviour but was disheartened because it seemed that these two disciplines which both held my interest, marine biology and behavioural ecology, were wholly disjointed. And this disjoint, between the theoretical nature of behavioural ecology and the applied focus of marine biology, seemed unreconcilable. However, the more I reflected on the two, the more I realised there didn’t need to be a disjoint. I began to feel confident that behavioural ecology principles had value in marine systems and could help us better understand the relevance of habitats for species of interest. And from this realisation grew my current doctoral research.



Molly, with a juvenile lemon shark in Bimini’s ‘SharkLand’ nursery.


My doctoral thesis project is in partnership with the Bimini Biological Field Station Foundation (BBFSF, Bahamas), where I worked as a volunteer and then research assistant in 2017 and 2018. The BBFSF is a field station on Bimini, The Bahamas; and Bimini is the only mangrove fringed island on the Great Bahamian Bank. The ecological importance of these mangroves cannot be understated. Bimini’s mangroves sit at their most eastern point less than five miles from the Gulf Stream, the highway of the North Atlantic, connecting the juvenile sharks, rays, and fishes in these mangroves to the open ocean. It’s likely that these mangroves contribute significantly to the North Atlantic’s biodiversity and biomass. When I began to consider a PhD, I contacted the BBFSF, and with their support I wrote a research proposal for a doctoral thesis based in this study system.


I wanted my thesis to accomplish a few goals: 1. Show how behavioural ecology can be applied in a marine ecosystem to permit greater understanding of species behaviour; 2. Show how these findings could then be applied in conservation planning; and 3. Support the BBFSF long term effort to quantify the biodiversity of the mangrove nurseries of Bimini, to help establish a Protected Marine Reserve.


To accomplish these goals, I needed a species that is trackable, well-studied, wild, marine, and, if possible, charismatic. Enter the juvenile lemon shark. The BBFSF has long studied the biology of the juvenile lemon sharks. And for good reason – this subpopulation of North Atlantic lemon sharks show natal philopatry, which means adult pregnant females come back to the same mangroves they were born in, to give birth to their ‘pups’. These pups live in the mangroves until they are over a metre in total length, which takes up to five years, sometimes even longer. My project will measure the environment in which these juveniles grow, socialise, and learn, to try and identify what is driving their habitat dependence – is it to avoid predation? Larger sharks are known to be nearby, and these pups are snack-sized to many oceanic predators. Or is it to access resources? The mangroves host a range of fishes, crabs, and other invertebrates, as well as other sharks and rays! These types of wildlife, in their juvenile stages, are ideal snacks for the pups. Or could the social environment of all these pups living together in the mangrove roots, explain their long-term dependence on this habitat? These pups form friendship groups and exhibit social learning; this is thought to facilitate survival, and help individuals grow to adulthood. Any or all of these theories could play a role in their preference for the mangroves; and, for planning and management, it is useful to know why.


My goal, with this work is to provide a new way to evaluate marine spaces for conservation planning and management, by incorporating theories of risk, resource, and sociality. I want to bring pure theory into the wild and define a new research framework for ecologists studying marine ecosystems, to hopefully facilitate their use of this idea in mangrove nurseries and other critical marine habitats all over the world.


And that is why I love lemon sharks, specifically the pups that live in these mangroves, because they offer me the opportunity to show the efficacy of this idea, while working in a habitat system that has been special and familiar to me since the first time I saw them, over twenty years ago.



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