My avid fascination with the ocean started at a young age. From my earliest memories of snorkelling I know I was captivated by the intricate beauty, elusiveness and complexity of marine ecosystems, and I have naturally been drawn to sharks as a manifestation of these traits.
While studying abroad and doing research in Australia, Palau and the Northern Line Islands during college, I experienced the unspoiled splendour of protected reef ecosystems and observed at first hand the impact of destructive human activities on them. I realised that I didn’t want to study coral reef ecology or sharks just for the sake of doing science, but rather to enhance conservation and management efforts, particularly within local communities.
However, it quickly became apparent that I was dealing with shifting baselines. Even the most pristine ecosystems I visited had not entirely eluded the human imprint. This prompted me to approach the issue of coral reef health and resilience from a different perspective. Instead of focusing solely on the modern ecology of the reefs, I turned to the past to gain a better understanding of their present predicament and thus ensure their future protection.
This led me to get involved with a marine palaeoecology project at the Smithsonian Tropical Research Institute – to which I currently belong – that had not yet begun to explore shark baselines. Having previously studied the tremendous role that sharks play in shaping modern-day reef communities and immediately recognising the conservation potential of this project, I sank my teeth wholeheartedly into the opportunity to investigate how shark numbers have changed over the entire timescale of human interaction with the ocean. I am now referred to as the ‘shark girl’ of the lab. Having been obsessed with sharks as a child, it seems I have serendipitously come full circle.
Mounting evidence demonstrates that shark populations worldwide have been decimated by overfishing, reef degradation and pollution. However, how can we effectively restore them if we don’t know what constitutes a natural, ‘pristine’ population? This question is particularly relevant in the Caribbean, where the human imprint on shark communities is ubiquitous. While diving on reefs around the Bocas del Toro archipelago in Panama for my research, I was struck by the absence of sharks. Reef surveys from myriad other locations across the Caribbean have reported a similar scarcity of sharks, contrasting sharply with the considerable body of anecdotal, historical and ecological evidence that suggests sharks there were once present in numbers unheard of today. To reconcile this age-old problem, our study takes a novel, paleoecological approach. It turns out that the story of the missing sharks could be in the sediment.
To unravel this tale, we venture back in time. Intriguingly, our ‘time machine’ is a suite of mid-Holocene reefs in Panama and the Dominican Republic that date back 6,000 to 8,000 years and have been exquisitely preserved in situ.
Imagine strolling along a dry, flattened coral reef. Although the living corals, invertebrates and fish are long gone, their remnants have been preserved like artefacts in a museum. Everywhere you look there are fragments of fossilised branching corals and massive coral heads marking the different zones of what was once the living reef. Shells are scattered about, and the occasional shark tooth or urchin test can be spotted. This reef structure continues as you dig deeper into the sediment, revealing layers of reef history deposited one on top of another. Off in the distance, mangroves and the sparkling turquoise waves of Panama’s Almirante Bay mark the horizon, demonstrating just how close in space this ancient reef is to its modern counterparts – and yet so distant in time.
Exposed and undisturbed sites such as these are rare. In fact, Panama’s Bocas del Toro and the Dominican Republic’s Enriquillo Valley are home to the only two in the entire Caribbean. Together they represent a unique ‘window into the past’, showing what pre-exploitation reefs were like when sea levels and other environmental conditions were similar to those of the present. By sampling sediment at these sites we are able to rewind reef history and assess what pristine reef communities were like before they were subjected to subsistence harvesting by indigenous communities or, more recently, the commercial extraction of fish.
Establishing baseline conditions for shark populations is central to understanding how humans have altered the abundance, species diversity and distribution of sharks over time, and can thus not only influence their conservation status, but also inform more accurate management practices when current ecosystem changes are put into an historical context. However, this requires data prior to modern surveys and historical evidence, prompting us to turn to the fossil record.
Being cartilaginous, sharks don’t preserve as complete fossils, and their teeth are too rare to document past populations comprehensively. Instead, we’re exploring whether shark dermal denticles found in fossil reefs can be used to reconstruct pre-human shark communities. Dermal denticles can be thought of as miniscule ‘skin teeth’ that line the bodies of sharks. As sharks swim and interact with their environment, their denticles fall off and accumulate in sediments, where we have discovered that they are well preserved.
The challenge lies in extracting them from this reef sediment. Not only are denticles very small – typically less than half a millimetre across – they’re also rare. Finding them involves excavating massive amounts of sediment from the fossil reefs. After collecting the sediment, the carbonate is digested using acetic acid and the remaining particles are picked under a microscope to isolate and identify the denticles. We’re also sampling comparable modern reefs across different habitats and shark abundances to document spatial variation in the denticle record and understand the process of denticle accumulation on reefs.
It’s a lengthy process, but worth the effort as this sediment has begun to reveal empirical data about the status of sharks prior to human harvesting, unfiltered by personal memories and biases. Already we have refined the laborious extraction methodology and are building a reference collection to facilitate denticle identification.
The Save Our Seas Foundation and the Smithsonian Tropical Research Institute make it possible for us to visit reef sites and undertake the large-scale sediment processing necessary to extract the denticles and hone this novel technique. This partnership has also provided the platform for us to disseminate our findings more effectively to a broader and more diverse audience, bridging the gap between science and conservation.