Whitetip reef shark cruising by our newly installed tile experiment. Photo Credit: Kristina Tietjen

Sharky Reefs

Coral reefs are amazingly diverse habitats, harbouring thousands of different species of fishes,  large charismatic sharks, and benthic organisms (i.e. ocean bottom-dwelling—including all the immobile organisms growing to make up the reef like corals, algae, sponges, etc.). These different species interact with each other, in particular through feeding interactions, which can determine which species, and how many, live on the reef. I’m interested in these feeding interactions, either herbivory (consuming plants, e.g. fish eating algae) or predation (consuming animals, e.g. sharks eating fish or fish eating corals), and how they influence the entire coral reef community. My dissertation research centers around the question: how do coral reef fishes influence the benthic community through predation and herbivory, and how this is potentially mediated by predation from large predators like sharks (Figure 1, 2)?

I’m extremely privileged to ask these questions and conduct research in the Chagos Archipelago, a beautiful and remote location in the British Indian Ocean Territory (Figure 3). Remote systems like this are ideal settings for questions about how species interact, especially when thinking about larger predators like sharks that are typically overfished and removed on human impacted coral reefs. For example, the Chagos has extremely high fish and shark abundance compared to other reefs in the Indian Ocean, primarily due to its remote location. This is definitely the sharkiest place I have ever worked- nearly every time I get in the water there is some friendly shark checking you out.

The logistics are pretty different from any research I have done before. On each trip, there are typically ~15 researchers from different universities all around the world, all working on different projects. My team is usually 2-3 people, either a fellow graduate student here at Hopkins or a research assistant. From Hopkins, it takes us nearly four days of travel to get to the vessel where we conduct all of our research and travel around to the various islands (Figure 4). We do everything on the boat- eat, sleep, prep for all the diving, and conduct all of our labwork. I love living on a boat, even with the highly limited internet, but I definitely get seasick when trying to do labwork while we are traveling across rough seas, so that has been an interesting experience! We typically take smaller zodiacs out to our study sites, and then conduct most of our fieldwork by scuba diving on these coral reefs (Figure 5, 6). It has been amazing to be able to integrate diving into my work, because it’s something I’ve loved since I started diving about ten years ago.

To address our research questions, my awesome collaborators at the Australian Institute of Marine Science and I use a mixture of video surveys and experiments. We survey the fish community using underwater video cameras, where we swim along the reef for 30 meters and then watch the videos afterwards to count and identify all the fish species (Figure 7). For predators like sharks, it’s better to use prey-baited videos (without divers being in the water!) to safely attract them and give us a better idea of what sorts of species are present (Figure 8).

To look at the benthic community, we deployed experiments on the reef in March 2019, and plan to leave them down for two years. We used underwater, battery-powered drills (super fun, highly recommend!) to install terracotta tiles on the reef, exactly like the ones people might use on their kitchen floors (Figure 9, 10).  These clay teracotta tiles mimic the natural substrate of the reef, and baby corals and other organisms like algae and sponges settle on them, allowing us to study how they grow without influencing the reef that is already there. We deployed pairs of tiles around the reef: one that is caged so that fishes can’t eat organisms growing on the tile, and another that is uncaged to allow predation and herbivory by fish (Header Photo). This way, we can compare the organisms who are living on the caged versus uncaged tiles to better understand the role of fishes in structuring benthic communities.

We ended up installing a total of 225 tiles across eight sites in the Archipelago. After allowing the organisms to settle and grow for a year, we surveyed the tiles by taking photographs, video and using UV light to find fluorescing baby corals (Figure 11, 12). After two years (March 2021), we will pick them up and use microscopes and DNA sequencing techniques to confirm the species. We’re interested in how the benthic community grows and changes, at sites with different amounts of sharks, as sharks can change how fish feed on the benthic community by 1) eating fish and decreasing their abundance and/or 2) changing the fishes feeding behaviour since they are scared of the sharks. This will help us understand the role of sharks in shaping coral reef environments, and how they might have cascading influences down to the benthic community.

Figure 9. JMI drilling a hole into a dead coral so we can use a dry wall anchor and screw to attach the terracotta tiles to the reef. Photo Credit: Kristina Tietjen
Figure 12. JMI taking underwater photos of the tiles one year after installation (March 2020). Photo Credit: Kristina Tietjen

Our overall goal is to combine all of this different data on sharks, fishes, and the benthic community to further our understanding of the role these fish and sharks play in coral reef ecosystems and to uncover and highlight what is lost when we overfish these systems and lose these feeding interactions. I’m very excited to go back to these reefs, pick up my experiment, and process all of my samples on the boat (even if weather conditions are rough) and see what I find.

Jamie McDevitt-Irwin is a 3rd year Biology PhD student at Hopkins. She studies the impacts of large predators on coral reef ecosystems. Funding for this fieldwork comes from the Bertarelli Foundation as part of the Bertarelli Programme in Marine Science, the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Women Divers Hall of Fame.