Lionfish research in the Caribbean waters of Curaçao
In a lot of ways, deep reefs are understudied. Too deep for divers to reach and only accessible by submarines, this zone of ocean habitat is often overlooked. Combining deep reef research with scuba diving around shallower reef areas, SAFS master’s student, Sarah Yerrace, is working in the waters of the Caribbean Island of Curaçao looking at one thing in particular: lionfish.
Red Lionfish (Pterois volitans) were previously thought to only inhabit the top 100 meters of the ocean, which is their usual range in their native Indo-Pacific Ocean. However, when studying deeper depths and coral reefs found between 100m and 300m in the Caribbean, lionfish were present and active. UW coral reef research in Curaçao has been underway for more than a decade in partnership with the Smithsonian Institution, conducted by the Fish Systemics and Biodiversity Lab of which Sarah is a member, led by SAFS Professor Luke Tornabene.
Why would the presence of lionfish on a deep reef be considered a problem? Named as a super predator, the species wreak havoc on marine biodiversity by preying on all fish inhabiting reefs and fringe areas, multiplying quickly in population. But when looking at understudied deep reefs, lionfish have also been found to be preying on marine species not even yet discovered, presenting a bigger problem.
One of the big questions Sarah is tackling in her work is if lionfish are swimming one way, from shallow to deep waters, or back and forth? Lionfish are unidirectional as they age, migrating from the shallows to the depths as they grow older. Usually, younger lionfish are shallow-dwelling creatures with a high tolerance for freshwater and estuarine environments. But is this downwards movement to deeper depths an extension of their natural migration in their lifespan, or because they have pressures in their invasive range, such as spearfishing?
Spearfishing with a single pole is the only legal way of capturing and managing lionfish populations in Curaçao, with the fish used in a variety of ways, from eating in local cuisine, to using their fins to make jewelry.
“Different species live in different depth ranges. If I collect a lionfish at 100m, with another fish that is only known to live in the top 30m of the ocean in its stomach, that is evidence that the lionfish moved from 30m to 100m within the last few hours. How do I know this? Because that is within the timeframe it takes for a lionfish to digest their prey,” Sarah shared. “The vice versa scenario is the same: if I collect a shallow lionfish with a deep-water prey species in its gut, the lionfish has moved deeper and then back up.” A diet study using lionfish collected below 100m hasn’t been done before, so seeing what they are eating at all below this depth will be new information for researchers.
Another facet of Sarah’s work is exploring the use of otoliths – the ear stones of fish – to see if lionfish are moving from shallow to deeper waters and back, but over longer time scales, i.e., longer than the time of digestion and therefore not detectable by looking at prey species. “Otoliths have rings like a tree, and as the fish grows, the otolith grows in rings. Each ring is made using minerals from the water the fish is living in and so we can sample the rings of the otolith for stable isotopes,” Sarah said. “Different stable isotopes tell us information about the water the fish was living in when that ring of the otolith was formed. From stable isotopes, we can tell how warm or cold the water was. So, as we sample from the center of the otolith, which is when the fish was young, to the edge of the otolith (most recently), we might see a steady progression of cooling temperatures, meaning the fish moved deeper over time, or a random, non-linear pattern if the fish is moving up and down,” she added. Sarah is working on this part of the project with UW Marine Biology undergraduate, Alyson Liu, who is doing a test run with a few pairs of otoliths in the lab, in the hope of providing some evidence for this theory.
First visiting Curaçao in 2019, Sarah is returning for the third time this summer in 2024 to collect more data. Carrying out her research both in a submarine and when scuba diving, Sarah conducts three dives a day with a partner while in Curaçao. “Lionfish are most active during dusk and dawn for hunting, so when we are trying to collect them while scuba diving, we dive at sunrise or sunset,” Sarah said. The submarine, called Curasub, spends 4-6 hours in the water during the middle of the day. During her dives, Sarah also works with another SAFS grad student, Juliette Jacquemont, conducting transects to characterize the bottom habitat, plus collecting fish data. “I’ll use some of the data Juliette is focusing on for her work to compare what is present on the reef versus what I find in the guts of fish,” Sarah shared. “During our last trip, in five days of diving, we counted more than 5,000 individuals of over 50 different species.”
When on the submarine, which reaches depths of 300m off the coast of Curaçao, the research process is a little different. “The submarine is equipped with hydraulic arms, and the submarine pilots attach a speargun to the front. Once the pilot has carefully maneuvered the 6-ton sub to line up the shot, using the arm, we can trigger the spear gun from inside the sub and then use an anesthetic to put the lionfish to sleep,” Sarah said. “Using a second arm, we remove the lionfish from the spear and scoop it into a holding basket below the sub. But it’s a delicate dance, as sometimes the fish wakes up and tries to escape in the process of putting it in the basket.” When back at the surface, the fish are placed on ice and immediately processed, with depth of capture, length, and weight recorded. “The next step is to dissect the fish and record if it’s a male or female, then remove the stomach. The stomach is put in 90% ethanol and kept frozen until I can do DNA extractions back at UW,” she added. “One of the benefits of conducting submarine research off Curaçao is because the ocean gets deep really quickly. We can essentially move down a wall versus needing to travel across a greater horizontal distance.”
Outside of her research, Sarah is an active diver – both recreationally and professionally. “There is so much to love about diving. The dive community is wonderful, and I’ve met lifelong friends through diving. I also think the physics, physiology, and science of diving is fascinating,” Sarah shared. “No matter what ocean I’m in, or the objectives of the dive, there’s always an incredible sense of adventure. Floating weightless and doing science underwater is the next closest thing to being an astronaut on another planet. It doesn’t get better than that.” Sarah is currently a TA for a scientific diving course offered at UW. Hoping to defend her master’s thesis by the end of 2024, Sarah’s currently writing her thesis and perfecting figures that will go into her paper.
