We named her Wendy, the unofficial mascot of the driven and intelligent students at my school. A long, silver flash in the water, Wendy was the largest barracuda I had ever seen. She had piercing eyes that observed every movement, and she loved to watch me and my classmates as we surveyed the reef for fish. After a week of being followed, we got used to what had originally been unnerving. But while Wendy would come up to us and stare us in the face, the smaller fish we were seeking would dart away, finding cover in little crevices of the coral. By the end of the week, we hadn’t all seen the same types of fish, but we had all seen Wendy. But what about those other fish? What if we had been looking for them specifically? A new study about fish communities in coral reefs explains how fish react differently to divers. The results have serious implications for how we study fish populations in coral reefs and how we understand the information we already have.
The paper, written by scientists at the Australian Institute of Marine Science and Dalhousie University, critically analyzes the process by which we study how many fish are in coral reefs. Underwater visual surveys (UVS) are one of the most common methods for doing this. During a UVS, a diver swims through a section of a coral reef, counting the number of fish they see, identifying them, and sometimes estimating how large the fish are. It takes an experienced diver who can identify a number of fish. However, the diver can only record the fish they see. If some species of fish are shyer or better at hiding, it is possible that the diver will underestimate how many of these fish are living in the reef, no matter how experienced they are. What Dr. Michael Emslie and his colleagues found was not only are some fish warier of divers, but they may hide long after divers have come into the reef.
In order to investigate how accurate these surveys are, the researchers decided to focus on five sites off the Great Barrier Reef. The areas, while not completely remote, were visited only infrequently be tourists and fishers. At each site, a lead diver would swim through counting and identifying fish, while a second diver followed, laying down the transect to mark the path the first diver followed. They did this ten times at each site. After waiting for a variable amount of time, the divers would come back, perform another survey, and count and identify the fish they saw once more. Fish were categorized within seven different groups (see Figure 1). The counts were then compared between the first and second surveys performed at each transect. The rates at which fish returned after the first divers were inferred from the relation between the two surveys.
What they found was that fewer fish and fewer species were identified on the second round of the surveys. The total abundance of the fish, the number of species identified, and the abundance of four of the families of fish were all smaller than what was estimated from the first survey. The most anxious fish groups that took the longest to recover included surgeonfishes, rabbitfishes, predatory wrasses, and parrotfishes. Not only this, but it could take hours for the number of fish seen in the survey to return to normal. Predatory wrasses, for example, returned to their original numbers after 222 minutes, but the total fish abundance wouldn’t recover until after 1022 minutes.
These findings have important ramifications for how we study coral reefs and assess fish populations. While some groups, like sea basses and snappers, may be less impacted by divers, other species are likely to be underestimated simply because they get scared and hide. Coral reefs will need to be monitored going forward as the threat of climate change looms, but just because we are monitoring closely doesn’t necessarily mean we are getting good data. If the reefs are in areas where tourism is more popular or fishing more extensive, it is possible that some of the shyer species will never be fully estimated. This could leave us bemoaning the loss of fish before they are even gone. Additionally, some studies will lay down tape along their study transect before they survey the fish. This small action alone could skew their results. If researchers want to gather the best data possible, they will need to keep these more bashful fish in mind. Otherwise, they may end up seeing all the Wendys, but miss the parrotfishes.
I am a PhD student studying Biological Oceanography at the University of Rhode Island Graduate School of Oceanography. My interests are in food webs, ecology, and the interaction of humans and the ocean, whether that is in the form of fishing, pollution, climate change, or simply how we view the ocean. I am currently researching the decline of cancer crabs and lobsters in the Narragansett Bay.