Grouper and snapper species are common in the deep-reefs of the GBR and form the basis of deepwater fisheries throughout the Indo-Pacific (Epinephelus morrhua and Pristipomoides filamentosus.) (Credit: Tiffany Sih)
Until recently, it’s been difficult for scientists to monitor, inventory, and study deep water fishes. Yet these species are critical to understanding threats posed by climate change, fishing pressure, and pollution, among other factors impacting marine life. Now, teams are using newer technologies to access and document fish abundance and diversity among deeper reef settings.
Tiffany Sih, a PhD candidate from James Cook University, has used Baited Remote Underwater Video Stations (BRUVS) with lights to sample deeper habitats (54–260 m), in the Great Barrier Reef (GBR), Australia. Sih corresponded with EM about her recent work, the first study of its kind looking deeper than 100m, and what inspired her to take the deeper dive.
“As scientists, we have to keep pushing the boundaries of what we study, by improving our techniques and using newer technology,” Sih explains. “These ‘Twilight Zone’ depths are receiving greater attention worldwide because of a fascination for the new and unfamiliar. These deeper extensions of the reef are below the traditional deepest threshold (30-50m) of SCUBA operations, so these ecosystems are harder to access and also harder to study. The Great Barrier Reef is one of the most visited and studied marine ecosystems on the planet—frequented by many fishermen, divers and scientists—yet by going deeper and taking a closer look, we are finding new species of fish and corals simply hiding in plain sight.”
Before beginning her PhD studies, Sih lived in Hawaii, whose deep reefs have long been targeted. Both fishing operations and those hoping to exploit precious resources such as black coral for jewelry have permanently changed these deep water reefs in Hawaii.
“When I spent time in Australia, I was surprised that few fishermen focused on deeper fisheries,” remarks Sih. “Australia has very abundant coastal fishing, but in other places like Tonga and Hawaii, the fishing pressure has moved further offshore and into deeper habitats below 100 m. I saw an opportunity and also a need for better information on the fish communities at deeper depths.”
Sampling in deeper water has its challenges. For one thing, it doesn’t work without equipment that can withstand major changes in pressure and compensate for light that diminishes rapidly with depth. Additionally, researchers end up sampling where they can’t know in advance what the conditions will be hundreds of meters beneath the surface.
“I could spend months fishing and tell you what comes up at the end of the hook, I could spend a lot of time in a submarine (I wish!), or I could use some video cameras and efficiently sample many depths and locations,” quips Sih. “I decided on the last option and used Baited Remote Underwater Video Stations (BRUVS) rated for depths up to 300 m. I was fortunate to collaborate with a mesophotic coral specialist Dr. Tom Bridge of the Queensland Museum, and marine geologists Dr. James Daniell and Dr. Rob Beaman, both of James Cook University, for this fieldwork.”
Thanks to her well-equipped deep dives, Sih found species that had not previously been recorded on the GBR. She was also able to make use of the video she captured, sending it online so she could immediately confer with other fish experts to identify unexpected species.
When I recorded fish I did not recognize, I was able to email images across the globe!” Sih exclaims. “From Australia to Europe, specific fish ichthyologists were consulted based on their area of expertise. I reached out to scientists who specialized in damselfish, wrasses, goatfish and sharks. For instance, Martin Gomon and Fenton Walsh were working on describing one of the species I recorded, a new species of deep-reef Bodianus commonly referred to as ‘hogfish.’”
The attention to the previously unexplored depths of the GBR—and the international collaboration—paid off.
“In this study we found over a dozen new species records or potential new species, which are fish no one could identify, but the experts believe is a likely candidate for a new species,” details Sih. “Some of the new species records were the Y-patterned Moray eel, the Japanese Puller or brightly colored Pink-banded Fairy Wrasse, which have been found in other locations, such as the nearby Coral Sea or on the other side of the country in Western Australia. Damselfish such as the Gold-rim Chromis and Okinawa Chromis have been found in other countries such as Yap or Japan, and while it is likely they could be found in many more places between these sightings, there is still a lot of ocean to explore.”
Sih identified quite a few species at deeper depths than previously reported; for some of these fishes the difference may be marginal, just a few meters, while for others there are extensions to previously recorded depth ranges that are notable, more than 20 meters. These might seem like nit-picky details, but in fact may be very significant.
“The truth is we still don’t know how many fish species perhaps use deeper habitats, and for what purposes,” Sih remarks. “Sharks are known to swim in between shallow and deep waters, and a few were seen in the videos. Deep reefs might be important ‘refuges’ where large fish, such as sharks, can go to reproduce or hide from fishing pressure. Fish may also swim deeper to avoid warming waters, or to follow sources of food, so good record-keeping may be useful if how or where fish are change over time.”
Sih also identified new species using BRUVS.
“We think we may have found evidence of at least three new species, from the genera Selenanthias, Bodianus, and Chromis,” Sih describes. “These are fish that did not match any existing known species; however, we may have seen more than that! For some records, the image quality was too poor for positively identifying potential new species; either the fish was too far away from the camera, the lighting was not ideal, or the fish swam too quickly, and we preferred to conservatively record what we saw, so we left out some ‘unknowns.’”
Of the specific aims Sih had for the research, she found many answers—and even more questions.
“We found that both richness and abundance decrease with depth, which we expected from deeper habitats with similar depth gradients from other locations worldwide, but what we found was that the fish community really did change over the range we sampled, but some species had more narrow depth ranges while other species inhabited a broader range of depths,” Sih clarifies. “In this way, not all depths are created equal, and there is a depth-specific stratification to the fish community. It would be reasonable to pick out a few of the fish we found to be good indicators as representative of the fish community to be able to monitor or sample similar locations and depths.”
Although this research achieved a great baseline, the sampling also suggests that there is more work to do in terms of describing diversity of fishes in multiple locations.
“Temperature along the thermocline was just one of the possible drivers of change along the depth gradient, and similarly, it shows that we have more work to do!” Sih asserts. “Sampling the local oceanography may help us to better understand where and how fish are distributed in deeper habitats.”
“Mesophotic” simply refers to how much relative light is available, so mesophotic reefs are typically at depths of 30 to 150 meters. The GBR is a mostly clear, tropical location that can be viewed without added light depending on the clarity of the water on any given day. Mesophotic reefs are strongly linked to shallow reef communities, which means that both fish and invertebrates in both kinds of reefs rely upon a healthy, interconnected ecosystem.
“While there is now some evidence of limited genetic connectivity between shallow and deep we do not know if they can survive without the other,” explains Sih. “I believe there is some truth that larger, more mobile fish use deeper habitats regularly, but this study was a first look, which opens up a lot more follow-up questions.”
As shallow water reefs are harmed, deeper reefs might become refuges to various species. If this becomes true, it isn’t entirely clear how this will affect the current ecosystems of these deep water reefs.
“Shallow reefs are generally thought to be under more pressure—from climate, from fishing, from boats setting anchor, et cetera,” Sih details. “They are more accessible and more easily fished. Mesophotic scientists have often recorded larger fish in deeper habitats, but what is not known is whether they were originally in shallower habitats and got ‘fished out,’ or whether larger/older fish move to avoid heavy fishing pressure at shallower depths.”
Furthermore, the idea of the “refuge” can mean more than one thing.
“Some mesophotic scientists hypothesize that mesophotic reefs can provide spatial refuges, places for fish to go if under pressure or if habitat loss becomes too great, but also there is the question of replacement refuges,’” Sih elaborates. This refers to the idea that if a reef bleaches, dies, or gets wiped out in a cyclone, will its neighboring reef be able to ‘seed’ it with new coral recruits, new habitat-forming sponges, and newly settled reef fishes? I use ‘refuge’ in both senses of the word.”
Every method has its inherent biases and limitations, and the BRUVS apparatus is no exception. The benefit of using BRUVS is that these biases and limitations are generally well-understood, and, of course, the system also offers many benefits. One of the potential biases using BRUVS is counting errors related to over-counting.
“We used MaxN, or the maximum number of individuals we could identify in one frame,” Sih explains. “This is a conservative estimate, and likely when we were counting fish there were many more swimming just outside of the frame or behind the camera, but in order to not over-count individuals appearing and re-appearing, we only used MaxN.”
The bait plume of the BRUVS apparatus can also present its own problems vis-a-vis the relative amount of attraction it presents to the video.
“To overcome this bias we placed the stations far apart so we would not re-count individuals,” Sih states. “We were also fortunate to have clear water, which made it easier to count and identify fish, and standardized the field of view. For other studies where the water clarity really varies it becomes more essential to estimate how much you can see.”
Sih’s goals also included comparing results from place to place, and sampling across various fish families.
“We knew we were not able to sample all species equally, especially shy, rare or cryptic species,” Sih admits. “Most methods would have similar biases, and BRUVS information could make a good comparison. Some of the most important aspects of the study are reproducibility and archiving information. BRUVS have been used all over Australia and it made the most sense to be able to compare what we have recorded here. By saving images to specific locations we or other researchers can go back and confirm previous sightings, compare images for species identification, describe the surrounding habitats, or answer different questions in a future study.”
In all, working in deeper waters has been an exciting experience, one that Sih recommends, with a few caveats.
“The most helpful piece of advice I would give about working at similar depths with this type of equipment is to have plenty of line!” Sih remarks. “Currents can and did change suddenly during our deployments, and while it originally felt like excess protocol to have ‘twice the amount of line’ per specified target depth attached to each BRUVS unit, at times the full load of line was carried by current and the floats strained to be at the surface. The surface currents often did not reflect what the current was doing at 100 or 200 meter depths. Also, temperature changed dramatically from the tropical air at surface-level to the depths we targeted. We taped desiccant into our depth housings to prevent condensation and fog in our units.”
Sih will soon be taking more deep dives to explore more reefs, and it will be fascinating to see what her team uncovers next.