Decompression is often given a bad wrap. Yes it is long, yes you do get hungry, and yes it can get cold after 4 hours in the water. But decompression isn’t all bad. Many of the exciting talking points after a day of diving are about what we saw during decompression. During the decompression phase of the dive there is a shift from the controlled chaos of the morning preparations and dash to reach maximum depth in order to get as much of your research accomplished in the allowed 10 minutes of bottom time; to one of calm, idleness, and the anticipation of moving to your next decompression ceiling.  Decompression can last 3 or 4 hours depending on your dive profile and is a time that, due to the nature of rebreathers (no bubbles), is also very quiet, allowing the marine life to acclimate to your presence. This leaves a lot of time for idle contemplation.

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Joshua Copus poses for the camera during one of his decompression stops. Photo by David Pence.

As I “ride my ceiling” (follow my decompression stops closely) back to the surface, I find myself considering Pohnpei’s reefs, both the physical structure and the fish community that occupies them.  Pohnpei is a high island with a barrier reef, shallow reef flats (often with heavy surge) and steep walls to 150-200ft (45-60 meters), which transitions into very steep slopes down to… well, further than I care to explore. It still supports a decent community of large predatory fishes such as jacks, groupers, and sharks, unlike many other places I have dived. It harbors apparently healthy reefs with good coral cover in the shallows that transitions to a habitat dominated by sea fans and other invertebrates at depth.


One of the resident white tip sharks swims by during decompression. Photo by Sonia J. Rowley.

But the interesting (and possibly most valuable) thing I have come to appreciate as I explore the reefs across the Pacific is that every location I dive harbors a unique community of fishes. No two reefs are the same. In fact, no two dives are the same. Habitat where you might expect to find one species of fish based on previous dives elsewhere may not have them. Species that are rare at other locations can be surprisingly abundant on the reefs here. This may be part of the allure for many of us on this expedition and part of what drives us to do these deep exploratory dives in remote locations across the Pacific; aside from the thrill of seeing fish that few if any human has ever laid eyes on, to dive on spectacular unexplored habitat, and the curiosity to unravel the mysteries of these deep ecosystems. But for me it can also be a humbling lesson as I, in frustration, try to piece together sample sizes I need to complete my dissertation.

Richard Coleman, Garrett Johnson, Josh Copus and Rob Whitton (clockwise, from left) process fish specimens collected in Pohnpei. Photo by Richard Pyle.

Richard Coleman, Garrett Johnson, Joshua Copus and Rob Whitton (clockwise, from left) process fish specimens collected in Pohnpei. Photo by Richard Pyle.

My part in this expedition, and the focus of my PhD dissertation at the University of Hawaii’s Hawaii Institute of Marine Biology is to complete a genetic survey of deep reef fishes of the Pacific with the goal of understanding the biogeography and population connectivity of the mesophotic communities. Not much is known about the population structure of the fishes on these mesophotic reefs (coral-reef habitat at depths of about 100-660 feet, or 30-200 meters), or their relationship to shallow species. With funding from the Seaver Institute to complete a preliminary study in the Pacific, we hope to start to address some of the questions we have about these unknown depths. Do mesophotic communities have similar distribution patterns and gene flow as shallow reef communities, or do they show different patterns, unique to the deep reefs? How do deep reefs contribute to the shallow coral reefs, and can deep reefs provide a refuge to repopulate shallow reefs? To address these questions and others, I am collecting genetic samples from a broad variety of fish taxa (with the help of the entire team) here at Pohnpei and other locations across the Pacific.  Although it will not be soon, I hope to be able to share the results as they become available, so stay tuned for updates and future expeditions.