I woke up yesterday morning excited to start the day. We had arrived at Pearl and Hermes Atoll, home of some of my all-time favorite dive sites in the Northwestern Hawaiian Islands. A glance out out port hole made it appear that the sea was flat and calm, but by the time of the morning safety briefing, the winds had picked up, the seas were choppy, and there were scattered squalls around. But even the prospects of inclement weather couldn’t sour my mood, especially as a rainbow formed over our deep dive team during the safety briefing.
With two full days of diving ahead of us at this expansive atoll, I was confident that we’d find all sorts of interesting things on the deep coral reefs. As Jason, Dan and I plunged off the side of Metal Shark, I was struck by how clear the water was! As we descended through the emerald-blue water, within a few moments I could see the bottom at a depth of about 200 feet (61 meters).
On our way to the bottom, we plunged through several thermoclines — abrupt changes in water temperature — and it got progressively colder as we descended. By the time we got to the bottom, the water was icy cold! Our dive computers registered 61° F (16° C), but we suspect that the water was actually colder, but our computers didn’t have enough time to reach equilibrium. Whatever temperature it was, it was certainly chilly! However, my concern for the cold quickly faded as I saw many “old friends’ swimming about the reef.
By “old friends”, I’m referring to species of fishes that are rare or extremely deep in the Main Hawaiian Islands, but much more abundant and at shallower depths here in the Northwestern Hawaiian Islands. First and foremost among these is the beautiful Masked Angelfish (Genicanthus personatus). This species is only found in the Hawaiian Islands, and while it is known from only a handful of collected specimens and observations in very deep water in the Main Hawaiian Islands, it is often encountered in the Northwestern Hawaiian Islands at depths of 50-200 feet (15-61 meters). It is highly prized by aquarists, and was recently bred in captivity by Karen Brittain (among the most exciting news in the marine aquarium would in years!)
Another “old friend” I often see up here in the Northwestern Hawaiian Islands is the stunning Dragon Moral Eel (Enchelycore pardalis). Another species highly prized by aquarists, this species is much more common up here than it is down in the main islands. In fact, I saw two of them yesterday.
Yet another species that is deep and rare in the Main Hawaiian Islands but much more frequently encountered (and at shallower depths) up here is the Whiskered Armorhead (or Boarfish; Evistias acutirostris). At the very end of today’s dive, I saw three juveniles grouped together hiding among some rocks near the base of the ledge. Adults are rare enough to find, but juveniles are even rarer — so this was a particular treat for me.
Old friends aside, however, the most exciting find here at Pearl and Hermes (indeed, of the trip so far) was a “new friend”. Half-way through yesterday’s dive, I saw a small wrasse that at first seemed to me to be an Elegant Coris (Coris venusta), but I quickly realized it was not that species. There was onle largeish individual about 5 inches long, and about a dozen smaller ones in the general area. The larger one had a different color than the smaller ones, which is typical of many species of wrasse. The larger color form is typically the male of the species, and the smaller ones are females. After sending photos of this mysterious fish to my ichthyological colleagues, the consensus seemed to be that it was either a Threespot Wrasse (Suezichthys notatus, a species common in Japan but dubiously reported from the Hawaiian Islands), or an undescribed species similar to S. notatus. Fortunately, I was able to collect three individuals today (one male, and two females), so perhaps we’ll be able to sort out the true identity of this “new friend” after we return from the cruise.
In general during our deep dives we are focused on the small, cryptic fishes that scurry about the reef. The reason for this is that the large predatory species are reasonably well known from a long history of fishing activity, so we focus on the more poorly-known small species when we are diving. But in some cases, it just not possible to ignore the larger fishes. This is true, of course, for the many sharks we’ve regularly seen on these deep dives. But it’s also true for the aptly-named Giant Trevally (Caranx ignobilis), known locally by its Hawaiian name “Ulua”. Populations of this species are much higher here in the Northwestern Hawaiian Islands than they they are in the Main Hawaiian Islands (likely a result of centuries of fishing pressure in the inhabited islands). They show absolutely no fear of divers and will swimm right up to us and circle around. The often follow us up as we ascend towards the shallows for decompression.
Speaking of decompression, I have referred to this phase of the dive often in these blog posts. After we descend to the deep reefs at depths of 200-280 feet (61-83 meters), the helium and nitrogen in our breathing mixtures dissolve into our blood and tissues at elevated concentrations, due to the increased pressure at depth. If we were to ascend directly to the surface after spending some time at depth, this dissolved gas could cause tiny bubbles to grow within our blood and tissues, causing anything from mild pain to rashes, nausea, visual disturbances, numbness, paralysis and death (among other possible symptoms). This malady is formally known as “Decompression Illness”, and popularly known as the “bends”. I suffered a severe case of the bends when I was 19 years old, so I’m particularly cautious when it comes to the decompression phase of the dive.
Decompression is complicated for two basic reasons. First of all, there is an imperfect understanding of the physiology involved with decompression sickness. It’s reasonably well established that gas molecules dissolved under pressure contribute to bubble growth when a diver ascends too quickly, but the exact mechanism by which those growing bubbles can lead to such a wide assortment of symptoms is understood only in the most basic ways. We use several different kinds of dive computers on these dives, with different algorithms for calculating decompression schedules, and we always follow the one that recommends the most “conservative” profile (i.e., longest decompression time). The second reason the decompression phase of the dive is complicated is because of logistical needs. In many cases, such as our recent Pohnpei expedition, we decompress from deep dives along a vertical reef drop-off, so it’s easy to stage emergency gas supplies on the reef itself. However, here in the Northwestern Hawaiian Islands, the decompression occurs in open water. This means we have to rely on the surface support team to provide us with additional emergency gas if needed. Thankfully, we have an EXCELLENT support team on this project, including safety divers Keo Lopes and Senifa Annandale, as well as our excellent coxswains, Scott “Scotty” Jones and Hadley Owen.
Without a coral reef to explore, these “blue-water decompressions” can be pretty boring. However, here in the Northwestern Hawaiian Islands, they can sometimes get pretty exciting. At Lisianski, we had the excitement of sharks on the decompression. Although there are plenty of sharks here at Pearl and Hermes, the Giant Trevally are much more prominent here. In fact, we had a bit of a deja-vu experience on the decompression line yesterday. Because there were nowhere near as many sharks here as at Lisianski, I felt confident that I could do my usual swim during decompression without fear that my unattended reel would be eaten. However, just as the Galapagos Sharks had taken a strong interest in my dangling reel at Lisianski, the Giant Trevally did likewise here at Pearl and Hermes! Fortunately, the large fish did not manage to eat or otherwise make off with my reel, and things settled down pretty quickly after I returned to guard it.
In truth, yesterday’s decompression was one of the most enjoyable in recent memory. We were swarmed by dozens of Giant Trevally, as well as Galapagos sharks and one of the largest schools of Bluefin Trevally (Caranx melampygus) I’d ever seen. The water was incredibly clear, and our decompression carried us over shallow water (~50 feet/15 meters), so we could see the bottom very clearly. I was almost sad when my decompression computer finally cleared, allowing me to ascend to the surface (almost…). As we were preparing the gear for the drive back to the Hi’ialakai, I noticed many more sharks circling the boat than I had seen just minutes ago while decompressing. Suddenly there was thrashing at the surface, with shark’s tails and dorsal fins cutting through the water at lightening speed. Several of us noticed strong odor of fresh fish in the air, so it appears that after we left the water, some poor fish became lunch for some hungry sharks.
Our next stop is Midway Atoll, site of one of the most decisive battles in navalhistory. It also happens to be one of my all-time favorite islands in the world! We’ll be back to Pearl and Hermes for three more days of diving on our return trip back towards Honolulu, so I’ll have more to report in a few days.