Sunday saw us retrieving the last of our acoustic equipment from the lagoon at Pearl and Hermes atoll, which went very well. All through the dive Lauren and I had been admiring the half-dozen or so galapagos sharks that had shown up. We sent the hydrophone to the surface and watched them swimming around for about 15 minutes. As we ascended the sharks became more curious, reducing the distance between them and us. Being in blue water and surrounded by sharks and big alua was very special – so special that our coxswain and one of the science outreach team got back in the water for some snorkeling. Soon, the coxswain suggested they be picked up as the sharks were becoming very curious. After they got in the boat, we dropped over the side for a look. Initially, there were around four sharks visible in the very clear water. The bottom could be seen clearly, easily 30 m below. Slowly, the number of sharks began to increase. In almost no time at all there were around twenty galapagos sharks below our feet, swimming close by and inspecting our flippers. Lauren reports that this was one of the most amazing experiences of her life. Looking down on the school of sharks was absolutely incredible, and the visibility was perfect so we could see them all the way to the sandy bottom.
We motored overnight to Kure and were back in the water first thing Monday morning. We’re back from our first day’s diving at Kure Atoll, the northernmost atoll on the planet.
Initially formed much further south in warmer waters, Kure has been slowly transported north by the tectonic motions of the pacific plate. Sometime in the future, conditions will presumably be too cold for corals to grow. Right now, it’s hanging on, barely at the edge of the tropical currents that sweep through the Hawaiian Islands. Consequently, the species we see here are like what you’d see in Northern New Zealand!
Our first day of diving at Kure was very nice (Lauren says it was idyllic). Both topside and underwater colors were vibrant and bright, and most of the views we saw looked like they could be used for postcards. The term “gin clear water” describes well what we saw off the southern side of Kure atoll. Underwater visibility is around 50 m, making the search for an ideal hydrophone deployment site a task we could do from the boat, rather than having to get in, dive down, and swim around.
We motored over an expanse of spur-and-groove coral habitat, and an ideal section of white sand straddled by two high spurs of coral reef was selected as a candidate site for our equipment. Diving down, a few galapagos sharks immediately came in to view. They stuck around but their numbers did not swell to what we had seen the previous day at Pearl and Hermes. The white sand was deep enough for our sand anchors – a critical aspect we needed to check out before the flyby array was bought down to the sea floor. The coral spurs adjacent to the sand were teeming with a large number of different fish species – there were hundreds of fish, but it seemed that there were very few which were alike.
The tropical species which are so familiar to us were there (surgeonfish, squirrelfish, angelfish, parrotfish, wrasse), but they were also joined by subtropical varieties (morwong, boarfish, endemic butterflyfish, etc). Interestingly, big snapper hung out near one of the overhangs, a good sign that fishing had not really taken place here in some time. The coral situation was an interesting one – the large spurs of rock on which coral was growing are clearly made from calcium carbonate, or dead coral. This suggests to us that at one stage massive reef-building corals existed here. Nowadays, however, small porites colonies dot the surface of the rock, too small and sparse to build reef, but enough to maintain habitat for the animals which rely on live coral growth. This was the first site on this cruise where Lauren spotted one of her favorite sea slug relatives, a neon yellow and black flatworm.
The flyby and another hydrophone were put in the water and bought down to the bottom in short order. Working on the sea floor here was a pleasure not just because of the stunning scenery, but because it was also so calm! Our time at Kure is the first time on this cruise we’ve experienced good weather. The surface was mirror calm, the swell was of low amplitude, long-period, and manageable. We took a number of photographs of the array deployment, which runs as follows:
1) Assemble the hydrophone cable, battery pack, and data acquisition computer together. 2) Power on (don’t forget!). 3) Simon and Lauren kit up and enter the water. 4) The coxswain places the cable in the water, and lifts the computer/batteries to the boat gunnel. 5) The array is lifted from the gunnel and lowered into the water, Simon guiding it in while the coxswain does the lifting. 6) A rope is tied to the array and the divers descend to one of the sand anchors. 7) The rope is strung through an anchor eye and the array (very buoyant) is slowly pulled to the bottom. 8) As the array reaches the bottom, Simon maintains tension on the line while Lauren ties the array off to the sand anchor using a shorter rope. As soon as this is done, Simon can let go and relax! 9) The array cable is strung out and fixed to other sand anchors, the array geometry being carefully measured so we’ll be able to process the data with greater accuracy later on. Retrieval is the reverse of deployment – the only difference being that it’s much harder to bring the array back on board the boat than it is to put it in the water. Hopefully Wednesday will be just as calm!
Once the array, another hydrophone, and our camera ‘tree’ were in the water, we moved site to deploy one more hydrophone and a couple of cameras in the inside of Kure lagoon. Arriving on site inside, it was immediately clear that things were very different here. The slow ocean swell was gone, replaced by some confused wind chop (wind and swell were in opposing directions). The clear blue water outside was gone too, and in its place was something more greenish. The
environment on the shallow lagoon floor was like what you would experience in a harbour. Fine silt, lots of dead coral, many small fish, including juvenile versions of what you’d find outside the lagoon. Visibility was worse but not bad. The temperature was considerably warmer at 28 degrees (as opposed to 25-26 outside). We positioned our equipment and began surveys. What initially looked to be piles of dead coral rubble turned out to harbour lots and lots of small critters – many kinds of reef fishes (including a boxfish!), cone shells (venomous), juvenile fish, rock lobsters, polychaetes, sea cucumbers, and some pufferfish! While definitely a dive where the small things were of greatest interest, the lagoon was an equally fantastic place.
Its our day off today due to the boat shortage. We’re taking advantage of the down time to sleep in 🙂 enter data, back up data and photos, update our notes, and of course write a new blog! We’re planning to head back out tomorrow morning for a half day to retrieve our gear before the Hi’ialakai steams back east towards Lisianski tomorrow night.