Monday, August 13, 2012

Exploring an Atoll

July 27-August 2, 2012

Silhouette at anchor in Tahanea atoll
Since this time is the first time I’ve ever been to an atoll, the novelty has not yet worn off. I’ve taken waaaayyy too many pictures of the mesmerizing hues of turquoise, blue-green, and jade green water and too many pictures of the picturesque motus (islands), surrounded by coral fragments, making up the border of the atoll’s central lagoon. 

You can think of the structure of an atoll as a series of concentric circles or ovals (depending on the shape of the atoll.) To get an accurate picture of the atoll, however, you must think of these concentric rings as ultimately being attached to one another by the coral reef; they only appear separate due to their relationship to sea level. The true, outer (ocean-side) border of the atoll is formed by a fringing reef. A chain of green motus sitting on top of the reef---strung together like beads on a necklace*---forms the next layer in and the more visible border of the atoll.  The motus are separated from each other by shallow lagoons with colors that remind me of geothermal pools, and the surf can sometimes wash over the low fringing reef into the lagoons between the motus. 

*Original source unknown:  I have seen this analogy in several places.     

Lagoon between motus
Black tip reef shark in shallow lagoon
The motus border a large, central lagoon, the sunken caldera of a former volcano, which can be over 100 feet deep and miles across. A good portion of the lagoon side of the motus is also lined with reef---but luckily, not all of it, so we can still land a dinghy ashore. 

View across the lagoon on a fine day

View of motu from lagoon
The dark blue shadows on the turquoise water are coral heads underneath the water
We spent a couple of days anchored on the northeast side of Tahanea’s lagoon following our arrival. After resting up from our passage, we explored the nearest motu. 

Patrick securing Shadow
We reveled in the colors of the water at the lagoon edge…

A reef on the lagoon side
….and observed the construction of the atoll from both the lagoon and ocean sides. The motus are fairly narrow in width, but the dense vegetation makes it difficult to cross through them in most places. Unless it is a really large motu, however, it is “easy” to walk all the way around the island. Easy is in quotes because walking around a motu requires walking over uneven ground composed of coral fragments and large, jagged blocks of dead coral. On Tahanea for the most part, these were no white sand beaches. 

Edge of reef on lagoon side

Coral fragments making up the motu

Fringing reef from the ocean side

Another view of the ocean side
Patrick finds a prize---a pearl farm buoy---washed up on the ocean side
We examined the vegetation on the motu. 

Sprouting coconut
The understory I mentioned in an earlier post turned out to by primarily different species of mangroves (both large and small), so the diversity of the flora is pretty low. Later, on some of the other motus, we also saw pandanus and some cycad-like palms.

The mangroves utilized a veritable freeway of bright orange aerial roots to find suitable places to colonize the inhospitable---low-in-nutrients, high-in-salt---coraline soil. I'm assuming these are aerial roots and not rhizomes (underground stems); I couldn't find any information online that told me for certain. 

Brilliant Nature

Sometimes those aerial roots go a little crazy... 

The coral heads on the northeast side of the atoll also had excellent snorkeling with a much greater diversity than the heads in the southeast anchorage. Here, I saw my first green moray eel! (The picture I took was too dark to post.) I never got tired of looking at the giant clams:  Their day-glo colors were something to marvel at. 

These are not the same species of giant clams you see while scuba diving, which can get up to four feet across. From the limited resources I have onboard, I believe these are Tridacna maxima, a species which gets up to 14 inches long and embeds in coral or sponge. The colors are not actually part of the clam but are from a symbiotic algae living inside the clam. The algae makes extra glucose (energy) for the clam and the clam gives the algae a home.

One of the most colorful denizens of the reef is the parrotfish. Their name stems not only from their brilliant palette of colors but from the parrot-like beak they have for feeding on coral. When you are snorkeling, you can actually hear their beaks crunching on the coral!

Since the lighting was not ideal, I used a little help from an Adobe photo program to bring out the colors of the parrotfish in the pictures below---however, these are their actual (not enhanced) colors!

In general, everywhere there is coral, there is life---living on it, feeding on it, or feeding around it.

 The coral gardens were spectacular in and of themselves.  

I was surprised by how few invertebrates (besides the coral itself) live in the coral reef.  The fish far outnumber the invertebrates (at least here in the Tuamotus.) One colorful invertebrate I discovered were these Christmas tree worms.  

Christmas tree worms come in pairs - are they two ends of the same worm?

Tridacna with Christmas tree worms
I also enjoyed a game of cat-and-mouse with a marbled grouper. Groupers are very shy fish, yet they are curious at the same time. They like to stay hidden in a hole or under the security of a rocky ledge or piece of coral, but they will occasionally come out into the open to stare at you, racing back to security if you make the slightest motion towards them.   

Marbled grouper 
Look at how this fish is camouflaged by blending in so well with the chalky coral bottom:

Brilliant Nature 2
We had entered the lagoon via the middle pass. We didn’t make it to the old village site (now deserted) at the western pass, where there is also supposed to be excellent snorkeling. There was a large shallow reef area in the eastern pass, Pasee Motu Puapua, which I would have liked to explore with a snorkel at slack tide, but the wind and currents were too strong when we took the dinghy there. I read of other cruisers who did a drift snorkel/dive through this pass---which under the right conditions, would be exciting, and under the wrong conditions, very dangerous.   

After a couple of days in the northeast anchorage, we moved to the southeast anchorage. Since the trade winds blow primarily from the southeast during this time of year, the southeast anchorage provides more protection from the wind-blown waves, which have nine miles or so of fetch before they reach the northeast side of the island. Unfortunately, after we moved to the southeast anchorage, the wind clocked around to the north, and we spent one night pitching quite energetically at anchor.

We transited across the lagoon to the southeast anchorage using waypoints from S/V Soggy Paw’s route (Tuamotus Compendium). We left at mid-day so the sun was overhead and we could see the coral heads. I kept watch from the bowsprit the entire time, while Patrick took the helm. Since the area is uncharted, we were looking for shallow depths where the coral heads might rise to the surface. Indeed, we saw several of these areas (only one which we had to change course to avoid), in which the water suddenly changed from a deep blue to jade green right in the middle of the lagoon. We could easily spot these areas from a distance given the conditions (wind 9-11 knots and clear skies); however, it would have been difficult with more wind and chop or if the sun was not behind us.

The following picture was taken on our way back across the lagoon, in winds under ten knots. 

The shallow spot pictured next to us was taken when Silhouette was in a water depth of 139 feet.  

The southeast anchorage is the tropical paradise that most people probably think of when they think of the South Pacific.

Approaching the southeast anchorage

Two motus separated by a lagoon in the southeast anchorage

Edge of the motu

Moonrise over the atoll

Sunset over the atoll

However, finding a good spot to anchor among the coral heads can be a challenge. If your anchor chain gets wrapped around a coral head, it reduces your scope, and you can snap your anchor chain while pitching up and down on the shortened chain. This had happened to another boat in the northeast anchorage the previous week, and we met them under calmer conditions retrieving their anchor from the GPS waypoint they had entered. Patrick immediately employed his find from the beach on the northeast side in buoying our anchor chain above a small coral head near our anchoring location. 

Anchor chain buoy

Even so, as the wind clocked around, we were forced to move our anchoring spot once due to the possibility of swinging over a larger, shallow coral head.

At the southeast anchorage, we usually spent the mornings doing chores and boat projects. Each afternoon, we explored a different motu, usually followed by a snorkeling excursion.

Today's colors
Although at first glance, all the motus look the same, there are subtle differences. During our first outing, we observed the geology of the coral sediments making up the lagoon and ocean sides of the motu.

Coral sediments

Coraline blocks in the lagoon between motus

Walking the motu
We also visited the large motu we were anchored next to and noticed a beach shelter left behind by previous visitors.

In general, I was a little disappointed by the lack of a “leave no trace” ethic on the motus. Although the various lean-to shelters we saw were quaint, the fire rings where cruisers had (in most cases incompletely) burnt their trash were not. The impact of humans was also visible in the flotsam, present on all the motus:  primarily debris from the fishing industry such as nets, floats, and even a hardhat, but also many empty plastic (recyclable) soda and water bottles that somehow made it into the waterways and found their way to the distant shore of a motu in the South Pacific. The statistic I learned while working for the Seattle Aquarium a couple of years ago, that eight out of ten plastic bottles never make it into the recycling stream, began to ring true.

We snorkeled the coral heads near the anchorage and found the waves choppy (the wind was still ten knots and above), the visibility poor, and the diversity low. 

Coral head
The next day, we explored a different motu which had a deep lagoon edged by a rock/reef shelf. Since the waves in the anchorage were even choppier than the previous day, I snorkeled the lagoon and found it calm with the clearest visibility we’d seen yet in the entire anchorage. 

My favorite atoll activity
There were probably over a hundred different species of fish, both large and small, and large black and gray-tipped reef sharks patrolled the lagoon. 

Lemonpeel Angelfish

Striped fish and coral

Peacock damselfish

Scissortail sergeant

Reef community
On one motu, we found a variety of beautiful shells we hadn’t seen on the other motus, including a “nest” of delicate shells filled with hermit crabs. The hermit crabs had all congregated together for some reason (maybe pure coincidence), but as we watched them, an interesting phenomenon occurred. At high tide (which is when it tends to be easiest for us to beach our dinghy)---when the distance between the hermit crabs and the water was the shortest---all the hermit crabs began stirring at once. 

This movement was what called our attention to them. In practically a single file line, the hermit crabs all started making their way from the beach towards the water’s edge. How did they know when high tide was? How did they know which direction to go? That’s another one of life’s mysteries, overlooked in the hustle and bustle of daily life, which the cruiser has time to notice and ponder.

Still another motu held more hermit crabs:  this time a small army of them creeping over the dead mangrove leaves in the forest in large, top-shaped shells. 

Motu hermit

Photographer being photographed
Many of the motus held bird life, but we didn’t seen any nesting birds at this time of year. There were frigates, boobies, delicate white and dark terns with needle-sharp features, and sandpipers. The birds seemed to favor the motus with pandanus as part of the flora. 

White tern in flight
As with all of the anchorages we have been in, Silhouette had its own under-the-hull fauna. Here, there were purplish-blue fish we called “needle fish” due to their syringe-like shape. They are four to six inches long and travel in large schools. There were also remoras:  Whenever I emptied the scraps from the galley sink strainer overboard, several of them darted out to feed. I guess eating scraps from a cruising boat is easier than attaching yourself to a shark with a suction cup and feeding on its droppings. There were also large fish of an indeterminate species:  the visibility generally was not clear enough to make them out at the bottom, where they tended to hang out.

With another low system moving in below the Tuamotus creating forecasted winds of 20-25 knots here, we decided to make the transit to Fakarava before the bad weather set in again. On our last day in Tahanea, we headed across the lagoon in the morning and temporarily anchored for the afternoon (giving me time for one last snorkeling expedition!) 

Patrick keeping the bow watch on the way back across the lagoon

In a school of unicornfish

Closer view of unicornfish

We waited until the high tide slack. Since the wind had come down significantly by then, Patrick coached me as this time I piloted the boat through the swirling eddies and currents and out of the pass.    

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