Monday, April 12, 2010

#8: Outlying Islands

Peter Gawne, Belize Expedition

Randi and I decided to head over to Twin Cay to gather some additional specimens for our experiments. We hopped behind the wheel of our trusty Panga (Pangas also made an appearance in Belize blog #2!), and made the crossing over rough water to Twin Cay.

Twin Cay is a small strip of land and seems to be completely covered in mangrove trees. (Explore Belize's mangroves further here and here.) It is an island split nearly in two by a saltwater river running straight through the land. The breeze was strong enough to keep any insects away, and the air was filled with the many birds that make their homes amidst the mangrove canopy. The saltwater river carried us past a Belize Fisheries Department station and toward the interior of the island. The water was murky, and very warm when I tested it with my hand.

Many invertebrates use the mangrove roots as structure. This looked like a good place for a crocodile to hide.

We slipped out of the boat and began snorkeling up the creeks that run through the mangrove forests. These creeks are little more than shoulder-wide where mangrove meets water, but under-cut a great deal beneath the surface. In effect we were swimming beside a great, dark cave in water where the visibility was less than three feet. I wished immediately that I had not spoken to Alex and Dan about saltwater crocodiles (Crocodylus acutus). Although these creatures are not known to be particularly aggressive, they do grow to twenty feet. My imagination caused crocodiles to be seen around every bend in the shallow creeks. I very much respect Alex and Dan for heading into the mangrove swamps day after day, listening to the croaking and splashing of the crocodiles.

We did not find what we were looking for in the creeks, so we decided to head into some of the lagoons formed by the mangroves, and try our luck in slightly more open water. The visibility was perhaps worse in open water than it was in the creeks, but for some reason crocodile attack seemed less likely.

We finally began having some luck finding what we had come to Twin Cay to find, and were shortly thereafter loading the boat with plant samples. I hopped in the boat and Randi remained in the crocodile infested (not really) water to hand me the samples. The water had become so cloudy from our efforts to collect samples that the visibility was reduced to nearly zero. I actually saw a small shark swimming to investigate the cloud of muck that we had stirred up. I shouted to Randi so that she might have a chance to see it, but she was unable to see it despite the fact that it was less than five feet from her. The shark got to within a few feet of Randi, and quickly bolted for deeper water as soon as it saw her.

Cassiopeia jellies decorate the bottom like sinister snowflakes. Each one is unique in the layout of its oral arms and tentacles, but disturb them and they will release stinging nematocysts cells into the water.

The animal diversity within the mangroves is far different from that of the open reef. While barracudas and snappers patrol the margins of both reefs and mangroves, animals such as manatees, Cassiopeia jellies, and crocodiles rarely stray from the protection and foraging grounds of the mangroves. We were lucky enough to see a manatee in the distance, and carefully approached. The manatee proved too wary for us in the end, and hid its massive bulk in the shallow, murky water.

A cloud of frigate birds looms over Man-o-War Cay.

Not far from Twin Cay was a very small mangrove island, Man-o-War Cay. As we approached Man-o-War Cay a great black cloud loomed over the isle. From far away it looked as if a huge swarm of black gnats covered the cay, but as we neared the island it was clear that this was a nesting colony of frigate birds.

Male frigate birds inflated their front pouch and beat a drum-like rhythm in order to attract a mate. The sight and sound of the frigate colony was overwhelming.

I had been in several large bird colonies before, but never with birds quite the size of the frigates. Off the coast of New England, I have worked with the Seabird Restoration Project, which involves working inside tern colonies. To see a couple thousand terns take to the air to defend their nests is a spectacular sight. While the frigates at Man-o-War Cay were not nearly as numerous as the terns, they made up for it in sheer mass. These birds were big. A full grown frigate bird can have a wingspan of over seven feet, and their silhouettes looked like something out of a Batman movie.

Bird colonies can be an all out assault on the senses. Visually, they are quite impressive, and usually recognizable from a long way off. As one nears a colony, other senses are engaged. Bird colonies in general, and Man-o-War was no exception, tend to stink a bit. I have never found it to be overpowering, but there is certainly a familiar smell associated with large aggregations of birds.

As we got a little closer, the sound of the colony begins to swell. By the time we were right atop the colony, the chorus of the birds had risen to a roar. Frigate birds are mostly silent in flight, but they make bizarre rattling, clicking, and grunting sounds in the nest. Add to this the inflated scarlet-red throat pouches of the males, and the scene can be otherworldly.

The closest island to Carrie Bow is to the north, and is referred to as South Water Cay. South Water has some modest habitation, with a few small resorts tucked along its shores. South Water is also home to Pelican Pouch Resort which served as a resupply for provisions on Carrie Bow, if and when the cupboards ran short. The staff at Pelican Pouch frequented Carrie Bow to be certain that we had everything that we required, and we all very much appreciated their attention and good nature. I have been told that Pelican Pouch will share duties with the Smithsonian field station at Carrie Bow for an upcoming SECORE project. I am certain that the researchers will be well taken care of.

Clockwise from top left: Osprey; brown booby and female magnificent frigate bird; yellow-crowned night heron; cormorant (probably double-crested)

South Water Cay had a lot more vegetation than Carrie Bow, which encouraged birds to alight on the island. Osprey, brown pelicans, cormorants, Sandwich terns, night herons, and various gulls are all residents at both South Water and Carrie Bow. We got the pleasure of watching ospreys take numerous fish from the back reef at Carrie Bow. The ospreys would then have to run the gauntlet of frigate brids who sought to steal the ospreys’ catch. Some really incredible high-speed chases ensued, with the result being in favor of the frigate birds more often than not.

This little bird was knocked out to sea after a particularly violent storm. We tried to identify it, but came up short. I'd love to know what it was.

During periods of high wind blowing out from the mainland, the cays can get some unusual visitors that get knocked out to sea. We had swallows, yellow warblers, nighthawks, and many other colorful little birds that stopped on the islands, presumably to rest after getting blown out to sea.

Those are the islands in the vicinity of Carrie Bow. Each of them seemed to have its own appeal, and I feel privileged to have visited them all.


Thursday, April 8, 2010

#7 - Taking a moment to enjoy the show

Peter Gawne, Belize Expedition

Randi and I have now been looking into the diets of reef fish for some time now. Most of our dives are spent with our noses in the reef, not really paying attention to what's happening over our heads.

This particular dive was too deep for us to get two full sets of data. When working underwater, the deeper one goes the less time that you can spend down there. It's a complicated physiological process that involves the accumulation of tiny nitrogen bubbles in the bloodstream, but it will suffice to say that we did not have time to record another data set. On this dive we knew that we would have a little time to lift our heads out of the reef, take in some of the sights, and hopefully snap off a few pictures.

Thousands of Creole wrasses engaged in an elaborate mating dance over our heads.

On a peninsula of reef, extending up from the sand channel, we found Creole wrasses (Clepticus parrae) in the thousands. In the days surrounding a full moon Creole wrasses gather in huge numbers in order to spawn. These small fish aggregate in a spawning rush in order to play nature's numbers game. Creole wrasses spawn in large schools, all at once, so that a huge number of fertilized eggs are in the water at once. Predators, disease, and lack of food are but a few of the obstacles facing the young Creole wrasses on their journey toward adulthood. If even a fraction of these fertilized eggs survive, the Creole wrasses have accomplished their mission.

Atop our thin finger of reef, one of these obstacles is very apparent. The spawning rush attracts predators of all shapes and sizes. Egg-predators, such as damselfish, are drawn from the cover of the reef in order to feed on the plentiful eggs suspended in the water. The Creole wrasses stick to their numbers gamble, and make no attempt to protect their spawn or to deter the egg-predators.

Large, fast-moving horse-eye jacks patrolled the periphery of the reef.

Egg-eaters are not the only predators to join the party. Permit, groupers, cero mackerel, horse-eye jacks, barracuda and skipjack tuna all patrol the blue water surrounding the peninsula. These large predatory fish are present in great numbers, presumably to take advantage of the many fish that have been drawn into the open by the prospect of a free meal.

Two large tiger groupers hold in the distance as horse-eye jacks encircle the reef.

Large in both size and numbers on this particular dive, it was unusual to see so many groupers. Tiger, black and yellowfin groupers all made an appearance. These animals have been hunted extensively as their firm white flesh and great size makes them especially prized by fishermen. One of the groupers on this dive allowed Randi and I to get close enough to see an old wound on its head, probably from an unsuccessful spear-diver.

This dive has been one of the highlights of our trip. It has proven difficult for me to capture the scale of Creole wrasse spawning, so I hope the pictures provide some glimpse into the magnitude of the event. It was very nice to take a quick break in data collection to just sit back and enjoy the show that nature sometimes puts on.


Sunday, April 4, 2010

#6 - Data speak louder than words

Randi Rotjan, Belize Expedition

Pete and I are not slogging through the mud in the mangroves like Dan and Alex. Instead, we face different challenges in the lab and on the reef. Every morning, we rush to check our fish feeding experiments, siphon, replace food cubes, and do whatever else needs doing in the wet lab. We then prep our data sheets and decide on the dive agenda for the day. Then, it's dive-dive-dive all day long, followed by a night of sample processing, microscope work, data transcription. Midnight comes, we crash, and then start again.

Pete in the field; Randi at the scope.

The fish could save us a lot of trouble if they could just talk to us directly (though then we might be out of a job!). All of our effort is aimed towards a single goal: why are fish eating what they are eating? Simple question, perhaps, but it’s important. Together with collaborators Dr. Michael Berumen (KAUST, Saudi Arabia) and Dr. David Raubenheimer (Massey University, New Zealand), we're studying some of the most important and charismatic fish on the reef -- parrotfish and butterflyfish. It is a deceptively simple question to ask why these fish are eating what they are eating, but we first have to figure out what they're eating. To that end, we follow fish around the reef, while counting the number of bites and recording the prey items. Then, we collect samples to bring home for nutritional analysis.

Chaetodon capistratus butterflyfish (L) and Sparisoma viride parrotfish (R)

But that's not enough – are the fish really eating what we think they are eating? It may look to us, for example, like a fish took a bite of macroalgae. But remember that coral reefs are among the most complicated ecosystems on the planet and algae is NOT just algae… it can be encrusted with sponges and hydroids, covered with epiphytes, or be host to lots of tiny crustaceans and other organisms that find refuge in the dense foliage. So, did the fish eat algae, or something ON the algae? The mystery continues.

A forest of Turbinaria macroalgae on a calm day

Next step: bring the fish into the lab and offer them more isolated food choices. This algae or that algae? With sediment or without? With extra protein? Or perhaps the fish prefer a fatty diet? Lean? Getting closer – fish make very deliberate decisions that we dutifully record in our lab notebook.

Sparisoma aurofrenatum parrotfish choosing different diets (cubes on chains) in a tank

But, what are the fish actually digesting? To answer that question, dear readers, we carefully collect fish excrement, place it in tiny vials, examine it under the microscope, and then preserve it to bring home for nutritional analysis. We will then do some basic arithmetic: food in - food out = food digested.

A photo of the lab at Carrie Bow

And just when you think you finally have the answer.... some smart blog reader will ask: Do juveniles and adults have the same diets? What if the preferred food type isn't available? Do fish in Belize have the same preferences as the same fish species elsewhere in the Caribbean? Do different fish species prefer different food types? Does food preference change with season? Reproductive period? Lunar cycle? What if food quality changes? What if fish density changes and competition regulates access to preferred prey? Does metabolic need change in higher temperatures? Lower temperatures? Etc etc etc.

The Belizean barrier reef

If fish could talk, I'm sure they'd laugh at us (after all, they probably know the answers!). Then again, they might also tell fish tales seeded only with a grain of truth. Thus, we are trying to find clever, indirect ways to understand how coral reef ecosystems function in a natural environment, and how reefs will change in changing environments. We'll keep you posted, but it's back to the lab for now since the fish are still silent. That's okay though, in the end, data speak louder than words. :-)