Sunday, 23 March 2014

Coral reef oases in the eastern Caribbean

Alaria at Tobago Cays Marine Park
Coral reef at Tobago Cays Marine Park

After Mustique, we sailed south to the drop-dead gorgeous Tobago Cays Marine Park (“TCMP”).  There, the Park Rangers took us to reefs they have been monitoring for some time. We spent a week there before we sailed on to Union Island from which we traveled to Petite St. Vincent again courtesy of the TCMP Rangers and their fast boats.  Finally, we sailed to Carriacou in the country of Grenada.  This was our most southern station where we surveyed the island’s reefs along its windward east coast and south coast.  So after over 170 coral transects and nearly 500 fish transects we had surveyed most of the coral reefs we had targeted in the eastern Caribbean.  Now, we can consider what, if any, patterns we’ve found.

The Grenadines with the most extensive coral reefs (right inset)
Fig. of eastern Caribbean Islands including Grenadines

A “SNAPSHOT” OF THE EASTERN CARIBBEAN CORAL REEFS

Living coral comprised less than 20% of the hard surfaces we surveyed from Anguilla to Carriacou.  This is a low number but it is comparable with most coral reefs of the Caribbean today.  Nevertheless, three regions -- St. Lucia, Mustique and Tobago Cays -- had higher than average coral cover and a higher than average abundance of baby corals.   Similarly, two of those three regions had lower than average abundance of harmful seaweed.  Grazing fish such as parrotfish and surgeonfish were most abundant at Mustique but they were either average of above average at those three regions.  The most prized reef fishes to eat are groupers and they were most abundant at both Mustique and Tobago Cays.

The relatively healthy reef at Mustique
Relatively healthy reef at Tobago Cays Marine Park
In short, it appears that all of the reefs with better than average conditions for coral reef health (abundant coral, baby corals, little seaweed, many and grazers and relatively abundant predators) were all found in no fishing “marine reserves” with effective enforcement.  In the larger view this included St. Maarten, St. Lucia (Soufriere Marine Management Area), Mustique and Tobago Cays Marine Park.

Average coral, seaweed and baby coral abundance.  The blue horizontal line represents the Antilles average.  Note that St. Maarten, St. Lucia, Mustique and Tabago Cays have enforced marine protected areas.

There are, of course caveats to the pattern that reefs with less fishing are “healthier”.  For example, it is possible that areas with high coral cover were targeted for protection.  That certainly happened in St. Lucia and possibly Tobago Cays Marine Park.  However, Mustique’s protection was established with little specific knowledge of what coral reefs existed there.

Specific reefs within managed sites vary greatly.  Sometimes this is due to natural variability found in any ecosystem.  However, in some places we think we see some patterns that may have caused the local variations.  To address why adjacent reefs differ, we targeted a few sites for more detailed studies with some more expertise.

OUR RESEARCH TEAM GROWS!

For help on why reef fishes and seaweed abundances vary so much among local coral reefs Dr. Peter Mumby (University of Queensland) and Ms. Natalia Rincón Díaz (Universidad Nacional de Colombia) joined our team.  Both of these scientists will spend a month working on these problems from Alaria. 

Dr. Pete Mumby (reef fish ecologist) sailing to our study site on Alaria.

Natalia Rincon Diaz (a seaweed expert) joins our group.
In the way of introductions, Dr. Peter Mumby is an Australia Research Council, Laureate Fellow and heads the University of Queensland’s Marine Spatial Ecology Lab.  He is the President of the Australian Coral Reef Society but most of his research over the past few decades has been on coral reefs of the Caribbean.  He is one of the world’s leading experts on the ecology of coral reef fishes (Google him if you are curious).   Natalia is a masters student at her university who is studying the relationship between herbivorous fishes and the algae they eat.  I saw this a great opportunity to help advance her career while helping us with our studies.  She will have full access to all of our data to help her in her masters thesis research.

Science discussions intensify during our sunset chats.
WHY ARE SOME CORAL REEFS SO DIFFERENT FROM OTHERS NEARBY?

While it is great that there are areas with higher than average coral cover and fish abundance, it is also interesting to consider why another reef nearby is different.  We suspect there are two big factors affecting reefs.  One is one is the variability of reef structure (that is, its habitat architecture), the other is how fishing pressure changes from place to place.  These are by no means the only factors causing the variability we observed but we think they may be factors contributing to the differences.

Corals create the architecture of reefs.  This includes those corals alive today and the skeletons of corals past.  This architecture may take centuries to develop but it provides places for fish and other critters to live or to hide.  The habitat architecture of reefs increases the surface area on which seaweed can grow and herbivores such as parrotfish can graze.  So, we wondered if differences in habitat architecture may explain the differences we see among adjacent and nearby coral reefs.

A simple and relatively featureless reef in Barbuda.
A complex elkhorn coral reef at Tobago Cays Marine Park.
To examine these questions in greater depth we set out to measure habitat architecture by measuring the length of coral structure under each meter along a 10 meter transect.  With our measurements we learn how high and how dense the coral structures thrust into the overlying water.  We get a sense of how much reef surface area there is in any given space.

Bob measuring spatial complexity of the coral reef (photo Pete Mumby).

We also measure how the topography of the reef is being used by grazing fishes.  We watch for 5 minute periods and via small video cameras, the bite rates on the reef by parrotfishes and tangs.   As the surface area of reefs increase, so to does the area on which seaweed can grow. So it may be necessary in high complexity areas for more parrotfishes grazing just to keep the reef clean of harmful seaweed.

Natalia quantifying the rate at which herbivorous parrotfish and tangs bite tops, sides and bottoms of complex reef surfaces.
Because it is also possible that fish behave differently on these reefs, Peter Mumby documented the grazing behavior of the dominant parrotfish including juveniles and adults.  We also deploy video cameras to film fish grazing at sites that range from high to low complexity

Pete quantifying bite rates for different size and species of grazing parrotfish.
One of several compact GoPro video cameras quantifying fish grazing without a human presence.
We will need to considerable analyses beyond what I’ve presented here.  However, we are beginning to see patterns that make sense.  This is also information that reef managers will want to know.  In a future blog I’ll describe how we have been meeting with managers at each of the islands we are studying.  All of them are eager to hear what we have learned.

The beauty of coral reefs today.  All these photos happen to be from reef protected areas.

Elhorn coral in TCMP

Neon gobies on a star coral.
Sponges in the Soufriere Marine Management Area

A green moray in Mustique's Lagoon reef.

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