Sunday, 17 November 2013

Anguilla, the northern most island of the Leeward Antilles

Sailing into Anguilla.  Obviously a "low relief" island
View from the Customs and Immigration dock.  Alaria is mast on the right.
We sailed north to Anguilla.  Unlike St. Maarten, Anguilla is a “low island” (no mountains or steeply sloping shores).  This geological feature is ideal for creating a platform on which coral reefs can grow… and grow they did.  Seal Island reef is one of the largest coral reefs in the eastern Caribbean.

The little island with a big reef.  Breaking waves along the Seal Island Reef shows the extent of the coral reef and the extensive lagoon behind it.

Unlike St. Maarten where the Nature Foundation supplied us with scuba tanks for our surveys, now we have to work as an independent floating marine lab.  So we deployed the compressor, charged the scuba tanks and got ready to dive Anguilla’s coral reefs.

The compressor filling tanks on Alaria's bow.  It takes 20 minutes to fill one tank
Anguilla’s best coral reefs are within their Marine Park.  To visit those reefs you must pay $60 per day (expenses go to buoys and park management).  There is a long list of “Marine Park Regulations” that limit activities in the parks.  No harming flora or fauna, no jet skis, no water ski but what interested me most was “Visitors are NOT permitted to fish within Marine Park boundaries.”  That’s terrific because seeing a big reef with the full complement of reef fishes would be very instructive.

Alaria at our Sandy Island reef site

Sandy Island Reef

A small reef surrounds Sandy Island so we made that our first site.  We did three dives to record fish, corals, algae and juvenile corals at several sites.  The coral cover was low and the seaweed was too abundant for this to be a healthy reef.  The fish fauna was more depauperate than we would have expected from an area having no fishing.  However, then we saw why fish are rare.  There was a string of baited fish traps!

Fish trap sitting on the shallow fore reef

Another fish trap on the fore reef.  

So, it was obvious that these reefs are fished.  We suppose that only visitors are not permitted to fish – the local folks can.  However, often it is more important how you fish than whether you fish somewhere.  Fish traps catch fish 24/7.  The very valuable grazing parrotfish have the unfortunate behavior of looking for caves in which to sleep and fish traps often become the parrotfishes death bed.  I’ve seen many parrotfish in fish traps and I’ve seen many dead parrotfish in fish traps. Other fish also get caught and die in fish traps.  So larger fish whether they are eaten or not die as the result of fish traps.  This is why Bermuda and Bonaire have outlawed the use of fish traps.

Georges’ surveys were showing relatively few large bodied fish (neither carnivores such as groupers and snappers, nor herbivores such as parrotfishes were very abundant).   My surveys were quantifying the low coral cover and the high seaweed abundance.  Juvenile coral densities were also low.  These are not very good signs of coral reef health.

Seaweed over growing coral

Quadrat for counting juvenile corals is dominated by a thick coat of seaweed (no juvenile corals here)
Along with the 10 m transects I use to quantify the reef dwelling critters and seaweed, I also take quadrats records of baby coral density.  We know fairly well how fast reef corals can die but we know much less about how coral reefs recover via baby corals.  Usually there are no baby corals with seaweed but often we see them on or near the pink coralline algae.

Juvenile coral on a patch of coralline algae (good for corals)

Seal Island Coral Reef & Algal Ridges

The next day we got up early and sailed over to the big Seal Island coral reef adjacent to Prickley Pear Islands (the reef is also known as Prickley Pear).  We had several priority sites we wanted to study but conditions suggested otherwise.

A large ocean groundswell had developed over night and huge waves were crashing on the reef.  Paul and I went to survey site after site and found the forereefs impossible to work.  We could have conducted our dive but the swell would have shifted us back and forth 6 to10 feet which is unproductive and gets very old after a few minutes.

Paul and me checking conditions where the coral reefs live (white breaker zone between the two islands)
Large groundswell eclipses all of Alaria except for the top of the mast.

The reef is famous for having heavy wave action most of the time.  The best biological indicator of this is the presence of algal ridges.  Algal ridges are solid thick caps of calcareous algae that grows on top of the coral reef.  They only can form in regions having high wave action over most of the year and low tidal amplitude (there is a one foot tide in Anguilla).  Algal ridges are impressive structures.  On rare calm days you can walk around on top of them.  Below there is “room and pillar structure” of large caves (rooms) and vaulted supports.

One of several algal ridges capping Seal Island's long coral reef

We looked for cuts in the reef by climbing up onto the boom and looking over the waves.  We found places we could motor to but they all had too much swell to work.  So our Plan-B is to study backreefs.

Looking for study sites and breaks in the reef (breakers are the reef)
The backreefs had been well developed in the past given the dead coral skeletons evident everywhere. However, there was some cause for optimism.  We found elkhorn coral (the once dominant coral in the Caribbean) juveniles and adults!  George found a collection of juvenile reef fishes so the backreef looks to be a viable nursery habitat. 

Healthy elkhorn coral (which is an endangered species in the US)
Baby elkhorn coral surrounded by sediment-trapping filamentous algal turf.

Wave action stimulates coral growth but also grinds up the produced limestone so the water flowing over the reef often carries lots of sediment.  When it hits the quiet backwater, sediment falls from suspension and drapes the backreef environment.  The fine turf algae traps the sediment and creates a blanket that few other critters can survive.  So the there are unique dynamics in backreef environments that can both promote and suppress reef health.

Even in the stressful environment of lagoonal sand, you can find critters that thrive there.  The heart urchin lives just under the sediment surface feeding on things that live within the sediment.  They then are an "island" habitat for small commensal crabs.  The crabs get a place to live and glean food and possibly parasites off the heart urchins.

A heart urchin just below the sediment surface.

Three hitchhiking commensal crabs (the heart urchin is their habitat)
The swells continue to plague us so rather than waiting for them to subside, we decided to head to Antigua which has by far the most diverse array of coral reefs in the Leeward Antilles.

A porcupine fish of no significance to our study but that it is cute

Anguilla sunset from our anchorage.

On to Antigua!  It's a 125 nautical mile sail which should take us about 31 hours.  Our destination is English Harbor.  It happens to be a harbor I remember very well from 40 years ago.  I wonder if anything has changed?  ;)

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