Marine biologist Keith Hiscock visited the Galápagos islands in April to discover marine ecosystems that were unique to the islands and that can provide further understanding of biogeography and evevolution.
Keith Hiscock is Programme Director of the Marine Life Information Network (MarLIN), part of the Marine Biological Association in Plymouth. For eleven years he ran the Marine Nature Conservation Review of Great Britain. Visiting other countries to view their marine biodiversity helps him to put the rich seas around Britain into perspective.
When Charles Darwin visited the Galápagos Islands in 1835 he collected information that would underpin his theory of evolution. He paid little attention to marine life apart from describing the remarkable marine iguanas and by bringing back specimens of 15 fish species – all new to science and five of which are now known to be found only in Galápagos. Now that scientists can dive around the islands, the peculiarity and fascination of the Galápagos underwater world is revealing its secrets – many of them of equal importance to our knowledge of the finches, tortoises and other well-known terrestrial species in demonstrating how life can become unique in such isolated and ancient locations.
As many as 18% of marine species living around the islands are only found (are ‘endemic’) in Galápagos. The fishes are probably the most thoroughly studied and, in a publication by Grove and Lavenberg (The Fishes of the Galápagos Islands, 1997), of the 460 species described, 11% were endemic to the islands. Compare that with Britain and Ireland where no marine fish species are endemic and there are 333 species known. Seaweeds are considered to have 23% endemism. The most recent count of marine species numbers in Galápagos suggests 3,000, compared to a recent count of 6,172 named species around the Marianas islands on the other side of the Pacific or 8,200 named species around Britain and Ireland. Surveys undertaken by staff of the Charles Darwin Research Station (CDRS) together with visiting scientists have catalogued species from a substantial number of marine groups of organisms and have identified three distinctive biogeographic regions. The CDRS has now brought all existing information together into a 500-page report cataloguing marine biodiversity.
The Galápagos Islands are the result of a geological hotspot a thousand kilometers west of Ecuador – an island chain spreading eastwards from a flaw in the earths crust that allows volcanic rock to erupt and build-up to above the sea surface. The islands are tens of millions of years old and the westernmost ones are still active. Although the islands sit astride the equator, the shallow waters that surround them are hardly warm – 19 to 25 °C – cold enough for a diver to need a full wetsuit and too cold for extensive coral reefs. The reason for such cold waters lies in the currents that bring waters to the islands. A cold oceanic current from the south and a warm current from the north converge east of Galápagos to create the west flowing South Equatorial current whilst an east-flowing cold bottom current, the Cromwell current, rises to the surface in the west of Galápagos. Nutrient rich waters from the depths hit the islands in the west – producing clear water for a short while but, once sunshine and nutrients get together, creating a plankton bloom which, to the dismay of divers, turns the water green.
From green water to blue water. Over a hundred miles to the north of the main island archipelago lie the isolated rocks of Darwin and Wolf. They have a reputation for exceptional underwater visibility and are one of the worlds ‘must-go-to’ destinations for divers. But not only for the visibility – it’s the sharks and other megafauna that the few visitors able to get here come to see. Our trip was lucky. Visibility in excess of 50m and sharks. Sharks that come not to feed but to be cleaned by small barberfish (Johnrandallia nigrirostris) especially. So, how to see the sharks. Well you can try lying on the top of an underwater escarpment and waiting for them to come-by, which they obligingly do in shoals of hundreds – but in the distance – they have most likely seen your bubbles and veered away before you see them. The best approach if you are in a group is to go out into ‘The Blue’ – to hang in open water just off the reef and when they come past, hold your breath and swim out to them. Amongst the hammerheads are Galápagos sharks and occasionally turtles or rays come past. Go there in August and September and you will most likely see the magnificent whale shark – the largest fish in the world.
However, if you are a marine biologist, there is more to be learned from what lives on the seabed than above it about island biogeography. But, prepare to be disappointed. Where the much sought-after blue water occurs, the rocks are ‘bare’. The surgeon fish that graze them ensure that a very short turf of algae persists with the occasional sea fan and giant barnacles. Deeper than 20 m, it does get richer with Galápagos yellow-polyped black coral, sea squirts and sponges colonizing the rocks.
To see rich marine life, find some green water. In areas where water wells up from the depths, where plankton blooms and where diving tourists dismay, there especially can you see some of the Galápagos specialties. In one location off the island of Daphne Minor, I was bowled-over by the variety and abundance of invertebrates on the underwater cliffs – for many other divers in the party it was probably the worst dive of the trip – we were in green water and visibility was about 5m – not enough to see barracuda, shark and rays.
The distribution of marine life around Galápagos is not static and the 1997-98 El Niño event demonstrated how seawater warming (generally by about 5°C in this case) can dramatically change the distribution of species through bleaching of corals and crustose coralline algae, mortality of barnacles and spread of fish species from the north to the south of the archipelago. Sharks largely disappeared into deep water during the event but returned once seawater temperatures fell. The marine iguanas suffered as the balance of algae shifted from their favoured green algae to less palatable brown algae and the Galápagos sea lions declined by up to 48% in some areas. Iguanas and sea lions have since recovered but populations of the endemic Galápagos penguin and corals are still low following the 1982-83 El Niño. However, there are positive effects including large recruitments of algae, invertebrates and some fish during El Niño events.
Much work remains to be done but the diversity of species around Galápagos may not be outstanding and the biodiversity highlight of the island might, like the land, be the large number that are only found there. Species such as the marine iguanas, the sea lions, the barnacle blenny, black-striped salema, blue-striped sea slug and many more are endemic to Galápagos. However, groups such as sponges, sea firs, sea mats and sea squirts have been little-surveyed so much remains to be discovered. But why such high endemism? Isolation plays a large part of course but there are equally isolated island groups such as the Azores that do not exhibit such a large number of species unique to them. Indeed, the sea is considered to provide a much more effective medium for migration than would occur for land animals between islands. Perhaps it is the age of the Galápagos archipelago that has enabled speciation to occur. Or, perhaps the direction of currents, bringing species to the island but not providing a route for them to the mainland. Mysteries remain to be solved but, in the meanwhile, the islands provide fascinating wildlife and some spectacular creatures.
A marine park was established around the Galápagos archipelago in March 1998. The Galápagos Marine Reserve is 133,000 square kilometers in extent. Diving tourism is carefully regulated although regulating damaging fisheries is much more of a challenge. In 1999, 4,401,657 ‘pepino’ (sea cucumbers: Stichopus fuscus) were harvested and exported from around the islands. The dry weight was 122 tons and the value $3.4 million. Sea cucumbers have been described as ‘earthworms of the sea’ – swallowing and processing sediments and, although not ‘key species’ to ecosystem function or structure, removal of such large numbers is doubtless having some impact on ecology. Slipper lobster and crawfish are also now infrequently seen (except in restaurants) and some species of fish, notably sailfin grouper (bacalao, Mycteroperca olfax), are in decline. As with all other parts of the world, achieving sustainable fisheries seems a challenging goal.
To find out more, the book Galápagos Marine Life by Pierre Constant is a description of ecology and a guide to the most conspicuous species. The Charles Darwin Research Station Web site (www.darwinfoundation.org) is rich in information.
Text and Photos generously provided by Keith Hiscock. Please follow this Link for more of Keith’s Photos
Thank you Keith
Rene & Sue