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Isabela is with 4'670km² the largest island of the Galapagos. Most of the diving is done around the north coast (Cape marshall, Punta Albermarle, Punta Vicente Roca) and at Roca Redonda (on the trip north to Wolf and Darwin). On extended trips Fernandina island is also visited but I have heard that diving in some dive sites there are not permitted anymore. Puerto Villamil in the south of Isabela also offers some interesting dive sites around several small islands in the surrounding. The west of Isabela is directly in the path of the Cromwell current, so there is a lot of upwelling cold water. Temperatures are around 20 to 24°C but can drop to 13 to 15°C (in El Niña years)!
This area is specially interesting, since several species of fish are more abundant here than elsewhere in the Galapagos islands because of the cold water, such as the Galapagos horned shark (Heterodontus quoyi / español: gato / Deutsch: Galapagos Stierkopfhai) or the Sailfin grouper (Mycteroperca olfax, español: bacalao). Fernandina is very isolated and has endemic species seen nowhere else on the Galapagos, such as the flightless cormorant or - in the water -the endemic Galapagos Grunt (Orthopristis forbesi) and the White Salema (Xenichthys agassizi).
Isabela and Fernandina sit directly on the mantle hot spot and are geologically very active with 7 large volcanoes. Fernandina is one huge shield volcano with a large caldera and many lava flows. On Isabela there have been at least 13 eruptions since 1911, five of them at Cerro Azul and just 1998 a kilometer long crack opened and fountains of lava erupted to several hundred meters height, and three eruptions at Santo Tomas (Sierra Negra - black mountain) volcano in the south and five of them at the Wolf volcano in the north.
Roca Redonda is located off the northwest tip of Isabela Island, separated by about a 30km stretch of very deep water. This is the tip of a submarine shield volcano that rises nearly 3000m from the sea floor and emerges from the water as an island. The underwater area of this volcano is said to be about 18 kilometers wide. The island is about 300 meters high with steep cliffs and a flat top. Several lava flows can be observed on the island and in the shallow water and there are various caves. Seabirds live here, for example the swallow tailed gulls (Larus furcatus). These gulls are endemic to the Galapagos and the world's only night-feeding gull. It has unusual large eyes and feeds on squids that come to the surface, usually about 15-30km distance from the nearest land.
This dive site is visited either on the way up to Wolf and Darwin in the north or as part of a tour around Isabela. The water temperatures are very low here, because of the cold Cromwell current. Diving can be difficult because of the strong currents, unpredictable downcurrents, eddies and the heavy surge. If conditions are not right, your dive guide might advise you against diving.
You dive around several underwater rocks and pinnacles. This is a good place to find schools of scalloped hammerheads, yellow tailed surgeonfishes, barracudas, jacks, Galapagos grunts and king angelfishes. Galapagos sharks and whitetips, mantas, and even the huge sunfish visit this remote place. With luck you can also find sea horses among the rocks in the shallows.
A special attraction is on the Southeast Side of the island. There are several underwater fumaroles (steam vents) in the shallows (12 to 18m) and you can see gas bubbles rising to the surface which indicate that the volcano is probably still active. I have visited a place similar to this in Siau, Indonesia, where there was hot sand and so many bubbles, it looked like a curtain, and found, that some species of marine animals grow much larger because of the abundance of some minerals. Anayway - it's also just interesting to touch hot rocks below the water and swim among bubbles...
Punta Albemarle lies on the very northern tip of Isabela. Rocky volcanic cliffs drop down to the ocean floor as almost vertical walls. You might see large animals like manta rays, marbled rays, hammerhead sharks and marine turtles, but also Chevron barracuda, snappers, yellow fin tuna, rainbow runners, wahoo and groupers. There are also a lot of smaller fishes like creole fishes, parrotfishes, scrawled filefishes, pacific boxfishes and tiger snake eels.
Cape Marshall (Puerto Egas) lies
on the eastern side of Isabela island. You dive where a huge lava stream from
the Wolf volcano stopped flowing into the ocean. The lava breaks off abruptly
and forms a very steep slope with boulders and lava ridges. Cover is sparsely
with some black corals. Dependig on the prevalent current you dive along either
from north to south or vice versa.
This is a good place for mantas, mobula rays and eagle rays but also for the occasional shark, yellowfin tuna and sealion. We saw several huge mantas close by - one of them black on the top as well as on the belly - but the most impressive sight was the school of mobula, probably at least 50 animals, that we met several times. As we swam out into the blue, they were suddenly all around us, with quick flips they sped past us and the whole school engulfed us, mobulas in front, above, behind and on all sides! They stayed around, formed a more compact group again and let us swim along for a while. A eagle ray which also joined us only got a glimpse from us, we were so entranced from the mobula rays!
Punta Vicente Rocalies at the northwestern point of Isabela close to the volcano Ecuador. There are two beautiful coves which lie on either side of the eroded remains of a volcanic cone. You start your dive on a shallow wall, that becomes a steep drop-off down to 50m. The wall is full of crevices and narrow shelves and is nicely covered with sponges and corals and you can find nudibranchs, crabs, slipper lobsters. There are several really interesting species of fishes living here, the red lipped batfish, frogfishes, seahorses,electric rays and the endemic camotillo. Here you can see schools of barracudas and salemas and even the occasional sunfish and marlin has been spotted here. Mostly drift diving. Night dives possible - good for crabs, shrimps and lobsters and other invertebrates. (In 2006 I heard night diving is not permitted anymore in the Galapagos islands).
Tagus Cove: This is a large and deep bay on the western side of Isabela with no beach. In the north, divided only by a small stretch of land is the Darwin lake, which is a saltwater lake. There is a lot of upwellings of cold water here, so temperatures can get very low. A lot of sponges and tunicates and non-reef building corals grow here and there are plenty of the black coral bushes here (Antipathes galapagensis) which hide animals like seahorses (Hippocampus igens) and the longnosed hawkfishes (Oxycirrhites typus). Frogfishes are also said to be seen here and stone scorpionfishes (Scorpaena mystes / Pacific spotted scorpionfish) and player scorpionfishes (Scorpaena histrio) have been found here. These are cryptic fishes which lie unseen on the reef top waiting for small fishes to pass by. Their spines are highly poisonous. Night dives possible (In 2006 I heard night diving is not permitted anymore in the Galapagos islands).
South of Tagus Cove lies Urbina Bay (or Urvina). This is not a dive site, but this place is interesting, because in 1959 the coastal area was uplifted 5m above sea level in a matter of hours. You can still see the remains of the marine animals that died, when it happened, really large coral heads surrounded by plants! Elizabeth Bay is a large sheltered bay with mangroves in the very west of Isabela where Whale sharks have been sighted. They seem to migrate south from Darwin and Wolf and pass this area in December / January. Punta Morena further down are rarely visited for diving, I have read, you can find the Pacific seahorse here. The place is very impressive with large lava fields from both Cerro Azul and Sierra Negra.
In the very south of Isabela are several dive sites situated close to Puerto Villamil: Los Hermanos (Crossman island) and Tortuga Island. This area is not very well known, there might be also some good diving around La Viuda and Punta Ventimilla in the north, Roca Union and Cabo Rosa in the south and around Roca Blanca in Cartago Bay even further north. Sunfishes have been reported from these waters. I heard there is a dive operator in Villamil, but couldn't find his address (write to me if you have more information!)
Since Fernandina lies to the very west of the Galapagos archipelago it has no introduced species, and as a consequence remains as the Galapagos were before being colonized. La Cumbre (the summit, about 1500m) is a highly active volcano with a large caldera and surrounded by several smaller craters. Most of the island is covered in lava flows, so there is not much vegetation. The island is surrounded by shallow rocky reefs that are covered in the green algae that is the only diet of the marine iguana.
There is only one dive site, Cape Douglas, because on Punta Espinoza which is located in the northeastern point of Fernandina island at the Bolivar channel between Isabela and Fernandina diving is not permitted anymore. The water here is cold all the time, since Fernandina lies smack in the zone of major upwellings from the Cromwell submarine current.