Socorro Island N18°48' W110°59' Reef Report
Coral Reef Study at Isla Socorro, Mexico
March 2004

Heather Logan, Lindsey Feldman and Carol Milner
Planetary Coral Reef Foundation

The Clarion Angel fish - endemic to the Islas Revillagigedo


Isla Socorro is a volcanic island located at 18° 48'N, 110° 59'W approximately 300 nautical miles from mainland Mexico with an elevation of 1130 m. An established Mexican Naval base on the island houses military personnel and visiting scientists throughout the year. Obtaining a permit to Socorro is difficult and there are subsequently few tourists besides the occasional dive boat. The terrain of Socorro is made up of mostly hardened lava and there is little freshwater. Vegetation is of low diversity and consists of predominantly cactus and sage, which are well adapted to the arid environment. The underwater life of the Islas Revillagigedo has been shown to have the highest diversity of fish and coral communities in the Mexican Pacific (Spalding et al, 2001) with up to three endemic species of hermatypic coral, six of mollusks and twelve of reef fish. The reef at Isla Socorro has been described as particularly rich (Wells, 1998).

The goal of this study by the Planetary Coral Reef Foundation (PCRF) was to assess the biodiversity plus health and vitality of the scleractinian corals in the coral reef ecosystem of Isla Socorro and the biodiversity of the reef fish population.


We surveyed approximately 70% of the coastal zone of Isla Socorro during the course of this study. We scouted along the coast to find the areas most abundant in coral life. Studies were conducted over the course of 6 days at 3 different sites. Vitareef was used, a methodology developed by Dr. Philip Dustan, College of Charleston and Principal Investigator for PCRF. This methodology has been used at coral reef sites around the world by PCRF and the crew of its Research Vessel Heraclitus.

Teams of between two and four divers identified coral colonies to genus level and evaluated the health and vitality of each colony using a coding system in Fig 1.

Fig. 1 Characteristic Codes used in Vitareef methodology

2Unblemished (healthy)
3Damaged but healed
4Edge damage (occurs when filamentous algae trap sediment)
5Fresh damage to tissue and skeleton (eg. fish bites, diver damage)
6Excessive sediment on live tissue
7Damage to tissue only (biological predation eg. snails)
8Tissue bleaching (resulting from expulsion or death of zooxanthellae)
9Excess mucous on surface
10Black band disease
11Obvious algal mat smothering
12Sediment damage with tissue necrosis
13White plague disease
14Healed with secondary algal colonization
15Recently dead colony (whole colony only)
16Overgrowth by macroalgae
17Colony decreasing in size
18Colony almost unblemished
19Overgrowth by invertebrates (ex. Sponges, tunicates, boring clams)

Each code represents a condition that has an affect on the health or vitality of a coral colony, vitality referring to the ability of the colony to reproduce. This methodology has a dedicated computer program, ‘Vitareef’, for the purposes of analyzing the results obtained in the field. In order to gain an overall impression of the health and vitality of the reef ecosystem, the characteristic codes can be grouped into four categories representing:

The Vitareef computer program calculates the percentage of coral colonies in each of these categories.

Fish species at each site were identified and recorded.


Three different types of substrate composition were found along the Socorro coastline. The majority of the seafloor on the western side of the island (site 1) is made up of boulders, while the eastern side (site 2) consists of patchy regions of hardened lava. The third site is an enclosed lagoon in the north (site 3), which is surrounded by sandy beaches and has a bottom structure comprising both boulders and sand. Each site varied in visibility, depth, diversity and coverage of scleractinian coral and diversity and abundance of reef fish.

A total of 556 coral colonies were studied across the three sites. Only two genera of corals were observed along the entire Socorro coastline – Pocillopora spp and Porites spp.

Visibility at site 1 was 19 m. The coral colonies were located between depths of 5-15 m. The majority of the corals in this site were Pocillopora spp. and colonies were patchy along the predominantly boulder covered sea floor. Colonies of Porites spp. were all of the encrusting formation and were between 0.5-1.5 m in diameter. Corals appeared to be relatively healthy, with damage mainly caused by fish bites and overgrowth of filamentous algae.

Site 2 was located on a lava shelf offshore of the island. Visibility here was 25 m and corals were located between depths of 3-8 m. Coral coverage was extensive. Pocillopora spp. was the dominant coral species and only 9 colonies of Porites spp. were observed. Of the three sites studied, the corals at Site 2 appeared to be the most healthy with any damage being caused by fish bites and overgrowth of filamentous algae.

Visibility at Site 3 was low (6 m) and coral colonies were found between 1-7 m. Although Pocillopora spp. was once again the dominant genus, there were a significant number of both encrusting and massive Porites spp. colonies. Corals at Site 3 had been affected by sediment flow into the lagoon from the surrounding beaches, as well as overgrowth by both macroalgae and filamentous algae.

Of the 556 colonies observed, Vitareef results indicate that 55.3% of these were healthy or almost healthy, 53.9% were overgrown by either filamentous or macro algae, 0.2% were covered by sediment and 49.7% were decreasing in size (due to algal overgrowth, sediment damage or tissue/skeleton damage from feeding). The greater part of algal overgrowth was by filamentous algae, with overgrowth by macroalgae occurring only in the lagoon region.

Almost all Porites colonies observed were impacted by boring clams, which make distinctive holes in the coral heads with their bioerosive action, and the edges of Pocillopora colonies were white in color to due fish feeding.

41 different species of fish were observed at Isla Socorro (Fig. 2) with the diversity level similar at each of the three sites. At least 10-15 fish that were observed could not be identified.

Fish abundances varied at each location. Lutjanus viridis, Xanthichthys mento and Acanthurus triostegus were more prevalent at Site 1. Holacanthus clarionensis is endemic to the Revilligegido Islands and was found to be common throughout all of Isla Socorro. Only one shark was observed during the study – a white tipped reef shark at Site 3. Manta rays, with pectoral fin spans up to 4 m, were observed at various locations around the island.

Fig. 2 Species of Reef Fish Observed at Isla Socorro
Lutjanus viridis Blue and gold snapper
Forcipiger longirostris Big long-nosed butterflyfish
Pseudochaetodon nigrirostris Blacknose butterflyfish
Holacanthus clarionensis (+ juv) Clarion angelfish
Abudefduf saxatilis Sergeant major
Microspathodon dorsalis (+ juv) Giant damselfish
Pomacentrus vaiuli Princess damselfish
Halichoeres nicholsi Spinster wrasse
Thalassoma duperrey Saddle wrasse
Thalassoma amblycephalum Twotone wrasse
Coris bulbifrins Bumphead coris
Scarus rubrovoilaceus Redlip parrot fish
Scarus spp Parrot fish
Epinephelus dermatolepis Leather bass
Epinephelus spp Grouper
Cirrhitichthys oxycephalus Pixy hawkfish
Cirrhitus rivulatus Giant hawkfish
Mulloidichthys vanicolensis Yellowfin goatfish
Zanclus canescens Moorish Idol
Acanthurus nigricans Whitecheek surgeonfish
Prionurus laticlavius Galapagos sawtail
Acanthurus leucopareius Whiteband surgeonfish
Acanthurus triostegus Convict surgeonfish
Sufflamen fraenatus Bridled triggerfish w/a yellow patch on each side of its belly
Melichthys niger Black trigger fish
Xanthichthys mento Blue cheek lined triggerfish
Cantherhines dumerili Vagabond filefish
Aluterus scriptus Scawled filefish
Kyphosus cinerascens Ashen drummer (golden form also observed)
Caranx melampygus Bluefin trevally
Carangoides orthogrammus Yellow spotted trevally
Cirripectes spp Blenny (brown, 15cm)
Myripristis spp Soldierfish
Fistularia commersoonii Cornetfish
Aulostomus chinensis Trumpetfish
Scorpiaenopsis spp Scorpionfish
Diodon spp Porcupinefish
Arothron meleagris Golden puffer
Arothron nigropunctatus Black spotted puffer
Ostrcion meleagris meleagris Spotted boxfish
 Moray eel (green, large)
Manta spp. Manta Ray

a well camouflaged octopus hiding in the reef


Diver impact on the corals of Socorro is minimal, as we did not observe other divers during our stay or any dive related damage to the coral heads.

The Naval base introduced sheep and cats to the island, both of which are rapidly increasing in number due to the lack of natural predators. It has been suggested that the cat population has caused a decrease in the numbers of shearwater birds that are endemic to Socorro.

A reduction in the shark populations may have been caused by overfishing which would indicate that fishing regulations should be strictly enforced.


Only two genera of coral were identified along the Isla Socorro coastline – Porites and Pocillopora. We estimate between two and four species within each of these genera. This study focused on corals in the shallows directly surrounding the islands. Although the majority of corals were healthy, results show that the major factor presently affecting their health and vitality is overgrowth by algae. Overall the Isla Socorro coral reef system is thriving.

Results show a high level of reef fish diversity at Isla Socorro with species abundance varying between the three sites.

Only one shark was observed. Visiting scientists studying the humpback whale population at Socorro for the past 10 years informed us that shark populations used to be more abundant. Waters surrounding Socorro have previously contained tiger, bull, mako, and oceanic white tipped sharks.

This study provides valuable data of the underwater coral reef life at Isla Socorro. Future studies using the same methodology would be able to show changes in algal overgrowth or sediment cover, as well as changes in the biodiversity of the reef.

Humpback Whales, Manta Rays, and Dolphins:

It would be impossible to complete this report of Isla Socorro without mention of the extensive humpback whale population which use this area as a breeding and mating ground. During our stay at Isla Socorro we made over 10 whale sightings daily.

There were two teams of scientists studying the whales from the University of Mexico, Mexico City and Cornell University, New York. During communication with the scientists we learned that these whales travel from North West United States (as far north as Washington State) to the Revilligegido Islands in order to breed and nurse their young. They estimate the present population at 100 humpback whales comprising males, females and calves. The male whales have been seen competing for female partners using singing, breeching and sometimes violence. Isla Socorro provides a rare opportunity for scientists to study breeding and mating patterns, as well as whale songs and general behavior of this incredible mammal.

There is also a significant bottlenose dolphin population here.


As a result of this study at Isla Socorro, it is our recommendation that this unique island ecosystem be protected for its healthy coral reef ecosystem, its endemic species and the sanctuary it provides to the humpback whale population.


Special thanks to Fernando Ortiz Monasterio and Jorge Cortina for their assistance in obtaining the permits to carry out this research at Isla Socorro.

We would also like to thank the Planetary Coral Reef Foundation, Orla Doherty and Abigail Alling for their help as well as the crew of the RV Heraclitus and her crew - Klaus Eiberle, Claus Tober, Eddie Zuna, Nicole Lovato, Ben Hartshorne, Sofia Alexiou, Priska Komaromi, Ben Rozsa, Nate Napierala, and Elena Monforte Garrido.


World Atlas of Coral Reefs
Mark Spalding, Corinna Ravilious, Edmund Green 2001
University of California Press

Coral Reefs of the World
Susan Wells 1988
IUCN Conservation Monitoring Centre, Cambridge UK

Reef Report
Vitareef Data
Coral& Fish ID
Video Transect
GPS Data