THINK ON THESE: Coral reefs’ “twihlight zone”

In the early part of the 17th century, Spanish religious missionaries founded a small settlement which was called Abre de Ilog (coming from the Spanish verb, “abre,” which means “open” and Tagalog noun, “ilog” for “river.” Later on, it was renamed as “Abra de Ilog” (its present name), a Chabacano-like terminology which is loosely translated as “bucana ng ilog” or “opening of the river.”

Abra de Ilog is one of the eight municipalities of Occidental Mindoro. Although it is far from Davao City, I am highlighting it here since the Department of Science and Technology (DOST) conducted a coral mapping project, which the University of the Philippines-Marine Science Institute implemented. Davao region might be interested to know the findings of the study.

The projected did some “detailed underwater surveys and mapping in the shallow euphotic reefs and deep mesophotic reefs” of the town, according to Rose Anne M. Aya and Ronald Olavides, authors of the report.

Euphotic reefs are those corals found at 1-30 meters depth; they have been extensively surveyed by many marine researchers and conservation managers using conventional survey methods.

The mesophotic reefs, on the other hand, are corals located at 30-150 meters depth. They are considered at the “twilight zone” and among the least studied environment due to the technical limitations and personal risks involved in deep surveys.

“The project aims to document the occurrence and biodiversity of mesophotic reefs, provide detailed seafloor maps and hydrodynamic models,” the two authors wrote.

Abra de Ilog is among the first sites in the country that has been surveyed quantitatively for its mesophotic coral reefs and fish communities.

“The reefs surveyed were severely disturbed by sedimentation,” the report said. “Many dead and live corals were covered in silt. Live hard coral cover in euphotic zone was poor with 21% cover, and even more so in the mesophotic zone with only 4%. Dead coral cover was 29% in the euphotic and 26% in the mesophotic.”

Despite this fact, the researchers found out that the place is teemed with biodiversity. “The area is surprisingly biodiverse, considering the poor coral condition, high turbidity, and heavy siltation,” the report said. “These indicate that the mesophotic reefs should be included in conservation priorities and that they are also vulnerable to large-scale impacts like sedimentation.”

As they differ in depth, the kinds of corals thriving in both zones. Shallow reefs were dominated by boulder-like massive and branching corals while the upper mesophotic was home to flattened encrusting forms.

“There are more gorgonians, black corals, sponges, invertebrates, and fishes in the mesophotic that remain largely undocumented,” the report pointed out.

A total of 66 fish species from 15 families were recorded during diver-based surveys. Forty-one species from 13 families were found in the euphotic zone and 45 species from 15 families were in the upper mesophotic zone.

A total of 39 coral genera was also recorded, all of which were found in the euphotic zone, and 25 genera in the mesophotic zone.

“Despite poor reef conditions in the site, overall fish community biomass was high and averaged 54 metric tons per square kilometer,” the report said. “The mesophotic zone had six times more fish biomass than the euphotic zone: 92.5 metric tons per square kilometer.”

Target fishes and benthic carnivores were particularly more abundant in the mesophotic zone, the study found. Starting at the edge of the forereef, the most dominant fish species is the banana fusilier (Pterocaesio pisang). At 30-35 meters deep, golden damselfish (Amblyglyphidodon aureus) occurred in schools along with smaller groups of squarespot anthias (Pseudanthias pleurotaenia), which are iconic species in the upper boundary of many mesophotic reefs in the country.

“A flurry of wrasse, surgeonfish, unicornfish, angelfish and butterflyfish species also occurred,” the report said. “However, the lack of large predators like the groupers, snappers, trevallies and sharks is symptomatic of overfishing that places selective pressure on these high-value fishes.”

There are three major types of coral reefs, according to Dr. Angel C. Alcala, former head of the Department of Environment and Natural Resources. These are fringing type (those found on the edges of islands and which constitutes 30% of the country’s coral reefs); the barrier type (best exemplified by the Dajanon Reef of Central Visayas); and the atoll (of which the Tubbataha and Cagayan Reef in the Sulu Sea are ideal examples).

Unknowingly, corals are the dried and bleached skeletons of soft-bodied animals that live in the warm, sunlit waters of tropical seas and look more like plants and rocks than animals.

The main part of the real coral is the polyp – the extraordinary flower-like animal with a tube-like body and finger-like tentacles. “Coral polyps get nutrition in two ways,” explains Lindsay Bennett, author of globetrotter island guide, Philippines. “They catch their food by means of stinging tentacles that paralyze any suitable prey – microscopic creatures called zooplankton – and also engage in a symbiotic relationship with zooxanthellae that live within the polyp structure.”

Coral polyps reproduce in two ways: asexually (by the division of existing individual polyps) and asexually (by combining egg and sperm from two different polyps). “This results in a free-swimming polyp that will be carried by ocean currents to find a new colony and commence a new reef,” Bennet writes.

The coral reef is the world’s most diverse marine ecosystem, and one of its most productive. It is home to some 4,000 species of fish (approximately one-quarter of all marine fish species), along with a vast array of other life forms – mollusks, crustaceans, sea urchins, starfish, sponges, tube-worms and many more.

In the Philippines, for instance, more than 40 million people live on the coast within 30 kilometers of coral reef. Approximately, two million people depend on fisheries for employment, with about one million small-scale fishermen directly dependent on reef fisheries. The country’s reefs yield 5 to 37 tons of fish per square kilometer, making them very important to the productivity of fisheries.

“The Philippines is a major supplier of fish to the live reef food fish trade, a billion-dollar industry in the Asia-Pacific region,” noted Reefs at Risk Revisited in the Coral Triangle, a publication published by the Washington-based World Resources Institute (WRI). “In 2007, the Philippines exported at least 1,370 tons of coral trout (Plectropomus leopardus), one of the trade’s most important species in terms of volume, which fetched an estimated retail value of about US$140 million.”

“Despite considerable improvements in coral reef management, the country’s coral reefs remain under threat,” said Dr. Theresa Mundita S. Lim, director of the Biodiversity Management Bureau of the Department of Environment and Natural Resources.

SHARE