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Surgeonfish,”Alogo” — ruler of Samoa’s reeftop

October 8, 2012

American Samoa’s coral reefs are truly a wonder of nature. Our sea is home to a diverse and colorful assemblage of plants and animals. Some 930 species of fish occur here, which is about twice the number of marine fish species found in Hawaii.

Surgeonfish, “Alogo”

At first glance, the reef seems to be an exotic panorama of mass confusion, complete with bizarre shapes of fish
painted in psychedelic colors. It’s like looking into an overstocked aquarium. But as you frequent the reef more often, you begin to notice some structure to the confusion. Each species is generally found only in certain habitats such as shallow reef flats, sandy bottom areas, or deeper waters.

Many individual fish even take up permanent residence at a particular site rather than roam around. One
particular fish I watched stayed near the same coral block for 3 years (the fish had a unique markings on
its body, so I could easily identify it). That coral block was home. Such stay-at-home behavior is
actually quite common among coral reef fishes. One abundant species on our reefs that does this is the alogo (Acanthurus lineatus), also known as the blue-lined surgeonfish because of its knife-like blade located near its tail. The blade is usually not visible because it is folded away into a groove in the fish’s skin. It is a bit poisonous, and careless handling of the fish may cause a puncture and painful swelling in your hand.

Alogo blade

The alogo grows about 8 inches long and weighs half a pound. It
is a very attractive fish, with bold yellow, blue, and black
horizontal lines on its sides, although its basic color pattern can be
swiftly altered depending on the alogo’s mood. For example,
when the alogo becomes aggressive and chases another fish, its
face and fins darken and it looks angry.

The alogo lives in the foamy surge zone where the waves crash against the reef. This is not an easy place to live, but the alogo is quite skilled at it. When a really rough wave hits, the alogo darts down into a
hole or over the reef edge into the safety of deeper water.
Like a lot of other reef fish, the alogo is a territorial animal, which means that it dwells at a particular
patch of reef and protects that site from all other fish. The territory of each alogo measures about 5 x 5
feet. There it feeds on the thin film of plant material (algae) that covers the reeftop and appears as a
greenish grassy turf. Because of their territorial nature, the alogo space themselves evenly across the
reeftop, and as they munch away on the algae-covered rocks, they remind me of a herd of miniature cows
feeding in a distant pasture.

Many other species of coral reef fish are also algae eaters, and two general patterns of feeding have
evolved among these species. One is for a species to become territorial and fiercely guard its own algal
patch, the other is to be non-territorial and roam around the reef looking for an unguarded patch of algae
to eat. To an underwater observer, this dual approach to feeding is readily visible — most of the reef is
picked clean of all edible algae and looks like bare rock, except where a territorial fish guards its lush
algal plot.

The feisty alogo defends its plot from all competitors, so the turf algae grows well there and provides all
the food the alogo needs. Protection of this garden doesn’t come cheaply, however. The alogo must
defend its territory every minute of the day from other fish that lurk nearby, waiting for a chance to sneak
in and chow down.

That’s where the alogo’s sharp blade comes in handy. The alogo will threaten to viciously sideswipe an
intruder with this weapon. Most other fish heed the alogo’s warning and back-off quickly. It’s mostly a
bluffing game played repeatedly through the day, and rarely does anyone get hurt.
Other aspects of the alogo’s behavior are fascinating. Every evening at dusk, all the alogo migrate off the
reeftop to deeper waters where they will spend the night sleeping in crevices to escape being eaten by
predators like sharks (malie) and jacks (malauli, ulua). At dawn, they return to their exact same reeftop
territories by the same route. Their migrations to and from the reeftop look like rush-hour traffic on an
underwater highway.
The alogo is a popular Samoan food fish and it is one the most important species of reef fish caught,
accounting in some years for up to 30% (by weight) of all reef fish caught in the nearshore subsistence
fishery. Most are caught by spear fishermen, particularly at night when the fish are sleeping in reef
crevices. Daytime spear fishermen have a much harder time catching them, because the alogo tend to
stay just out of spearing range.

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