Ocean Watch

Monday, November 10, 1997

The shifting times of sand
due to people and nature

I received some interesting questions about sand lately.

One reader, an avid spearfisherman, has observed that parrotfish have become pretty scarce around Oahu over the years.

He asks: Since parrotfish make sand, is their decline in numbers decreasing the amount of sand we have on our beaches?

It's true that parrotfish make sand. Some parrotfish bite off pieces of dead coral rock (limestone) in order to get the algae growing on it. Other species eat live coral, biting into the limestone skeletons that support the living coral animals. Still others eat algae growing on the surface of the sand, swallowing some sand in the process.

Special bones in the fish's throat grind up both food and coral rock. The food gets used by the fish's body; the ground-up coral gets discharged through the anus.

If you go to places where fish are protected, it's easy to watch both ends of this process in action. While snorkeling in the sanctuary waters of Midway Atoll recently, I watched enormous parrotfish (2 feet long) take noisy bites from living coral.

With loud crunching sounds, they scraped their beaklike teeth over the surface of coral heads, leaving characteristic tooth marks.

At the same time, passing parrotfish excreted clouds of fine, white sand that fell to the reef floor.

So. We know parrotfish make sand. And we also know that fishing has depleted parrotfish stocks in many unprotected areas of Hawaii. It seems obvious then, that a lack of parrotfish means significantly less sand for Hawaii's beaches.

However, it's not true. And that's because ground-up coral makes up such a small proportion of Hawaii's white beach sand.

In many cases, tiny seashells called foraminifers compose 50 percent of the grains on a Hawaii white sand beach. The other components are snail shells, coralline algae pieces, sea urchin and starfish shells and, running a distant fifth, coral fragments.

In rare cases, at its most concentrated, coral makes up 20 to 25 percent of a white Hawaii beach. On the average local beach, however, the number is less than 10 percent.

So where is all this sand that parrotfish produce?

On the ocean floor. Much of the ground-up coral that comes from parrotfish is so fine and powdery, it rarely reaches shore. Rather, most of it falls into cracks and channels and remains near the reefs.

So, in answer to my reader's question, depletion of parrotfish doesn't do much to Hawaii's beaches, but it does lessen the amount of sand found in and around our reefs. Thus, these fish play an important role in the health of Hawaii's coral reefs.

Another reader wants to know if the beaches off Aina Haina were ever sandy.

Probably not, at least not since humans have been here.

The Wailupe Peninsula, a residential site seaward of Aina Haina, was once an enormous fishpond.

Ancient Hawaiians built their fishponds in areas where freshwater mixed with salt water, usually meaning some silty runoff from the land. Such conditions often produce muddy shores rather than sandy beaches.

This is true also of Waikiki, where the white sand beaches are mostly man-made.

Sand is periodically trucked there to replace that which is carried away both by water and people.

Beach changes often occur through human interference, but some changes are natural.

During my last trip to French Frigate Shoals, a pristine atoll in Hawaii's northwest chain, I was shocked to find one of my favorite islands, Whale-Skate, completely gone.

Biologists there assured me it would be back in its own good time, that is, at the whim of ocean currents.