Room at the Top

School of bigeye travally in the Phoenix Islands. Photograph by Stuart Sandin.

It is simply common sense. Look around you at any familiar ecosystem and you’ll see that there are few top predators, lots more prey, and still more vegetation for munching. This seems intuitive because we know that at each trophic (“feeding”) level, the organisms eat many times their own weight in food, roughly ten times their weight. It takes many mice to feed one hawk. Only 10% of what is consumed becomes new biomass that can then pass on up the food web to the next higher level. About 90% is spent in just staying alive.

But there are an awful lot of sharks at Kingman Reef. So how does this common sense apply to a pristine coral reef ecosystem? For simplicity, let’s picture a reef as having just four trophic levels. At the bottom are the primary producers, those being the photo-synthesizing algae and bacteria. Next level up are the herbivores—the fish and the invertebrates such as sea urchins that graze the algae. Next are the small predators that prey on the grazers, and at the top the large-bodied predators such as sharks, travally, and groupers. Since only 10% of the food eaten passes on up to the next level as biomass, this means that every pound of shark swimming about represents at least 1000 pounds of algae, 100 pounds of grazers, and 10 pounds of smaller predators. These approximations are minimums because there can be more mouths between the algae and the sharks, each taking their 90% cut.

A generic trophic pyramid.

 

Looking at a coral reef this way it seems perfectly natural for there to be only a few sharks on a reef. Even though we can recall when there used to be a few more around, as long as we saw an occasional shark cruising past above the corals, we had a “healthy” reef. What about the huge numbers of sharks described in the logs of the first European explorers to visit these regions? Exaggerations to impress the folks back home. Unscientific anecdotal evidence.

An inverted trophic pyramid.

This view was turned upside down in 2002 when Alan Friedlander (one of The Fish on the Line Islands Expeditions) and Edward DeMartini published a careful study of the fish assemblages in the Hawaiian islands. They compared the remote northwestern reefs with the heavily fished areas around the urbanized main islands. Where man had not been fishing, there were sharks, jacks, and other top predators in abundance. Combined, they made up more than half of the total fish biomass. In the fished areas, they were less than 3% of the total. The usual trophic pyramid was instead inverted. Was this just a peculiarity of the Hawaiian island chain? Or had we ourselves fallen prey to shifting baselines?

A mere quirk of The Hawaiian islands it was not. The Fish on the 2005 Line Islands Expedition came up with even more striking numbers for pristine Kingman where the obviously abundant sharks and other predators made up 85% of the total fish biomass—a figure that held the record for some time until a later expedition to Starbuck Island in the Southern Line Islands topped it.

How could this be? The explanation involves differences in the rates of growth and reproduction of the small-bodied grazers and small predators as compared to the massive, longer-lived top predators. The principle involved is a familiar one: it is more expensive to raise kids than to maintain adults. Still many questions remained, some of which are being investigated during the current expedition. For example, when the top predators have been removed, one would expect there to be more small fish and that they would live longer, grow larger. Indeed there are more, but not as many as you would expect. Why is that? We already know that removing the top predators has other profound effects on the reef community. One effect that will surely be discussed more later in the expedition is the death of the reef-building corals and overgrowth of the bottom by the algae. We don’t expect simple answers, as reefs are complex interacting systems.

An inverted trophic pyramid.

Determining the amount of fish on a reef gives us only a snapshot from one point in time. What matters more to us, from the perspective of our fisheries, is how many pounds of fish are being produced per day. That can help us determine how much we could harvest on an ongoing basis, without fishery collapse. Which reef produces the most fish? A pristine reef with lots of sharks, one where all of the sharks have been fished out, or something in between? To begin to answer this, the current expedition is measuring productivity on the reefs at all six locations, reefs that vary in the amount of local human disturbance as well as in their natural oceanographic conditions. Measurements are being made in situ on the reef using the benthic tents and algal cages (see The Science page); other samples are collected on the reef and brought back to aquaria on the ship where the experiments will be run. The net result will be comprehensive data about the activities of the benthic algae and corals, the fish, and the microbes.

If collectively we make use of the information gained, we might yet find ways for humans to live with coral reefs, to enjoy them and to use them, but without destroying them.

Meanwhile, you don’t go swimming at Kingman after dark.