On Becoming Part of the Food Chain

by Scott Hamilton and Brian Zgliczynski, members of the Fish Team

At Kingman, curiosity comes bearing teeth. Photograph by Jen Smith.

A few hours after dark, we arrived at Kingman Reef—a U.S. Fish & Wildlife Refuge and Marine National Monument at the northwestern end of the Line Islands chain and arguably one of the most pristine coral reefs remaining on the planet. As the Hanse Explorer turned to enter the mouth of this horseshoe-shaped reef system, we were greeted by jumping schools of needlefish and flying fish illuminated by our bright ship lights. We set anchor and within minutes the needlefish congregated around the ship’s lights, looking for prey. Just as quickly, the apex predators (grey reef sharks) began encircling the boat, looking to prey on unsuspecting needlefish. Over thirty large sharks patrolled nearby, tentative at first—making quick little strikes to gauge the reaction of their prey. For the next several hours we watched the sharks chase and attack needlefish right at the surface of the water. Each successful strike was accompanied by a feeding frenzy as sharks rushed into a roiling mass, fighting over the remains. This exact scene was to recreate itself over the successive three evenings at Kingman, but all we could think about at this moment was that in the morning we would be diving amongst all those sharks as part of the Fish Team.

Indeed, early the next morning we headed out through La Paloma Pass to a site on the south shore to make our first dive at Kingman. We were excited as we prepared to enter the water while dolphins played in the surf only fifty feet away. Our goal this cruise is to examine how apex predators change the growth rate of fishes from a variety of trophic levels: the herbivores who eat seaweeds, the planktivores that consume zooplankton, the omnivores that prey on invertebrates and others, and apex predators that target other fishes. To do this, we have to collect individuals and then record their size, reproduction, energy stores, diet specialization, and—most importantly—age and growth using ear-stones (i.e., otoliths). Otoliths act as historical recorders, forming annual bands of alternating light and dark color that reflect events throughout a fish’s life, similar to tree rings.

A shark in his element, at Kingman Reef. Photograph by Jen Smith.

We descended onto a beautiful shallow reef, covered in dense thickets of staghorn Acropora corals, where we had to fight against a raging current to maintain our position and begin our task of fish collection. Employing a variety of collecting implements ranging from hand nets to spears to traditional fishing gear, we began sampling eight common species representative of major trophic groups. Instantly, twenty grey reef sharks, six whitetip reef sharks, and over a hundred two-spot snappers–all apex predators on these reefs–assembled in our vicinity. All this attention made for an action-filled first dive that was a bit unnerving. The reef sharks—much larger here on average than at Palmyra, and very curious—drifted just beyond the tips of our fins. After seeing how effortlessly the sharks swam in the current last night, we never felt lower on the food chain.