One Fish, Two Fish

by Stuart Sandin, head researcher on the Fish Team

Snapper and school of convict tang on Palmyra. Photograph by Stuart Sandin.

If you listen to what our colleagues onboard have to say, you’d think we in the Fish Team were a simple bunch. In their eyes, we swim about on the reefs with only the most mundane of equipment—a dive slate and a pencil—and we record the fish that we see. I have even received a copy of Dr Seuss’s One Fish Two Fish Red Fish Blue Fish from Forest’s five year-old daughter as a birthday gift. Thanks for the scientific training manual, Willow!

Yes, we have counted a lot of fish. And yes, we have used simple equipment. But the data collected have been far from simple. As opposed to simply counting all fish to the closest color, we also record the species identity and the size of each individual within our survey area. Given this information, we can rely on established mathematical relationships between length and weight to estimate the weight of each fish counted. Further, based on natural history knowledge about the behavior of each species, we can categorize the fish into functional groups. We typically classify them based on what they eat: algae-eating herbivores, plankton-eating planktivores, crab- (and other invertebrate-) eating omnivores, and the top-dog, fish-eating apex predators.

Fish, and more fish, above a coral garden at Palmyra. Photograph by Jen Smith.

 

The fish counts and related data from our first trip to the Line Islands in 2005 yielded some surprising results. On the inhabited Kiribati islands with active reef fisheries, we found fish communities that looked good relative to many reef locations across the globe. For example, the total biomass of the fish community was higher than that found at popular dive destinations in Hawaii, the Caribbean, and the Indian Ocean. Of this total biomass, there was a seemingly healthy contribution from the apex predators, species that are particularly attractive for local fisheries. In fact about one fifth of the biomass was made of these yummy predators like snappers, trevally, and groupers.

 

 
However, when we finally made it to the uninhabited islands—Palmyra and Kingman—our perspective shifted. Sure, there was more total fish biomass where there was no fishing. That was not unexpected. But the distribution of this biomass across the fish community was shocking. There were not simply more of every species of fish, but rather there was a lot more of just a few groups. Most prominently, the apex predators were much more abundant in the absence of fishing. In fact, the apex predators made up more than three-quarters of the total fish biomass on Kingman. Yes, you read that right. The predators outweigh the prey in what we call an inverted biomass pyramid. Imagine walking through the western plains of the US and finding more wolves than bison. This is what we have been swimming in for the past week in the underwater US West.

One fish, two fish. Photograph by Stuart Sandin.

So now the Fish Team has returned to Palmyra and Kingman to try to work out how this is possible. How can a reef support more predators than prey? If there are that many predators, what are they eating? We know that they are eating ‘prey,’ but how can they find enough prey to stay alive? To get a handle on this, we are using a new approach, and a new set of tools. We are selecting a few species for a closer look—three herbivores, two planktivores, two omnivores, and one apex predator. We are collecting individuals from each species to determine what they eat, how fast they are growing, and—most importantly—how this differs at the various islands. We think that prey may eat differently and grow differently when they are surrounded by tons of predators. An entire academic community of environmental physiologists works to determine how stressful environments affect the health of children during their formative years. Why shouldn’t the stress of being eaten day in and day out affect fish similarly? And perhaps what we learn from fish could inform doctors working with children.