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Fat Fish, Sick Fishery?

Walsh, Sheila 3/14/2012

Walsh, S.M., S.L. Hamilton, B.I. Ruttenberg, M.K. Donovan, and S.A. Sandin. 2012. Fishing top predators indirectly affects condition and reproduction in a reef-fish community. Journal of Fish Biology 80:519-37.

Kiss good-bye to dry land and travel as far as you can from any continent and you will eventually find yourself on Kiritimati (Christmas) Island in the Pacific Ocean - 4,200 miles from Sydney, 3,300 miles from San Francisco, and the first inhabited place on Earth to greet the New Year.
Beneath the waves that surround this island, colorful reef fish roam the coral reef all day, live long lives, and are nice and fat.
Sounds like fish paradise, right? Not so fast. New research conducted by myself and my colleagues at Christmas Island and nearby Palmyra Atoll (and just published in The Journal of Fish Biology) suggests that fat fish are a sign of a fishery out of balance.
It’s well documented that marine predators like sharks are disappearing due to overfishing, even in certain spots of places as remote as Christmas Island. Losing these predators seems as if it should send ripple effects down the food chain and cause small prey fish to explode in numbers. Yet, strangely, there have been few signs of a population boom on coral reefs.
That’s because the enormous diversity of species on coral reefs can buffer these ecosystems from big shocks like removing a predator species. But, I still had to wonder: aren’t prey fish noticing that they are no longer being hunted by hungry sharks? Were the previous studies measuring the wrong thing?
My colleagues and I set out to examine this question.
Our research found that, while the population numbers of prey fish in overfished spots on Christmas Island don’t show that the predator-prey relationship has been altered, indicators of fish health tell another story. On coral reefs with high fishing and few predators, the prey fish are fat. On coral reefs with predators still intact, the prey fish are skinny….and a little bit nervous.
Trying to Catch an Escape Artist
One of the challenges we faced in looking at the predator-prey relationships on coral reefs is that there are few places where predators are still truly abundant. But Palmyra and the remote southeast coast of Christmas Island are like traveling back to a place in time when predators were more abundant than their prey and corals were lush and resilient. We compared these locations to the heavily populated northwest coast of Christmas Island, where predators have been overfished.
After months of painstaking preparation shipping weighing scales, dissecting tools, spearfishing equipment and SCUBA gear, we arrived in Palmyra to start our monthlong expedition that would conclude on nearby Christmas Island. We had a plan to collect samples of the five most-abundant species representing all key types of reef prey fish, from damsel fish that sit in the current snacking on passing plankton to surgeonfish that roam the reef grazing like cattle.
Although the numbers of sharks and other top predators on the un-fished and lightly fished reefs of Palmyra and southeast Christmas Island was enough to keep us researchers on edge, it was the prey fish that seemed to be really worried. All we had to do was dive, catch fish all day, dissect them all night and avoid sharks. But it wasn’t so easy — these fish had the defensive moves of professional escape artists! When we came back with barely a fish in hand, we knew something was different about Palmyra.
Skinny, Nervous Fish — But Not the Ones That Should Be Worried?
Previous studies have shown that prey fish on Palmyra spend a lot more time hiding and don’t live very long compared to Christmas Island. It turns out that these changes have consequences for the health of the individual fish and the population in terms of body condition and reproduction.
Our hypothesis? If you’re spending all your time trying to not get eaten, you probably can’t put much energy toward getting bigger and fatter. And, if your chances of making it to tomorrow are low, it might just make more sense to hurry up and have babies to pass on your genes rather than save up for having babies later.
We eventually managed to complete our collections. (Ironically, we lost a few of our samples to sharks and hungry locals on Christmas Island.) However, our reward was a pretty interesting result. These data, representing the diverse community of reef prey fish in Palmyra and Christmas Island, showed that prey fish are consistently heavier and fatter on reefs with fewer predators. However, the patterns in reproduction were less clear (isn’t reproduction always complicated?).
Okay, so what? Well, we used to think that fishing top predators, while not so great for the predators themselves, really didn’t affect the prey fish on coral reefs. After all, we didn’t really have evidence of their numbers changing. But, in fact, it turns out that loss of predators does affect the prey fish’s health in surprisingly consistent ways — they’re fatter.
Although being fat doesn’t sound like such a big deal, it may indicate that prey fish are growing more slowly and the productivity of the reefs is actually decreasing. The jury is still out on these questions, but some ongoing studies may reveal the answer. Marine conservationists’ bigger concern may be that the prey fish will be the next to disappear — because once predators are overfished, fishermen tend move down the food chain to fish prey.
In the meantime, fisheries managers may now be able to use these indicators of body weight and fat to detect whether predator-prey relationship they previously thought were fine are really out of balance.


* Sheila Walsh is a senior scientist for the sustainability science program at The Nature Conservancy.
Image: Sheila Walsh trying to catch a fish on the trip. Image credit: Kevin Lafferty.