One Man’s Swamp Is a Fish’s Nursery

A parade of schoolmaster snapper swims by me, their neon yellow fins directing traffic. Echoing in the background is the rhythmic crunch of striped parrotfish nibbling on coral polyps. I’m chasing brightly colored coral reef fish through turquoise waters during a childhood vacation to the Cayman Islands. Back on land, the hotel I just left begins construction of a new tennis court where a mangrove swamp had flourished.

Like most tourists, I’ve come for the main show: magnificent coral reefs and the rainbows of fish that dance through them. As I diligently scour the reef for every fish on my checklist, I never stop to question where those fish came from or how they got there. Who would have guessed that it’s what  goes on behind the scenes, in mangrove swamps like the one my hotel just paved over, that keeps the main show on coral reefs going.

Mangroves are like nursery schools for many of the “celebrity fish,” such as snappers, parrotfish, and barracuda, that attract millions of tourists to the Caribbean each year. Amid the mangroves’ tangled undersea roots, juvenile coral reef fish are well-fed and protected from bullies and predators until they mature and can migrate out to populate coral reefs. These valuable nurseries are disappearing at an alarming rate, and so are the fish they support. It’s ironic: The sweltering, mosquito-filled mangroves being so eagerly destroyed may be more important to tourism than the tennis courts and martini bars that replace them.

Scientists estimate we’ve already lost 30 to 60 percent of the world’s mangroves. Meanwhile, more than 70 percent of all reefs are threatened by human activities—from farming and fishing to boating and coastal development. Coral reefs offer people living resources, from fish to ecotourism, worth more than $375 billion each year, according to the United Nations Environment Program. Such dollar figures emphasize the need for management practices such as marine reserves to protect these reefs and their resident fishes.

Which swamps should we protect?

It’s been more than a decade since my childhood vacation to the Cayman Islands, and I’m still diving in the Caribbean. This time, however, it’s as a Ph.D. student in the MIT/WHOI Joint Program. I study the migration of schoolmaster snapper from their juvenile nurseries in mangroves to coral reefs. Unfortunately, we can’t save all of the mangroves. My goal is to determine which mangroves and how many mangroves need to be included in marine reserves to safeguard coral reef fish for generations to come.

One of the biggest obstacles to protecting coral reef fish is identifying which juvenile nurseries supply the most fish to coral reefs. Let’s suppose there were several schools that had fantastic teachers and ample resources. However, no one knew which of these schools graduated the most students into successful adults. That’s the problem we are facing right now with coral reef fish. I am developing a method, using chemical tags in the ear bones of coral reef fish, to identify which juvenile nurseries are supplying fish to the adult populations on coral reefs.

Tracking the movements of marine animals has led to incredibly inventive techniques that would make Sherlock Holmes proud. Scientists studying big animals such as whales, sharks, and giant tuna can use sophisticated electronic tags that beam data directly to their offices via satellite as these animals make spectacular ocean migrations.

The fish I study are far too small for that: If I put such tags on a juvenile snapper, it would sink to the bottom like a rock. To find out where my fish have traveled, I need something a little more subtle.

Chemical clues in fish’s ear bones

Studying biology and chemistry as an undergraduate at Bates College in Lewiston, Maine, gave me the idea to approach ecological questions such as fish migration from a chemical perspective. Mangrove swamps contain carbon, oxygen, and sulfur with distinctive stable isotopes—natural, nonharmful variations in elements that act like a unique chemical address. Following the old adage “you are what you eat,” the tissue of snappers living and feeding in a mangrove system get imprinted with that mangrove system’s stable isotope values, like a chemical address label.

The tissue I’m interested in is called an otolith. It’s an ear bone made of calcium carbonate and protein that helps fish maintain their balance. As the fish grows, its otoliths form sequential rings, much like a tree trunk, corresponding to different times in the fish’s life. The chemical makeup of each ring tells us where the fish had been living during that period of time—a fishy chemical address book.

Over the past two years, I have been figuring out how to decipher the chemical addresses stored in otoliths using stable isotope chemistry. As it turns out, some of the smallest compounds may tell the biggest stories. I am identifying specific compounds in otoliths whose stable isotope signatures will say, “This fish grew up in a mangrove system before it migrated out to the coral reef.”

The non-tropical side of research

It all starts with some very small fish and some even smaller otoliths. Armed with a scalpel and microscope, I carefully remove the delicate otoliths, each smaller than a grain of rice. I subject the otoliths to a series of chemical processes to identify compounds that reflect the chemical address of the nursery in which that fish lived. This typically involves countless hours injecting samples into a Gas Chromatograph-Combustion-Isotope Ratio Mass Spectrometer.

This often-temperamental machine tells me the stable isotope values of the otolith material. I match these values to those of the potential nursery habitats to determine where that fish spent its juvenile life. If I do this enough times, I’ll be able to say how important those mangrove swamps are to supporting coral reef fish populations.

I used to think studying coral reef fish would be all fun in the sun. However, for every day I spend in a wetsuit under the golden sun of the tropics, there are at least 10 more in a white coat under the fluorescent glow of a windowless lab. I’ve often run samples for 72 hours straight with only 20-minute naps in my office chair, using my keyboard as a pillow, to keep me going.

A little sleep deprivation is well worth it if it means I can provide the scientific basis to design marine reserves that protect coral reef fishes. With dynamite fishing on reefs and the demolition of mangroves to construct tennis courts, it’s not surprising that coral reefs and mangroves have become some of the most degraded environments on Earth. I hope my research will guide the conservation of coral reef fishes, so that by the time my children take a Caribbean vacation, they can pack their snorkels as well as their tennis rackets.

Kelton McMahon’s research is funded by a National Science Foundation Graduate Research Fellowship,  the Ocean Life Institute at WHOI, and an International Society for Reef Studies/Ocean Conservancy Fellowship. This article was written during a science writing course for graduate students at WHOI, supported by funds from The Henry L. and Grace Doherty Professor of Oceanography.