Dr. Becca Selden loves being a scientist. She spends a lot of her brain energy thinking about predators and prey. Becca was curious about what happens to fish as their body sizes change. Body size is very important to fish: it controls what prey they can eat, and what predators can eat them! In some parts of the ocean, people have fished out most of the large fish, changing how predators and prey interact with each other.
Becca wanted to know how the size of predators (sheepshead fish) changed when the size of prey (sea urchins) changed. To figure this out, she created an experiment in the Channel Islands of California, where sheepshead fish live in
kelp forests. There were some areas around the island where people weren’t allowed to fish, called Marine Reserves. Pumpernickel, where Cher and Emi ended up was a marine reserve, and that’s why there were so many other big fish there. Becca tested what prey sheepshead liked to eat when they lived in marine reserves. She also tested what they could eat when they lived in other places around the islands that weren’t protected from fishing. To test the fish, she put out different sized urchins in a quadrat, like putting a bunch of food out on a table. Then she watched what different sized sheepshead ate.
Becca discovered that until sheepshead get bigger than a half a foot long, they don’t eat urchins. She also found that smaller sheepshead can only eat smaller urchins. In marine reserves where all the big sheepshead have been fished by people, this means the smaller sheepshead left behind can’t each the big urchins. Urchins eat the kelp forests where sheepshead live, so if predators like big sheepshead don’t eat them, urchins will eat too much kelp and destroy the forest.
predator: an animal that eats other animals
prey: an animal that is eaten by other animals
Channel Islands: islands off the coast of Southern California that are home to many kelp forests and the animals that live in kelp forests
Marine Reserves: an area of the ocean where fish is either not allowed, or limited to protect the ecosystem
quadrat: a portable square grid that makes a border around ground where scientists do experiments
Scientists used to think that when baby fish (called larvae) hatched, they
floated in the plankton to wherever currents take them. Recently, scientists have actually measured how far larvae disperse, and it turns out, they almost never settle far from home! The scientists who did this research began to wonder, maybe larvae are really strong swimmers. If this was true, then larvae could fight the currents so they stayed close to their home reef, or could return after being carried away by currents.
The scientists tested this by putting larvae on fish treadmills, called flumes.
They collected Elacatinus lorilarvae as they hatched on the reef in Belize, Central America, and raised them in a wet laboratory on the island of South Water Caye (put in picture of lab). As the larvae developed, scientists put them in the flume and measured how long they could swim at different water current speeds. They found that E. lori larvae actually can’t swim long at all! When scientists tested larvae at speeds close to the current speeds on the reef, larvae can’t swim longer than a few minutes. What a discovery!
Now the scientists are wondering how these tiny larvae, who can barely swim, are settling so close to their home reefs. Stay tuned as scientists test their next question: maybe these larvae are skilled navigators? This summer the scientists will test the orientation abilities of larvae, as nobody before ever has. Eventually, their research will help answer the question of how larvae disperse. Once we know that, we can make marine protected areas that keep of our fish and our oceans healthy.