http://tbn0.google.com/images?q=tbn:eGGOFwc9ui9IZM:http://www.taipeitimes.com/images/2004/10/17/thumbs/20041016155912.jpeg

This is a double cardioid fish trap. This particular trap is in Penghu, China; as you can see from its beauty it has been maintained for centuries. It works like this:
* There are walls not shown above sticking out from the shoreline leading to the outer opening of the trap like a spade: http://kooderi.acm.jhu.edu/code/java/21/spade.gif
* Fish swimming along the shoreline as the tide ebbs meet one or more of these walls. They follow the wall until they swim into the trap.
* Some of these fish are small enough to fit through the narrow second opening. And the second cardioid is shallow enough to catch these fish with your hands or a little net at low tide. 
* Larger fish are predators - sharks, stingrays and so on. They're too big to make it through the second opening. They force the smaller fish to follow the walls and escape.
* Fish do not turn sharp angles; they prefer open spaces. So escaping fish that follow the walls leading back to the outer opening are redirected toward the opening to the second cardioid.
* At low tide then the second cardioid contains a shallow pool full of fresh, delicious tiddlers. And a pleasant little beach where you can comfortably stand in bare feet.
* The walls of the traps are the right height so that at high tide the sea washes all the fish back out of the trap. Otherwise it would fill up with stagnant water and starving fish.

-- PeterMerel