14 DAY TRIAL // Studies on the bottom dwelling toadfishes (photo) off the Florida coast, main prey for dolphins, revealed that these fishes often listen to dolphin calls to avoid getting caught. "Probably a lot of fish can do this," said Luke Remage-Healey, a behavioral neuro-endocrinologist at University of California, Los Angeles.
In turn, dolphins themselves often listen to toadfish calls to locate them and, in fact, 80 % of bottlenose dolphin diet is composed of sound-producing fish. Remage-Healey first noticed toadfish's behavior two years ago while recording the mating calls of the male toadfish off the Gulf coast of Florida. "Then, they all stopped calling," Remage-Healey recalled.
"My field assistant noticed dolphins foraging right over the toadfish site, and we heard we were recording dolphin sounds instead."
Then, the biologists placed toadfishes in cages on the seabed in the breeding yard. They played underwater recordings of neutral, common background noise in the bay, like snapping shrimp sounds or high frequency ("whistle") dolphin communicating sounds, but also low frequency "pops" used by dolphins to locate prey. The toadfishes ignored the "neutral" sounds, continuing their mating calls. But hearing dolphin pops or pops and whistles combinations, the fishes drastically decreased their calling rates.
Previous researches showed that toadfish is very sensitive to low frequency sounds, in order to react to low frequency pops of the dolphins. Measurements of toadfish blood taken immediately after the dolphin sounds were played revealed increased levels of the stress hormone cortisol.