Dolphins appear to catch their prey selectively. They select the fish that contain the most energy and leave the less interesting (less calorie-rich) prey swimming. It was always assumed that dolphins eat anything that swims by.
This research was performed with common dolphins that live in the warm coastal waters of the Atlantic Ocean. Researchers studied the stomach contents of dolphins that got trapped in fishing nets and compared these contents with the kinds of fish that occur in the coastal waters. This revealed that the common dolphin is quite a picky eater. The stomach contents contained no fish that was worth less energy than five kilojoules per gram. Favourite prey were the different species of lantern fish. These contain 6 to 8 kilojoules of energy per gram. The results of this research are easy to explain. Dolphins are warm-blooded animals that swim fast and they are social animals. These are traits that cost a lot of energy.
Cold-blooded sharks make no distinction in their prey. They catch any prey they can get.
The results of this research were published in the Journal of Experimental Marine Biology and Ecology.