Sharks: The Animal Answer Guide

7 Foods and Feeding

135 Chapter 7 Foods and Feeding What do sharks eat? The most general statement that can be made about feeding patterns in sharks is that they are opportunists, eating just about anything. Except plants: sharks don’t like brussels sprouts, perhaps attesting to their intelligence . Actually, chondrichthyan fishes don’t like any vegetables or fruits. They are as a group entirely carnivorous, eating all types of marine animal life that they can catch, or scavenging on dead animals that other sharks have dismembered. Bony fishes make up 70% to 80% of the diet of most shark species, although a variety of species also eat lobsters and crabs, squid and octopus, porpoises and whales, seabirds, sea turtles, and other sharks. Opportunism means eating what’s available. Galapagos Sharks in the Galapagos Islands feed on marine iguanas, a prey type not available anywhere else in the world. But marine iguanas are large, tasty, and slow-swimming. An example of a shark with a diverse diet, highlighting a species about which we know comparably little, is the Broadnose Sevengill Shark. Among food items found in the stomachs of Sevengills in South Africa have been at least 11 species of sharks; 2 each of skate, stingray, and torpedo ray species ; shark egg cases; a chimaera; a hagfish; more than 14 kinds of bony fishes; and squid, octopus, snails, crustaceans, seals, and dolphins. In other places, Sevengills also eat sea lions, rats, and people. The human remains were scavenged from a suicide victim who had jumped off the Golden Gate Bridge. Scavenging is an important feeding mode in many sharks. Their scavenging habit, coupled with an exquisite sense of smell that allows them to detect baited hooks from great distances, is part of what makes sharks so 136 Sharks: The Animal Answer Guide vulnerable to fishing. What they eat depends mostly on place and shark species. Among sharks attracted to dead whales and dolphins (and humans) are Sevengill Sharks, Tiger Sharks, and of course White Sharks; dead seals attract White Sharks and Sleeper Sharks; Blue Sharks feed on dead squid, as do some very deep-living species such as Velvet Belly Lanternsharks. The classic opportunist, in terms of predation and scavenging both, is the Tiger Shark. Tiger Sharks are efficient predators and capture other sharks, bony fishes, crustaceans, mollusks, dugongs, seabirds, sea snakes, dolphins, and sea turtles. But the list of items found in Tiger Shark stomachs reads like an ad for a yard sale: license plates, oil cans, tires, deer antlers , boat parts, jewelry, clothing, tires, books, and baseballs. Tiger Sharks also take advantage of opportunities afforded by shortterm periods of prey abundance. Best studied are Tiger Sharks that feed on albatross chicks in the northwestern islands of the Hawaiian chain. Every June and July, Tiger Sharks arrive at Midway Island and French Frigate Shoals just as Laysan and Black-footed Albatross chicks are making their first attempts at flying. Many of these first flights end quickly in the water, where the sharks are waiting. Tiger Sharks aggregate similarly at Raine Island in Australia to feed on a seasonal abundance of egg-laying sea turtles that become stranded at low tide and die of heat exhaustion (see “How do sharks navigate?” in chapter 5). Recent discoveries in the Gulf of Mexico have shown that Tiger Sharks feed opportunistically on birds much smaller than albatrosses. Songbirds migrate every spring from South to North America. Those flying across the Gulf of Mexico pass over a string of oil and natural gas platforms. The platforms are lit up at night, and many birds are confused by the lights and circle the platforms for hours until they become exhausted and fall into the sea. This “nocturnal circulation” may involve as many as 100,000 birds circling a single platform. Tiger Sharks have learned about this bonanza. Nearly half of 50 Tiger Sharks caught over a two-year period near these oil rigs contained a birdwatcher’s list of small species, including woodpeckers, Scarlet Tanagers, Brown Thrashers, Meadowlarks, catbirds, kingbirds, and swallows. Tiger Sharks also stalk prey, very much the way that terrestrial tigers do, relying on the element of surprise. Video footage taken via “Crittercams” attached to Tiger Sharks in western Australia show the sharks approaching green sea turtles that are feeding on sea grasses. If the turtle looks up as the shark approaches, the shark veers off. But if the turtle is caught unawares, the shark attacks. Studies of Bengal tigers in India that attacked fishermen in...