Biologists with the Division recently completed a study of common thresher shark food habits and have begun examining the diet of large shortfin mako sharks, as samples of large fish become available. While sharks are known and feared for their occasional attacks on humans, the majority of species prefer a diet of small fishes and invertebrates. A few (like the adult white shark) appear to prefer larger prey such as marine mammals, large fishes and other species of sharks. Some, like the tiger shark and blue shark, even include seabirds in their diets.
Thresher Shark Food Habits
In fall 1999, contract work began on a common thresher shark food habit study, and as of July 2000, work has been completed examining approximately 185 stomach samples collected from the California/Oregon (CA/OR) drift net fishery by federal fishery observers during 1998-99. The following prey items have been identified: northern anchovy, Pacific sardine, Pacific mackerel, California barracuda, Pacific hake, pelagic red crab, louvar, grunion, jack mackerel, shortbelly rockfish, market squid, and euphausiids.
Mako Shark Food Habits
The prey of the shortfin mako is known to include a variety of pelagic fishes including mackerels, bonito, anchovies, herrings, grunts, lancetfishes, cod, salmon, dolphinfishes, small sharks and squid. In-depth trophic studies have not been undertaken off the West Coast or elsewhere, and information on the feeding habits of large adult shortfin mako is scarce. Higher research longlining catches during the day versus night suggests the species feeds predominantly during the day. Recently, biologists Dave Holts and Darlene Ramon of the Division's shark research group examined stomach contents of two large sport-caught makos caught off southern California in 1999. The stomach of an 11-ft (3.4 m), 986 lb female taken near Santa Barbara Island contained remains of a young harbor seal, a small common dolphin, and vertebrae of one or more unidentified small sharks. The stomach of a 9 ft (2.7 m TL), 402-lb male caught near Santa Monica Bay, CA, was found to contain remains of a billfish, tentatively identified as striped marlin, plus another unidentified teleost.
Blue Shark Food Habits
In coastal waters off the U.S. West Coast, blue sharks reportedly feed on anchovy, mackerel, hake, dogfish, squid and pelagic crustaceans including euphausiids (Tricas 1979; Harvey 1989; Brodeur et al. 1987). Elsewhere also known to feed also on small sharks and seabirds (Compagno 1984). Sciarrotta and Nelson (1977) noted that species seemed to feed around the clock but was more active at night, with highest activity in the early evening. These authors observed a twilight movement from offshore to shallower waters around Santa Catalina Island, California , during March and June, but a shift to offshore waters during late June to October, suggesting a response to a change in prey availability.
White Shark Food Habits
According to Compagno (1984), large white sharks above 3 m long tend to prey heavily on marine mammals, while smaller sharks below 2 m long generally feed on bony fishes and small sharks. In-between sizes evidently feed on a mix of both (e.g., Fitch 1949). McCosker (1985) and Tricas and McCosker (1984) have pointed out that the tooth shape of juveniles and sub adults (less than 3-4 m TL ) is longer and narrower than that of adults, which allows them to grasp small bony fishes and elasmobranchs, but lacks the cutting ability of the broader tooth of the adult. The smaller young are also considered more agile with better turning abilities with which to better capture quickly moving fish prey. Juvenile northern California white sharks are known to feed on California bat ray, spiny dogfish, leopard shark, soupfin shark, smoothhound sharks, lingcod, Pacific sardine, king salmon, Pacific hake, white seabass, rockfishes, and cabezon, while the adults feed heavily on northern elephant seals, also harbor seals and California sea lions and occasionally some fishes and crab, including Pacific hake and sharks (including the basking shark)and Cancer spp crabs.
White sharks are also known to attack northern fur seals, Steller sea lions (Eumetopias jubatus), sea otters (Enhydrya lutris) and leatherback sea turtles (Dermochelys coriacea), but these are not considered regular diet items (Bonnot 1928; Orr 1959; Ames and Morejohn 1980; Long 1985). They occasionally prey on small odontocetes along the U.S. West Coast and scavenge on carcasses of large cetaceans, but in general predation on cetaceans is uncommon (Long and Jones 1985). Because of the apparent reliance of U.S. West Coast white sharks on pinnipeds, especially the northern elephant seal, major rookeries are considered vital fish habitat sites for this species. The two largest colonies (for both elephant seals and sea lions) are located on the Southeast Farallon Islands and Aсo Nuevo Island (Long et al 1985; Stewart et al 1994, Mark Lowry, NMFS, pers. comm. 5/00) These and other northern elephant seal rookeries represent a massive expansion of the population over the last century, which was presumed extinct by 1892 owing mainly to commercial harvesting for blubber oil that began in the early 1800s. A small residual breeding colony survived at Isla de Guadalupe off central Baja California Mexico, and with legal protection, rebounded rapidly through the early 1900s, with immigration to other sites in California and Baja from which additional colonies were also established as far north as the Point Reyes Headlands, California (Stewart et al 1994). The population is thought to be still expanding with haul-out locations now extending as far north as Cape Arago, Oregon and Vancouver Island, British Columbia --possible locations of future breeding colonies (ibid). While this expansion has likely contributed to an increase in U.S. West Coast white sharks, even the largest rookeries support relatively small white shark populations that have the capability of being swiftly reduced by very little fishing effort (Ainley et al 1981).