The blue shark is a pelagic species having a virtually circumglobal distribution in tropical and temperate seas. In the northern hemisphere, during the warmer months, they extend their range northwards in both the Pacific and Atlantic (Strasburg, 1959; Bigelow & Schroeder, 1948; Templeman, 1963). Between May and October they occur regularly and in considerable numbers around the south-west peninsula of England. Strasburg showed that in the central Pacific blue sharks are opportunistic feeders as were the other pelagic species he studied, their diet consisting mainly of squid and small fish. Bigelow & Schroeder suppose herring (Clupea harengus L.), mackerel ((Scomber scombrus L.), sardines (Sardina pilchardus Walbaum) and a variety of cephalopods to be the chief diet in northern waters, together with Squalus acanthias L. and various bottom fish on the fishing banks. In warmer waters anchovies (Engraulis encrasicolus L.), flying fish and the occasional sea bird caught resting on the water are known to occur in their diet. Bigelow & Schroeder also quote the well-known stories of blue shark following boats for offal and feeding on the carcasses of harpooned whales. Le Brasseur (1964) examined the diet of blue sharks caught by gill net in the Gulf of Alaska and in general confirmed Strasburg's observations. However, he stated that when salmon (Oncorhynchus sp.) were available in gill nets blue shark might selectively feed on them, while by-passing other readily available species occurring in their normal diet. Couch (1862) examined the stomachs of two specimens from Cornish waters, finding herring, mackerel and garfish (Belone belone L.), as well as Squalus and Conger conger L. Lo Bianco (1909) found anchovies and cephalopods in one of two specimens caught in the Bay of Naples, the other of which contained large numbers of flying-fish eggs (Exocoetus sp.).
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