In this paper a rare case of a double swordfish mortal attack against an adult blue shark (Prionace glauca) is reported. A female blue shark, with a total length of 3 m, was found stranded along the southern Sicilian coast (Strait of Sicily, Mediterranean Sea) on 30 May 2018. The analysis of this carcass revealed the presence of two swordfish bill fragments, impaled in the shark head; the former on the snout, the latter near the eye. The results of anatomical and computed tomography scanning analysis on the head of the blue shark showed that the larger bill fragment (19.7 cm) probably determined the death of this animal, having been impaled in a vital point, just behind the right eye. The analysis of both these events and other similar swordfish-shark interactions reported in the literature makes possible the hypothesis that young swordfish specimens put in place a precise defensive strategy against their potential predators or competitors, aimed at hitting vulnerable and vital points and delivering a mortal blow.

A case of bicephaly in embryos of the blue shark Prionace glauca in the Mexican Pacific Ocean is described. Malformed female shark embryos were found in pregnant females caught in the Gulf of California (Punta Arenas) and off the western coast of Baja California Sur, Mexico (San Lázaro). Abnormal sharks showed a symmetric bicephaly that could be caused by the high number of embryos found in the uterus of the blue shark, which is the most fecund species of shark in the world. The abnormality probably began during the embryonic development.

Blue sharks (Prionace glauca) and oilfish (Ruvettus pretiosus) are frequently caught incidentally in the swordfish surface drifting longline fishery of the eastern Mediterranean Sea. An information theoretic generalized additive model approach, modelling separately: (1) the probability of making a catch; and (2) the positive catch rates, suggested the presence of intra-annual variations in their abundance and revealed interesting associations with some environmental features. Blue sharks were more frequently encountered during late summer in cool water masses. However, the likelihood of making a larger catch peaked in warmer waters through late spring in the vicinity of land. Occurrences of blue sharks were more frequent to the south-west (Ionian Sea), while positive catch rates were higher in the eastern-most regions (Levantine). Oilfish occurrences, as well as catch rates, were always above average to the east. Intra-annual variation was analogous to that of blue sharks. The species showed a preference for cooler water bodies and its distribution was strongly dictated by the lunar cycle. Annual time series showed a decreasing trend of blue sharks abundance. Catch rates for both species depended on fishing gear configuration. Deeper settings (>20 m), more resilient lines and use of fish attractant light-sticks increased the probability of capturing a fish.

A juvenile female blue shark, Prionace glauca, 98 cm of total length and weighing 3 kg, was caught on 18 October 2008 off the Altınoluk coast. An adult female, 350 cm of total length and weighing 100 kg, was caught on 16 August 2009 off the Ayvacık coast. Available data suggest the following facts: (1) the rarity of the blue shark in the studied area; (2) P. glauca enters fishery in the studied area in the first year of life; and (3) coastal trammel- and gill-netting is a serious threat to the survival of the young populations of the species in Edremit Bay.

Stomach contents of 893 blue shark (Prionace glauca) caught off Ensenada, Todos Santos Bay, Baja California, from 1995 to 1997 were analysed. 614 had identifiable food remains. A large variety of pelagic cephalopods occurred in 55.5% of all stomachs and accounted for 46.2% of all prey and 84% by estimated weight of stomach contents. The most abundant (17%) and frequent (32%) was Histioteuthis heteropsis and the most important by estimated weight (22%) was ‘Gonatus californiensis’. Argonauta spp. and Vampyroteuthis infernalis were also abundant prey. A pair of beaks of giant squid Architeuthis sp. constitutes the first record for Mexican waters. Feeding habits and the probability of scavenging on mesopelagic cephalopods is discussed. The single most abundant prey was the pelagic, red crab Pleuroncodes planipes (41.7%). Other prey included teleost fish, amniotes and floating items. Diet varied greatly between months with no clear seasonal pattern. Tiny diet differences due to shark size or sex were inconclusive. A thorough review of studies on blue sharks stomach contents analysis is given. Blue shark feed on a large variety of passive pelagic prey, mainly mesopelagic cephalopods, that could be preyed upon as well as scavenged. Depletion of this predator due to overfishing may be leading to unknown cascading top-down effects in the mesopelagic realm.