Blue shark

Prionace glauca is the blue shark, a shark of the family Carcharhinidae, in the order Carcharhiniformes ^[1]. It is a slender oceanic shark, blue above and white below, that lives in temperate and tropical waters almost worldwide ^[2]. In Peru it is the most landed shark: between 1996 and 2010 it made up 42 % of shark landings ^[3]. Its global category on the IUCN Red List is Near Threatened ^[4].

Linnaeus described it in 1758 as Squalus glaucus with a vague type locality, the European ocean, and no holotype; the genus Prionace dates from Cantor, 1849, and is regarded as monotypic ^[2]. The FAO catalogue gives it the name blue shark in English and tiburón azul in Spanish ^[2].

It is a slender shark with a long snout, large eyes and very long, narrow, somewhat sickle-shaped pectoral fins ^[2]. Its upper teeth are broad, triangular and serrated, without small cusplets in adults, and it has 24 to 31 tooth rows above and 25 to 34 below ^[2]. The first dorsal fin sits much closer to the pelvic fins than to the pectorals, the second dorsal is small and the tail fin has a long lower lobe ^[2].

The back is deep blue and the flanks a brighter blue, and the belly is white with an abrupt boundary and no pattern at all ^[2]. It reaches 383 cm on good evidence, males mature between 182 and 281 cm and adult females measure from 221 cm ^[2]. In the small-scale longline fishery of southern Peru, the mean fork length of the blue sharks caught in the shark season was 119.9 cm ^[5].

It is probably the most widely distributed chondrichthyan in the world: it lives in temperate and tropical waters of every ocean, and in the eastern Pacific from the Gulf of Alaska to Chile ^[2]. It is an oceanic, epipelagic shark recorded from the surface to at least 152 m deep; it prefers relatively cold water, 7 to 16 °C, and in the tropics it dives deeper ^[2].

In Peru, 45 % of what was landed between 1996 and 2010 came ashore at southern ports and 38 % at central ones ^[3]. According to the authors of a study in northern Peru, about 70 % of blue shark landings come from the coastal small-scale longline fishery, especially in the south, where cold to temperate waters support higher abundances of pelagic sharks ^[6].

It is viviparous with a yolk-sac placenta; litters range from 4 to 135 pups, gestation lasts 9 to 12 months and pups are born at about 35 to 44 cm ^[2]. Adult females develop skin about 3 times thicker than that of males, which the FAO catalogue links to the bites they are presumed to receive during courtship ^[2].

In northern Peru, a study of 143 stomachs collected from February to December 2015 measured sharks of 110.0 to 299.4 cm in total length ^[6]. In the longline fishery of Ilo, on average 71.1 % of the blue sharks measured in the shark season were below the minimum landing size ^[5].

It feeds mostly on bony fishes and squid, and squid are a very important prey throughout its range ^[2]. In waters off northern Peru, cephalopods made up 87.4 % of the diet (by the %PSIRI index) and fishes 8.5 %, with 42 prey taxa in 115 stomachs that held food ^[6]. The genus Argonauta contributed 16.1 % and the squid Ancistrocheirus lesueurii 6.2 % ^[6].

The diet changed with size and with distance from the coast: Argonauta fell from 28.5 % in sharks of 150.0 to 229.9 cm to 8.6 % in those of 230.0 to 309.9 cm, and it was 33.2 % in the coastal zone against 8.3 % in the oceanic one ^[6]. The presence of mesopelagic and bathypelagic prey suggests that, at least in that area, it also feeds at greater depth ^[6]. In Peru, monthly landings showed a weak but significant relationship with the multivariate El Niño index ^[3].

Its global category on the IUCN Red List is Near Threatened ^[4].

Peru sets a minimum landing size of 160 cm total length, but it has not been fully enforced and many fishers are unaware of it ^[5]. In the Ilo fishery, on average 71.1 % of the blue sharks measured in the shark season were below that size, and the authors warn that catching sharks below the minimum size can leave slow-living species exposed to overexploitation ^[5]. Between 2000 and 2010, its landings showed no significant trend ^[3].

It is the most landed shark in Peru: between 1996 and 2010 it made up 42 % of shark landings ^[3]. Of 11,166 sharks identified to species on 145 longline trips from Ilo, 70.6 % were blue sharks ^[5].

Between 1996 and 2000, 79 % of what was landed of the species was caught with longlines ^[3]. Worldwide it is used fresh, smoked or dried and salted, and its fins are used for shark-fin soup ^[2].

In Peru the available information comes from fishing: the Ilo data come from observers aboard small-scale longline boats ^[5]. In 2015, sharks landed at Máncora and Salaverry were sampled ^[6], and between 1996 and 2010 most landings of the species came ashore at southern and central ports ^[3]. In the Ilo longline fishery, high catches were spread across the whole fishing area, without concentrating in specific areas ^[5].