The blue shark, Prionace glauca, is possibly the most beautiful of sharks.  It is a slender, fast and graceful shark, but it is the vivid, almost electric blue colouration that is most striking.

A blue shark passes close by.

Blues are oceanic sharks, uncommon in shallow coastal waters.  In the tropics they are normally found in deeper, cooler waters, but in temperate seas they are more likely to be found in surface waters (thus are described as being epipelagic).  This does not mean that in temperate waters they occur only in the warmer surface layers.  Recent data from satellite tags have shown that they blues may regularly undertake dives to more than 1000 metres.  One male blue shark, nicknamed Bodi by the researchers, was logged as having dived to 1250 metres off the Bay of Plenty (New Zealand) three weeks after being tagged in 2013.  An earlier joint British- Portuguese study recorded a female blue shark diving to 1160 metres off the coast of Portugal (Queiroz et al, 2012).   This behaviour is probably linked to hunting activity.  Blue sharks are quite catholic in their diet, eating a wide range of mid-water fish and cephalopods,  but appear to be particularly fond of squid, and of course squid will often undertake marked vertical migrations, with many species occurring at considerable depth.  That this deep diving behaviour of blues is primarily foraging for food is supported by a preponderance of deep water quid species found in the gut of contents of blue sharks caught by long-lines, in particular the wonderfully named vampire squid, Vampyroteuthis infernalis, a small deep-water squid those scientific name literally translates as the vampire squid from hell.

In the northern Atlantic at least, they appear to undertake seasonal migrations. They are unusual amongst ocean sharks in that they will sometimes aggregate in groups, often all same sex, as they roam.  This pack-like activity has lead to them being dubbed the wolves of the sea.  Around Southwest Britain blue sharks start to appear in June each year, and hang around until late October or early November.  Although there have been attacks on humans by blue sharks, aggressive behaviour is relatively rare.  Certainly in British waters, where large blues are rarely seen, most are quite timid and easy to scare off accidentally.  The largest blue caught in UK waters was approximately 2.5 metres long and weighed 107kg. This was caught off Penzance, Cornwall, in 2012 (but was also released).  Blues can grow up to 3.5 metres or more (the largest on record was 3.83 metres long) the females being significantly larger than males.

Blue shark, Prionace glauca. A female blue shark swimming close to the surface off Southwest Cornwall, UK.

An underwater encounter with a blue shark is a wonderful experience, and provided one takes sensible precautions (i.e. wearing gloves, not wearing shiny bits of equipment and NOT trying to feed them) is normally quite safe.  Blue sharks have been one the shark species hardest hit by the practice of shark fining in various parts of the World and there numbers appear to have declined markedly.  Consequently it’s worth remembering that most blue sharks have far more cause to fear us that vice versa.

The above, and more of my blue shark images, can be found on my stock image website www.colinmunroimages.com. They can be licensed for publication, or purchased as fine art prints and canvas wall art.

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References cited: Queiroz N, Humphries NE, Noble LR, Santos AM, Sims DW (2012) Spatial Dynamics and Expanded Vertical Niche of Blue Sharks in Oceanographic Fronts Reveal Habitat Targets for Conservation. PLoS ONE 7(2): e32374. doi:10.1371/journal.pone.0032374