March Elasmobranch of the Month: Blue Shark

Author: Bee Smith

Spring is just around the corner here in the UK, and I am excited to soon be swimming with the blue shark, one of the shark species that visits our waters in the spring and summer.

Credit: Andy Murch

• Key Features & Appearance

Blue sharks are named after their colouring, with a white underside and a striking blue dorsal side. This colouration is a form of camouflage known as countershading, which helps them blend in, making them harder to spot from above (as they merge with the deep blue ocean) or below (as they blend into the downwelling light from the surface). They are also easily recognisable by their elongated body shape, long, pointed pectoral fins, conical snout, and large eyes. 

They are known to reach as large as 4m/13ft but generally reach around 1.8-2.2m/6-7ft at maturity. They exhibit sexual dimorphism, with females typically growing larger than males. The largest blue shark ever recorded in UK waters was 2.74m!

Credit: Andy Murch

• Habitat & Distribution

Blue sharks are a pelagic species with a wide circumglobal distribution, found in all temperate and tropical oceans (ranging from surface waters down to depths as great as 1000m!). They are thought to form a single global, interconnected population. However, some studies suggest there may be distinct genetic groups, including one in the North Atlantic and Mediterranean Sea and another in the Indian and Southwest Pacific Oceans, as well as subtle genetic differences between the North Atlantic and Mediterranean populations. 

There are some regional patterns in the spatio-temporal distribution of blue sharks based on size and sex. For example, juveniles may reside in nursery areas for several years, they may shift from colder waters as juveniles to warmer waters as adults, and males and females appear to show sexual segregation due to temperature preferences, with large females tending to be found in higher latitudes with colder waters than males.

They are highly mobile and are known to make seasonal reproductive migrations following changes in water temperature and currents. In the North Atlantic, they follow a clockwise route, using the Gulf Stream and the Atlantic North Equatorial Current. This is why blue sharks are found off the south-west peninsula of England between May and October. Sightings primarily consist of mature females and juveniles, likely due to mature females showing a greater habitat overlap with juveniles than mature males, as a result of their preference for temperate waters.

• Diet

Blue sharks have a very diverse diet, with a preference for fish and cephalopods, primarily squid. 

• Reproduction

Blue sharks reproduce using viviparity, meaning the babies develop inside the mother’s uterus, where they are nourished by a placenta and are then born as live pups. Litter sizes typically range from 25 to 35, but can reach as high as 135!

• Threats

Globally, blue sharks are the most heavily exploited shark species, accounting for 60% of all reported shark catches. In 2019, 7 million blue shark landings were reported, though estimates of total landings are as high as 10 to 20 million annually. Historically, they were caught as bycatch in fisheries that were targeting other species, particularly tunas and swordfish, but today they hold significant economic value and so are deliberately targeted. They dominate both the fin trade (around 41% of the trade) and the shark meat trade (around 36%). In fact, the global blue shark fishery trade exceeds the value of the trades for each of the three species of bluefin tuna. What is more, they dominate recreational shark fisheries. While blue sharks are one of the most abundant shark species, with high reproductive and growth rates, their populations are believed to be in decline due to fishing pressure

Thinking about the sharks that visit UK waters, blue sharks in the North Atlantic have seen a decline of 53-80% over three generations due to fishing pressure. However, steps have been taken to manage this population, as a catch limit has been set for commercial fisheries. In the UK, although there are no commercial fisheries targeting blue sharks, they are a common target for recreational anglers. Historically, heavy recreational fishing led to declines in abundance in UK waters. Thankfully, recreational fishing has decreased, and a recent study found that UK anglers have a positive appreciation for blue sharks, respecting them, believing they are important for our oceans, and being keen to do more for their conservation and to contribute to research! 

• Status

The International Union for Conservation of Nature (IUCN) categorises the global population of blue sharks as “Near Threatened” and the Mediterranean population as “Critically Endangered”.

• Fun Fact

Off the south-west peninsula of England, from May to October, you can go snorkelling with blue sharks through a reputable shark diving operator. These sharks are incredibly inquisitive and often come up very close! Not only is this an incredible experience that will make you fall in love with the species, but supporting shark eco-tourism helps to show that sharks can be worth more alive than dead! 

• Works Cited

‘Prionace Glauca: Rigby, C.L., Barreto, R., Carlson, J., Fernando, D., Fordham, S., francis, M.P., herman, K., Jabado, R.W., Liu, K.M., marshall, A., Pacoureau, N., romanov, E., Sherley, R.B. & Winker, H.’ (2018) IUCN Red List of Threatened Species [Preprint]. doi:10.2305/iucn.uk.2019-3.rlts.t39381a2915850.en.

Campana, S.E. et al. (2006) ‘Effects of recreational and commercial fishing on blue sharks (prionace glauca) in Atlantic Canada, with inferences on the North Atlantic population’, Canadian Journal of Fisheries and Aquatic Sciences, 63(3), pp. 670–682. doi:10.1139/f05-251.

Coelho, R. et al. (2017) ‘Distribution patterns and population structure of the blue shark (prionace glauca) in the Atlantic and Indian Oceans’, Fish and Fisheries, 19(1), pp. 90–106. doi:10.1111/faf.12238.

Druon, J.-N. et al. (2022) ‘Global-scale environmental niche and habitat of blue shark (Prionace glauca) by size and sex: A pivotal step to improving stock management’, Frontiers in Marine Science, 9. doi:10.3389/fmars.2022.828412.

Druon, Jean-Noel & Campana, Steven & Vandeperre, Frédéric & Hazin, Fábio & Bowlby, Heather & Coelho, Rui & Queiroz, Nuno & Musyl, Michael & Lopez, Jon & Block, Barbara & Afonso, Pedro & Dewar, Heidi & Sabarros, Philippe & Finucci, Brit & Zanzi, Antonella & Bach, Pascal & Senina, Inna & Garibaldi, Fulvio & Sims, David & Travassos, Paulo. (2022). Global-Scale Environmental Niche and Habitat of Blue Shark (Prionace glauca) by Size and Sex: A Pivotal Step to Improving Stock Management. Frontiers in Marine Science. 9. 10.3389/fmars.2022.828412.

Nikolic, N. et al. (2022a) Stepping up to genome scan allows stock differentiation in the worldwide distributed blue shark Prionace glauca [Preprint]. doi:10.22541/au.165648248.86647399/v1.

Pacoureau, N. et al. (2021) ‘Half a century of global decline in oceanic sharks and Rays’, Nature, 589(7843), pp. 567–571. doi:10.1038/s41586-020-03173-9.

Scotts, G.L. et al. (2023) ‘Socio-cultural relationship between recreational sea anglers and blue sharks (Prionace glauca) in the United Kingdom’, Marine Policy, 157, p. 105831. doi:10.1016/j.marpol.2023.105831.

Silva, Thaísy & LESSA, ROSANGELA & Santana, Francisco. (2021). Current knowledge on biology, fishing and conservation of the blue shark (Prionace glauca). Neotropical Biology and Conservation. 16. 71-88. 10.3897/neotropical.16.e58691.

Vandeperre, F. et al. (2014) ‘Movements of Blue Sharks (prionace glauca) across their life history’, PLoS ONE, 9(8). doi:10.1371/journal.pone.0103538.

Vas, P. (1990) ‘The abundance of the blue shark,Prionace glauca, in the western English Channel’, Environmental Biology of Fishes, 29(3), pp. 209–225. doi:10.1007/bf00002221.