Taxonomy

Rarity

Abundance rates how many are in suitable habitat. Find difficulty rates how hard it is to locate one on a daylight snorkel or freedive through 50 ft. A higher find score means a harder search. The rarity category also reflects the kind of encounter; it is not an average of the 2 scores.

Seasonality

Location

Blue sharks are an open-ocean animal that comes close to land where the land runs out quickly. FishBase describes them as oceanic but findable close inshore where the continental shelf is narrow, which is a fair description of the southern California coast, and they have even been recorded inside estuaries. They usually hold in the top 720 feet and have been recorded to roughly 3,550 feet.

Temperature sets the boundary. Blue sharks live between about 45 and 70 degrees Fahrenheit, which is a cooler band than the shortfin mako's, and it is why the 2 species do not overlap as neatly as their shared bag limit suggests.

They travel absurd distances. One blue shark tagged in New Zealand was recaptured 1,200 kilometers off the coast of Chile, which is roughly 750 miles, and that was only the distance between 2 known points rather than the route it swam.

The diet is broad and includes some things that say a lot about the animal. Blue sharks eat herring, hakes, cod, haddock, pollock, mackerel, butterfish, sea raven and flounders, along with small sharks, squid and pelagic red crabs, the swarming crustaceans that wash onto southern California beaches in warm years. They also eat dead whale and dolphin, the occasional seabird, and garbage. That last item is in the scientific record, not an editorial comment.

Deep Dive

The blue shark is built for covering ground on very little fuel, and every part of it shows that.

Start with the shape. This is a slim, long-snouted shark with unusually large eyes and pectoral fins that are long and narrow rather than broad. Where the shortfin mako is a thick muscular cylinder, the blue shark is long and slim. The upper teeth are curved triangles with saw edges, and there is no ridge of skin running between the 2 dorsal fins, which is the quickest way to separate this species from several similar requiem sharks.

The color is the name and it is unusual among fish. Most open-ocean animals are some version of gray or steel. A blue shark is dark blue across the back, bright blue along the sides, and white underneath, with dusky tips on the pectoral and anal fins.

Reproduction is where the species diverges sharply from the mackerel sharks. A mako or a white shark grows its pups inside the mother with no placenta, feeding them on unfertilized eggs. A blue shark builds a placenta and feeds the pups directly, the way a mammal does. The payoff is numbers. Gestation runs 9 to 12 months and a usual litter is 15 to 30 pups, each 14 to 17 inches long at birth. Recorded maximum litters run far higher, and the published figures disagree with each other at the top end, with counts of 63, 80 and 135 all appearing in the literature. Even the low end of that spread is an order of magnitude past what a mako produces in a cycle 3 times as long.

Producing that many pups is what separates this species from the mackerel sharks on paper, and it is worth holding alongside the Near Threatened listing rather than instead of it. A shark can be both highly productive and in measured decline, and this one is.

One measured oddity has no explanation attached to it in the record, and it is worth stating carefully for that reason. Female blue sharks have a thicker skin layer than males of the same size. The observation is published and repeatable. The source consulted here reports the difference and does not say what causes it, so neither does this entry.

Where to Find Them

Researched habitat and range candidates

These locations have suitable habitat within the species range. An assignment is a possible encounter, not a confirmed sighting at every beach. Season, depth, and local conditions still matter. Historical habitat without present-day evidence is excluded.