Blue whale

Balaenoptera musculus is a rorqual of the family Balaenopteridae, described by Linnaeus in 1758 ^[1]. It is the largest of the rorquals and the largest animal ever to have lived on the planet; it feeds almost entirely on krill and occurs in every ocean except the Arctic ^[2]. In Peru, the 2026 master plan of the Mar Tropical de Grau National Reserve calls it ballena azul and counts it among the cetaceans sighted in the Cabo Blanco – El Ñuro sector ^[3].

It is a long, slender whale with a mottled blue-gray colour that looks light blue underwater, which gives it its name; the mottling pattern varies between individuals and can be used to recognise them ^[2]. In the North Atlantic and North Pacific it reaches about 90 feet (close to 27 m) and more than 100,000 pounds (about 45 t); in the Antarctic it reaches about 110 feet (close to 34 m) and more than 330,000 pounds (about 150 t). Females are generally larger than males ^[2].

Five subspecies are recognised ^[2]. In the southeastern Pacific there is a subspecies that has no name yet ^[2]. Several studies have suggested that these whales are a separate population, possibly a subspecies, with their own patterns of distribution, migration, genetics and song ^[4].

The blue whale occurs in every ocean except the Arctic. It usually spends summer feeding in polar waters and migrates towards the equator as winter arrives, although some individuals appear not to migrate at all ^[2]. Its distribution is driven mainly by where krill is concentrated ^[2].

In the southeastern Pacific, the unnamed subspecies occurs mainly in the Chiloense ecoregion and migrates to lower latitudes, including the Galapagos Islands and the eastern tropical Pacific ^[2]. Of ten whales fitted with satellite transmitters in northern Chilean Patagonia in 2013, 2015 and 2016, several stayed near the Chiloé archipelago; those that migrated converged north-westward along a corridor wider than 2,000 km, and one reached the Galapagos, a region about 6,000 km from the tagging area ^[4].

The blue whale is usually found alone or in pairs. While feeding or travelling it moves at about 5 miles an hour, and in short bursts it exceeds 20 miles an hour ^[2]. It produces pulses, groans and moans that are among the loudest sounds on the planet; under the right oceanographic conditions they might be heard about 1,000 miles away ^[2].

The southeastern Pacific song repeats highly stereotyped low-frequency units, and two types are distinguished, SEP1 and SEP2; the latter is now dominant in Chilean Patagonia. The frequency and pulse rate of SEP2 declined between 1996 and 2017, and the frequency of SEP1 between 1970 and 2014, at similar rates ^[5].

Lifespan is estimated at 80 to 90 years, with a gestation of 10 to 12 months and weaning at 6 to 7 months. Sexual maturity is thought to come between 5 and 15 years, and females probably calve every 2 to 3 years; most mating and births take place in winter ^[2].

Its diet is based on krill, and occasionally includes fish and copepods. It filters food by swimming with its mouth open towards schools of krill, then pushes the water out with its tongue and keeps the prey inside the baleen; a large adult may eat up to 6 tons of krill a day ^[2].

Off Peru and Ecuador, whales tagged in the Galapagos in 2021, 2022 and 2023 appeared to travel south along the edge of the continental shelf, with a feeding hotspot where it converges with the Nazca Ridge. Northern Peru receives nutrients from the Humboldt Current, and the Nazca Ridge has raised productivity ^[6].

Commercial whaling greatly reduced blue whale populations in the early 1900s, and today they are a small fraction of what they were, although they are increasing worldwide ^[2]. The main threats are vessel strikes and entanglement in fishing gear; other possible threats, less well understood, include ocean noise, habitat degradation, pollution and climate change ^[2].

In the Galapagos, where 16 whales were tagged between 2021 and 2023, the highest concentrations were around Isabela Island, which is increasingly visited by tourist vessels, raising the risk of collisions ^[6]. For the southeastern Pacific, joint conservation by Chile, Ecuador and Peru has been proposed, with measures against ship strikes and marine protected areas ^[4].

In Peru the information is scarce and old. A 1983 study reviewed by Hucke-Gaete and colleagues reports that at Paita blue whales were seen and caught all year, with a peak between December and March in 1961–1966 and 1975–1982; a cruise in November–December 1982 recorded 10 sightings of 14 animals within 60 nautical miles of the coast ^[4].

The 2026 master plan of the Mar Tropical de Grau National Reserve states that the species has been sighted in the Cabo Blanco – El Ñuro sector, and that the area is important for highly migratory species such as the blue whale ^[3]. One whale tagged in the Galapagos moved south to Peru ^[6].