South American sea lion

Otaria byronia is a sea lion of the family Otariidae ^[1]. In Peru it is known as «lobo chusco» or «lobo marino chusco» ^[2]. For almost 200 years zoologists used two names for this species, Phoca flavescens Shaw, 1800 and Phoca byronia de Blainville, 1820, because, in the view of some authors, the description of the type specimen of the older name does not fit this genus; in 2019 a neotype was designated for Otaria flavescens to settle the dispute ^[3]. WoRMS accepts Otaria byronia and treats Otaria flavescens as an unaccepted synonym ^[1]. The IUCN Red List classifies it as Least Concern in its current assessment ^[4], and Peru's official list of threatened wildlife, approved in 2014, includes it as Vulnerable ^[5].

IMARPE's censuses sort the animals by appearance. The adult male is larger than any other individual in the colony and carries a thick mane; the subadult male is also large but lighter, its mane not yet fully formed, and it tends to stay at the edge of the group ^[2]. The adult female is paler than the males and is recognised by her broader hips; juveniles are smaller and less robust than females, and pups, the smallest of all, are dark ^[2].

Two males stranded in 2018 at Malabrigo (La Libertad) and San Pablo (Piura), one subadult and one juvenile, measured 2.13 m and 1.77 m ^[6].

On the Atlantic side it lives from southern Brazil to Cape Horn, and on the Pacific side southward from northern Peru, in a range with few gaps ^[3]. It usually feeds in coastal waters and is closely tied to the continental shelf and to upwelling areas ^[2].

In Peru, the March 2026 census of protected natural areas counted at least 72651 animals ^[2]. The central zone, from 11° to 14° S, held at least 44731, against 17567 in the northern zone, from 5° to 10° S, and 10353 in the southern zone, from 15° to 18° S ^[2]. These are minimum figures: counts are made early in the morning and late in the afternoon, when most animals rest on rocks and beaches, and animals that have gone out to feed are left out of the count ^[2].

Isla Foca, in the Mar Tropical de Grau National Reserve, has 2 breeding colonies of the species that do not exceed 2000 animals, on inaccessible beaches at the foot of cliffs ^[7].

The breeding season falls in the austral summer, between December and March ^[8]. At the Ballestas Islands, from 1998 to 2002, mothers spent a perinatal period of 7.46 days on average with their newborn pup before their first trip to sea; after that they alternated feeding trips averaging 1.56 days with stays ashore of 0.90 days to nurse ^[8].

Over five years of observation in Peru, males guarded neither females nor a fixed territory but a spot on the shore, on the daily route of females going down to the water to cool off, where most matings took place; in temperate parts of the range, by contrast, a male keeps a group of females to himself or commands the way into tide pools ^[9]. With 1 male for every 17 females, 14 % of males obtained half of all copulations and 25 % never mated; females move freely and pick their partners, a system the authors liken to a lek ^[9].

Food supply sets the success of each season. Pup mortality was 13 % before the 1997-1998 El Niño and 100 % during it, and abortions were more frequent when prey was scarce; births also came later after lean years and earlier after years of plenty ^[10]. In 1998, 22 % of the marked females never came back to the beach after their first feeding trip ^[8].

In the 2026 census, adult females made up 48 % of the animals counted, pups 29 %, juveniles 15 % and adult and subadult males together only 3.3 % ^[2].

Scats collected on San Gallán Island between 1998 and 2002 held remains of fish, crustaceans and cephalopods; outside the El Niño year their mean frequencies of occurrence were 76.7 %, 88.3 % and 17.9 % ^[8]. In those years the Peruvian anchoveta (Engraulis ringens) accounted on average for 75 % of fish occurrences, and every crustacean found was Pleuroncodes monodon ^[8].

During the 1998 El Niño, with anchoveta and Pleuroncodes monodon scarce, the diet took in more fish species, mostly bottom-dwellers, and Cynoscion analis reached a high frequency, 22.2 %, in that year alone; females stretched their trips to sea to 2.37 days, against 1.12 days in 2000, a La Niña year ^[8]. Because of these responses, the authors suggest the species could be used to track relative changes in prey abundance in the Peruvian upwelling ^[8].

On arid islands of the Peruvian coast, the common vampire bat (Desmodus rotundus) feeds on the blood of this sea lion. Between 2003 and 2005 bats were seen at night approaching lone sea lions or animals at the edge of the colony and biting them on the neck, and stable isotopes in their hair were consistent with the sea lion being their main food source ^[11].

At Punta San Juan, the respiratory mite Orthohalarachne attenuata was found in 3 of 8 dead sea lions examined, without the lesions it caused in fur seals, and stomach and gut parasites did not damage their tissues in any important way ^[12].

Its global category is Least Concern ^[4]. In Peru it is classified as Vulnerable, and the regulation that lists it prohibits the hunting, capture, possession, trade, transport and export for commercial purposes of the species included ^[5]. It is also listed in Appendix II of the Convention on the Conservation of Migratory Species ^[2].

IMARPE's annual censuses counted 144087 sea lions in 1997 and only 27998 in 1998; in 2020 the figure was 105774 ^[13]. In 2026 the count in protected areas was 21.1 % higher than in 2025 ^[2].

Between January and April 2023, surveillance in marine protected areas found 5224 sea lions dead or dying on beaches, about 5 % of the country's population, 1112 of them on San Gallán Island; the authors attribute this die-off to highly pathogenic avian influenza A(H5N1) ^[14]. The only necropsy performed showed pneumonia and encephalitis, and the authors point to contact with infected birds or their carcasses as the likely source ^[14].

IMARPE lists among its threats El Niño, disease, predators, interactions with fisheries, pollution and habitat degradation ^[2]. In 2019, IMARPE's fishing logbook programme recorded sea lions in 75.47 % of 3054 sets made by the industrial fleet, with a maximum of 1250 animals recorded ^[13].

Scats collected at Punta San Juan contained 47 plastic particles, both fibres and fragments ^[15]. On San Pedro beach, in Lurín, the necropsy of one animal found nylon net filaments mixed with the fish it had eaten ^[16].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

Sea lions sick with H5N1 influenza in Peru showed disorientation, loss of coordination, seizures, heavy nasal discharge, drooling and laboured breathing ^[17]. In 2 samples from sea lions in the south of the country the PB2 D701N mutation appeared, which is linked to the virus adapting to mammals and was also found in a human case in Chile; the authors suspect direct spread between sea lions, though this is not yet confirmed ^[17].

SERFOR warns that a stranded sea lion can be very dangerous even when it looks weak or calm, and that it may react aggressively ^[18]. Anyone who has to handle one must wear gloves and guard against catching diseases through bites or contact with body fluids ^[18].

A pup found alone on the beach should not be put back in the sea: it may be waiting for its mother, who has gone to feed, and only trained staff should handle it ^[18].

Waste that fishers throw into the sea at docks draws sea lions, birds and other animals; in addition, many docks on the Peruvian coast are tourist attractions where birds and sea lions are fed on purpose so that people can take photos, and these crowds raise their contact with people ^[17].

SERFOR's 2026 technical criteria allow sea lions to be watched from land or from boats, without touching, feeding or scaring them, and do not allow swimming or diving with them in open water ^[19]. From a boat, the approach must be from the side at no more than 4 knots, no closer than 30 m to the nearest animal, with at most 2 boats at a time and 20 minutes per spot, and never in front of breeding areas ^[19]. On land, observers must move at least 60 m away if animals are moving close to the viewing spot ^[19].

In the March 2026 census, the largest colonies inside protected areas were Morro Quemado (19102 animals) and San Gallán Island (6193), in the Paracas National Reserve, and Lobos de Afuera Island (12394), the Chincha Islands (11335) and Punta San Juan (6080), in the Guano Islands, Islets and Capes National Reserve System ^[2]. Most breeding colonies lie between 13° and 14° S, and 39.4 % of the pups counted that year were born at Morro Quemado ^[2].

On Isla Foca 682 sea lions were counted in 2026 ^[2]. There the 2 breeding colonies occupy inaccessible beaches beneath cliffs ^[7], and the island's strict protection zone allows no entry by people, no boating, no swimming and no diving ^[7].