Black porpoise

Phocoena spinipinnis is a porpoise of the family Phocoenidae ^[1]. It lives in coastal waters of the Southeast Pacific, from northern Peru to southern Chile, and is the only member of its family in Peruvian waters ^[2]. It is one of the small cetaceans most often caught incidentally in Peru's artisanal fisheries, where it ranks second by number of catches ^[2]. Its global category on the IUCN Red List is Near Threatened ^[3].

Burmeister described it in 1865 under the original name Phocaena spinipinnis, and it is now placed in the genus Phocoena; its old synonyms include Acanthodelphis spinipinnis and Phocaena philippii ^[1]. In English it is called Burmeister's porpoise ^[1].

In Peru it is known as «marsopa espinosa» ^[2]. The 1996 law that protects several marine mammals names it «tonino» or «marsopa espinosa» ^[4].

It is the only porpoise in Peruvian waters ^[2]. It is hard to see at sea except when the surface is calm, because its dorsal fin is low and set far back, and its swimming is shy and elusive ^[5].

The 70 specimens examined at ports of central and northern Peru between 1987 and 1999 measured 135 to 183 cm in females and 64.5 (a foetus) to 182 cm in males ^[2]. Calves are about 86 cm long at birth ^[2].

It produces narrow-band, very high-frequency clicks, with a mean modal frequency of 133.9 kHz recorded on fishing nets off northern Peru ^[5]. On the Peruvian coast it is the only species that makes this kind of sound, unlike southern Chile and Argentina, where it shares the sea with dolphins of the genus Cephalorhynchus ^[5].

It is a South American species: in the Pacific it ranges from northern Peru to southern Chile, and in the Atlantic from Tierra del Fuego to southern Brazil ^[5]. In the Southeast Pacific it is distributed from the Bay of Paita to Valdivia, in Chile ^[2]. The Peruvian population appears genetically distinct from those of Chile and Argentina ^[5].

It is mainly a coastal animal ^[5]. Between 2009 and 2012, acoustic detectors on nets off Salaverry recorded it in waters up to 200 m deep, with the highest probability of presence at 50 m or less and within 50 km of the coast; it spent more time in water of 17 to 18 °C and avoided waters above 19 °C, and its foraging behaviour (buzzes) was recorded above all in the first 20 to 30 km ^[5]. A survey in February 2023 over an area of 3500 km² found the highest densities within 25 km of the coast and in waters of 50 m or less ^[6].

A study of 70 specimens (34 females and 36 males) obtained at 6 ports of central and northern Peru between 1987 and 1999 estimated that half of the females reach sexual maturity at 152.7 cm and half of the males at 157 cm ^[2]. In males, sperm production was observed throughout the year, with no visible reproductive seasonality, and no sign of reproductive senescence was found ^[2].

An earlier analysis, made with specimens collected between 1983 and 1989 and cited in that same study, estimated maturity at 159.9 cm for males and 154.8 cm for females, with the peak of the mating season in summer (February and March), a gestation of 11 to 12 months and a pregnancy rate of 60 % ^[2]. In addition, 34 % of the pregnant females were also lactating, which suggests annual reproduction ^[2]. The authors suggest that the length at first ovulation may have decreased, a possible response to heavy fishing mortality, although their sample is small ^[2].

Stomach-content analyses of cetaceans caught incidentally in Peru indicate that their main prey are forage fish, such as the Peruvian anchovy Engraulis ringens, and squid ^[5]. Anchovy occurred in 88 % of the stomachs of caught porpoises, a greater reliance than in other small Peruvian cetaceans ^[5].

The shelf off Salaverry holds large densities of anchovy, above all from December to April, and this distribution of prey could explain part of where the species is detected ^[5]. It also lives in an environment that is highly unstable because of the El Niño-Southern Oscillation ^[2].

Its global category on the IUCN Red List is Near Threatened ^[3].

For decades, deaths in fishing gear have endangered the species in Peru ^[6]. In a study of the Salaverry artisanal fleet, 66 % of 32 fishers interviewed said they had caught porpoises the previous year, with a mean of 6 porpoises per vessel per year, equivalent to about 420 a year (95 % confidence interval: 210 to 700) across the port's whole gillnet fleet ^[6]. The population of the study area, 3500 km², was estimated at 1696 individuals, and that catch far exceeds sustainable limits ^[6]. In 116 sets observed between 2009 and 2012, porpoises were caught in 2 % of the sets and detected acoustically in 22 % ^[5].

A 1996 law declared the species legally protected and banned its extraction, processing and sale ^[4]. Even so, Peru's artisanal drift-net fishery keeps one of the highest rates of incidental capture of small cetaceans in the world ^[5]. In a trial of acoustic deterrent alarms on that fleet, the capture rate of small cetaceans was 37 % lower with alarms, which suggests they may be effective ^[7].

For decades the species has been caught in large numbers on the Peruvian coast, and it ranks second among the small cetaceans caught by artisanal fisheries ^[2]. Between 1995 and 1999, evidence of cetacean exploitation was found at 25 of 30 monitored ports in the centre and north of the country, and the Burmeister's porpoise ranked first ^[2].

Between 2000 and 2017, of 942 cetacean specimens stranded along the Peruvian coast (873 identified), 66.3 % belonged to this species, an indirect indication of how much is removed from the population ^[2].

It is an elusive species that is seen rarely and only briefly at sea, and most of the few published observations come from the coast of Peru ^[5],[2]. In surveys off Salaverry it was placed close to the coast, within 25 km and in waters of 50 m or less, in February 2023 ^[6], and in 2009 to 2012 mainly in a coastal area somewhat offshore from that port and in another about 50 km to the north ^[5].

Much of what is known about it comes from caught or stranded animals: of 942 cetacean specimens stranded on the Peruvian coast between 2000 and 2017 (873 identified), 66.3 % were Burmeister's porpoises ^[2].