The South American Sea Lion is the most abundant marine mammal occurring along the southern part of South America (Cappozzo 2002). The population of the coast of Uruguay consists of two main reproductive colonies, Isla Lobos (35º01’S) and Cabo Polonio (34º24’S), and is estimated to be about 12,000-13,000 individuals (1,200-2,675 pups born per year; Páez 2006, Pedraza et al. 2012, Franco-Trecu 2015). On the northern coast of Argentina, there are only four haulouts (about 2,500 individuals), while the Patagonian region has both reproductive and non-reproductive colonies (about 120,700 individuals). An additional 7,500 animals are found in the Falkland Islands (Crespo et al. 2012). Baylis et al. (2015) reported a minimum estimate of 4,443 pups born at the Falklands in 2014. The Chilean population is estimated to be approximately 197,000 animals (Venegas et al. 2001, Bartheld et al. 2008, Sepúlveda et al. 2011, Oliva et al. 2012, Contreras et al. 2014). There are at least 105,000 individuals in Peru (IMARPE 2013), and no more than 200 on the Brazilian coast (Sanfelice et al. 1999, Pavanato et al. 2013). Therefore, the total global population is approximated to be at least 445,000 individuals.
South American Sea Lion numbers are increasing in northern Patagonia, in the Rio Negro (41º03’S) and northern Chubut (43º34’S) provinces, at 5.7-6% per year (Dans et al. 2003a, Grandi 2010). In central Patagonia, in the central (43º57’S) and southern Chubut (45º23’S) province, they are also increasing at 6% annually (Reyes et al. 1999, Reyes 2004). In Chile between 15°56’S and 48°40’S, the population increased from 137,000 to 197,000 in 7 years (Oliva et al. 2012, Contreras et al. 2014). The population trend in the Magallanes Region is unknown. On the other hand, abundance has been decreasing in Uruguay. Negative trends for all sex and age classes of the breeding population were reported by Páez (2006) as -1.4% per year for adult males, -2.1% for adult females, and -4.5% for pups. Results from population modeling by Paez (2006) showed a 2% per year decline for total population size and a 3% decline in birth rates. This coincides with recent findings from Franco-Trecu (2015) that estimated a -2% (CI -1.1% to -2.5%) decline in pup production using pup count data from 1956-2013. Although the reasons for the population decline in Uruguay are still unknown, it is suspected that it could be related to interactions with fishing activities (Crespo et al. 2012, Riet-Sapriza et al. 2013) and with the long-term effect of harvest (Franco-Trecu 2015). The cumulative effects of population extractions, including pup harvesting (~50,000) and zoo and aquaria sales (144 young males and 285 young females), not only reduced the local population size, but also could have disrupted its social structure to the point where Allee effects could be limiting the post-harvesting population recovery at Isla de Lobos (Franco-Trecu et al. 2015). In southern Patagonia, in the Santa Cruz (46º01’S) and Tierra del Fuego (54º88’S) Provinces, the trend is unknown because data are insufficient to estimate a rate of change (Schiavini et al. 2004); however, the current numbers are clearly less than the estimates reported in the late 1940s. Sealing activities, performed mainly at northern Patagonia and at Tierra del Fuego, are likely responsible for the depletion (Schiavini et al. 2004). At the Falkland Islands there was a 95% decline in the population from >380,000 animals to <30,000 (from 80,555 pups in the mid 1930s to 5,506 pups in 1965; Hamilton 1939, Strange 1979). The number of pups estimated in 2014 for the Falkland Islands was 6% of the number estimated in 1930s (Baylis et al. 2015). Different hypotheses have been proposed to explain the decline, include commercial sealing and environmental change (Strange 1979, Thompson et al. 2002, Baylis et al. 2015). However, the trend has been positive since 1990; with an 8.5% annual increase from 1990 to 1995, and a 3.8% annual increase between 1995 and 2003 (Crespo et al. 2012).
South American Sea Lion population trends along the Chilean coast are not homogenous. In northern Chile the populations are increasing (Barthled et al. 2008, Oliva et al. 2012) whereas the trend is unknown for central and southern Chile (Sepúlveda et al. 2011).
Due to the 1997-98 El Niño Southern Oscillation (ENSO), the Peruvian population of South American Sea Lions declined from about 144,087 animals in December 1997 to 27,991 in December 1998, a reduction of 81% (Arias-Schreiber and Rivas 1998, Arias-Schreiber 1998). This was probably due to a combined effect of mortality and dispersal from historically surveyed breeding and haul out sites. After this dramatic reduction, there was a recovery of 76.3%, with an estimated 118,220 individuals by 2006 (IMARPE 2006). The recovery of the population of Sea Lions on the coast of Peru is due to improved reproductive levels as a consequence of an increase in food availability as well as migration from the colonies in northern Chile (Oliveira et al. 2012). However, the stronger and more frequent ENSOs that appear to be occurring along the Peruvian coast may put the population in Peru at greater risk (Soto et al. 2004).
The majority of subpopulations in the southwestern Atlantic Ocean are increasing, although the trends are not homogeneous. However, in contrast to what is observed on the Peruvian Pacific coast, the population sizes do not show large inter-annual fluctuations (Crespo et al. 2012). In Chile, the population is increasing steadily (Oliva pers. comm).
South American Sea Lions are stocky, heavy-bodied otariids that are strongly sexually dimorphic (Cappozzo 2002). Adult males reach 2.1-2.6 m in length and weights of 300-350 kg; females reach 1.5-2 m and 170 kg (Grandi et al. 2012a, Rosas et al. 1993, Cappozzo and Perrin 2009, Riet-Sapriza et al. 2013). At birth, pups weigh 11-15 kg and are 75-85 cm long. Pups are born black above and paler below, often with grayish-orange tones on the undersides. They undergo their first molt 1-2 months after birth, becoming dark brown. This color fades during the rest of the first year to a pale tan to light brown, with paler areas on the face (Vaz-Ferreira 1975).
Sexual maturity is attained at 4-5 years for females and 4-7 years for males, but males cannot hold and defend a territory and maintain a harem until they reach 9-11 years old (Grandi et al. 2012a, Vaz-Ferreira 1982). Gestation lasts about one year. Longevity is considered to be about 20 years. Mortality rates for adults are unknown (Reijnders et al. 1993). Pup mortality estimated for some Peruvian colonies ranged from 13% before ENSO events to 100% during ENSO, and was negatively correlated with prey availability (Soto et al. 2004).
Breeding takes place during the austral summer, starting in mid-December. The start of the breeding season varies somewhat by location and latitude, with longer seasons occurring at low latitudes and shorter seasons occurring further south at high latitudes (Campagna 1985, Soto 1999). At most breeding sites, both sexes arrive in mid-December, with peak numbers of males and females ashore during the second half of January. Females give birth to a single pup, 2-3 days after their arrival at the rookeries, and remain onshore to nurse for approximately 7 days. Pups are born from mid-December to early February, with a peak in mid-January, coinciding with the timing of peak numbers of females ashore. Estrous occurs 6 days after parturition, and females make their first foraging trip 2-3 days after estrous. From this point on, a cycle of foraging and pup attendance starts and lasts until pups are weaned at 8-10 months old (Ponce de León and Pin 2006, Vaz-Ferreira 1982). As is the case for many Sea Lions, it is not unusual for females to continue to care for a yearling while they are nursing a new pup, as lactation can be extended up to three years although that is rarely observed (Campagna and Le Boeuf 1988a, Soto 1999). In Chile, pups gather in large pods on the rookeries while waiting for their mothers to return from 1-4 day long foraging trips. Females usually stay ashore for 1-2 days between trips (Muñoz et al. 2011). In Uruguay, trips have an average duration of 1.5±0.9 days and visits ashore are 1.1±0.8 days (Riet-Sapriza et al. 2013).
South American Sea Lions are a highly polygynous species. Social groups are composed by a dominant male and 4-10 adult females, although some solitary couples are found. This variation in female numbers depends on the various strategies employed by males and females during the breeding season that are related to colony substrate, thermoregulatory requirements imposed by weather conditions at the site, or avoidance of male harassment (Vaz-Ferreira 1982; Campagna and Le Boeuf 1988b; Cassini 1999, 2000; Cappozzo et al. 2008; Franco-Trecu et al. 2015). In Argentina, adult males tend to establish territories through vocalizing, posturing, and fighting when rookeries provide shade, have tidal pools that can be used for cooling, or funnel interior areas through narrow beaches between rocks or ledges to the sea. At more homogeneous locations with long shorelines, the male strategy focuses on identifying, defending, and controlling individual females in estrous, wherever they are found. Bulls actively and aggressively work to keep estrous females close to them by grabbing, dragging, and throwing them back inland, away from the shoreline (Campagna and Le Boeuf 1988b). On the Peruvian coast a lek-like mating system has been described in which males maintain positions along the shoreline where females pass each day (Soto and Trites 2011). According to these authors, the daily movements of females toward the water accentuate the difficulty for males to monopolize females and thus males are not able to defend females. In Uruguay, researchers combining behavioural and molecular data found that the reproductive behaviour actually involves the coexistence of two types of polygyny each occurring in different parts of the same rookery (Isvaran 2005, Taborsky et al. 2008). On one hand, males at the tide line monopolize relatively stable groups of females (female-defense polygyny) within floating territories (i.e., a territory that changes position over time) whose locations change with the tidal variation at the study site (Wilson 1975, Alcock et al. 1978, Barrows 1983). On the other, males at the internal pools defend fixed territories (defined as territory having a stable location during the tenure by its holder (Dewsbury 1978) and established a resource-defense polygyny (Emlen and Oring 1977).
At sea, South American Sea Lions frequently raft alone or in small to large groups. They have been reported in association with feeding cetaceans and seabirds (Duffy 1983). On the Atlantic coast most lactating females have been described as benthic divers and forage in shallow water within the continental shelf. Mean depth of dives at Isla Lobos, Uruguay, were 15-25 m and they lasted 1.0-2.5 minutes (Riet-Sapriza et al. 2013), and females from northern Patagonian rookeries made dives in the range of 2-30 m lasting < 4 minutes (Campagna et al. 2001). However, high levels of variability in foraging patterns have been found, as some lactating females from northern Patagonia and the Falkland Islands also behave as pelagic predators (Werner and Campagna 1995, Thompson et al. 1998, Campagna et al. 2001). The deepest dives recorded for female South American Sea Lions (>60 m) off Patagonia, Argentina, are similar to the depth of the shelf in that area (Campagna et al. 2001). Other deep dives of 100 m have been recently recorded in individuals off the coast of Argentina by Drago, Crespo and Franco-Trecu (unpublished data).
Adult male South American Sea Lions have been observed to reach distances of more than 300 km from the coast, both in Argentinean and Chilean waters (Campagna et al. 2001, Hückstädt and Krautz 2004). Juvenile Sea Lions in central Chile rarely ventured into offshore waters, reaching a mean distance from the coastline of 20 km, with a maximum of only 80 km. They show a clear pattern of epipelagic foraging, with dives usually shallower than 20 m, but sometimes reaching depths of 240 m (Hückstädt et al. 2014). ). In southern Chile a mesopelagic foraging behavior has been described, with mean dive depths of 100-120 m lasting 2.0-2.5 minutes, with a maximum depth of 320 m and duration of 5 minutes (Sepúlveda et al. in preparation). Hückstädt and Krautz (2004) observed Southern Sea Lions in the Pacific Ocean in association with a fleet fishing for Jack Mackerel (Trachurus symmetricus) outside the continental shelf, suggesting different behavior than that observed in the Atlantic Ocean, where the diving pattern is likely related to the depth of the continental shelf (Werner and Campagna 1995, Thompson et al. 1998, Campagna et al. 2001, Riet-Sapriza et al. 2013).
South American Sea Lions are considered non-migratory, although many individuals make seasonal movements away from rookeries during the non-breeding season (Rosas et al. 1994), and some southerly locations such as the Falkland Islands are largely abandoned during the winter. Although there are no breeding colonies in Brazil, many Sea Lions are found there throughout the year, grouped in specific places to rest (Refúgio de Vida Silvestre da Ilha dos Lobos, Torres – 29°20’S and Refúgio de Vida Silvestre Molhe Leste, São José do Norte – 32º11’S), or swimming in coastal waters in winter and spring months. Since many Sea Lions make seasonal movements away from their reproductive colonies in search of feeding grounds, it has been suggested that individuals in Brazil come from the breeding colonies off Uruguay after their breeding period (Rosas et al. 1994, Pinedo 1990). Among the continental and island colonies of the Argentine coast there is evidence of seasonal movements (Lewis and Ximénez 1983; Giardino et al. 2008, 2009). Animals that reproduce at Península Valdés (northern Argentine Patagonia) move to Uruguay and vice versa (Szapkievich et al. 1999).
As generalist feeders, South American Sea Lions take a wide variety of prey that varies by location. Their diet includes many species of benthic and pelagic fishes and invertebrates, some of them of commercial value. Forty-one prey species (including fishes, cephalopods, crustaceans, gastropods, polychetes, sponges, and tunicates) were identified in stomach contents of individuals found dead on beaches and from animals recovered in incidental catch of the fisheries of the Patagonian continental shelf (Koen Alonso et al. 2000). The most important items were Argentine Hake (Merluccius hubbsi), Red Octopus (Enteroctopus megalocyathus), Argentine Shortfin Squid (Illex argentinus), Raneya (Raneya brasiliensis), Patagonian Squid (Loligo gahi) and Argentine Anchovy (Engraulisanchoita). Differences in diet were found between sexes. Females fed mostly on coastal and benthic species, like Red Octopus and Argentine Shortfin, whereas males fed mostly on demersal-pelagic species, such as Argentine Hake and Patagonian Squid (Crespo et al. 1997, Koen Alonso et al. 2000). As expected from differences in body mass, Sea Lion males from northern Patagonia had been reported to exploit benthic and deeper foraging grounds than females (Campagna et al. 2001, Drago et al. 2009), although differences in foraging habits between the sexes are not constant over time (Drago et al. 2009). In Uruguay, carbon and nitrogen stable isotope values of skin and bone were used to infer the trophic relationships between the sexes during the pre-breeding period and year round. The study revealed that male and female Sea Lions used a variety of foraging strategies throughout the year and that no differences existed between the sexes. However, the diversity of foraging strategies was strongly reduced in both sexes during the pre-breeding period, when all individuals increased their consumption of pelagic prey over benthic prey, and isotopic niche space of males and females did not overlap at all (Drago et al. 2015). These results indicate that sexual foraging segregation only takes place during the pre-breeding season, when crowding in the areas surrounding the breeding rookeries increases and per-capita resource availability declines. At Isla de Lobos, Uruguay, the most abundant prey species during summer are cephalopods (Family Omastrephidae) and Striped Weakfish (Cynoscion guatucupa). However, the principal contribution by biomass is accounted by Whitemouth Croaker (Micropogonias furnieri), Large Head Hairtail (Trichiurus lepturus), Brazilian Codling (Urophysis brasiliensis), and Argentine Croaker (Umbrina canosai) (Riet-Sapriza et al. 2013). In Peru, Sea Lions prey mostly on Anchoveta (Engraulis ringens), Mote Sculpin (Normanichthys crockeri), Lumptail Searobin (Prionotus stephanophrys), Peruvian Hake (Merluccius gayi), Red Squat Lobster (Pleuroncodes monodon), and cephalopods (Fam. Loliginidae) (Paredes and Arias Schreiber 1999).
In Chile, temporal and spatial diet plasticity was found by Muñoz et al. (2011). In northern Chile the main prey species for South American Sea Lions are Anchovy, Patagonia Squid (Loligo gahi), Cabinza Grunt (Isacia conceptionis), and Corvina (Cilus gilberti). In central Chile the main prey are South Pacific Hake (Merluccius gayi gayi), Snoek (Thyrsites atun), and Araucanian Herring (Strangomera bentinckii), whereas in southern Chile the main species were the Chilean Jack Mackerel (Trachurus murphyi) and Snoek. In southern Chile farmed-raised salmonids are also important in the diet, suggesting that South American Sea Lions are capable of modifying their dietary habits in response to variation in abundance and/or accessibility of prey (Muñoz et al. 2011, Sepúlveda et al. 2015).
Diet and maternal care patterns reflect inter-annual fluctuations in food availability. In the unpredictable Peruvian upwelling ecosystem, females appeared to adjust their diets and maternal attendance patterns in response to annual changes in the abundance and distribution of prey (Soto et al. 2006). Short times onshore nursing and prolonged times at sea foraging are observed in Peru during ENSO events when prey are not abundant near the rookeries. As a result, the fasting ability of pups may be exceeded causing high mortality due to starvation (Soto et al. 2004, 2006). A larger diversity of prey species (particularly of demersal fishes) is consumed during ENSO, when Anchovy and Squat Lobster are less available. These observations suggest that South American Sea Lions may be good indicators of relative changes in the distribution and abundance of marine resources.
A small percentage of sub-adult and adult male South American Sea Lions regularly attack and kill South American Fur Seal (Arctocephalus australis) pups in Peru (Harcourt 1993), Argentina (Campagna et al. 1988b), and in Uruguay (Franco-Trecu, pers. comm). In Peru, attacks occurred more frequently in the nonbreeding season, when Fur Seal males are not actively defending the breeding colony (Harcourt 1993). Adult or sub-adult male Sea Lions hunt alone and focus their attacks on Fur Seal pups and juveniles up to two years of age that are consumed when caught. Sub-adult males also attack, but tend to abduct Fur Seals to serve as female Sea Lion substitutes, herding them and attempting to mate with them, usually killing them in the process. Sub-adults never consumed the pups they abducted (Harcourt 1991, 1992, 1993). Sea lions directly increase pup mortality when they take and kill young Fur Seal pups. These actions also indirectly increase mortality by creating disturbances on the beaches. When Sea Lions enter a beach with Fur Seals severe localized disturbances occur, animals in the immediate vicinity will stampede, and separations of mother-pup pairs are frequent. Pups may be crushed by older animals, or by rocks dislodged by the stampede. Although rare, Sea Lions will also kill adult female Fur Seals, and if the female has a pup it will then die of starvation (Harcourt 1992). Sea Lions have been observed killing young Southern Elephant Seals (Mirounga leonina) at the Falkland Islands. They are also known to take several species of Penguins, but the importance of Penguins in the diet is unknown (Boswall 1972, Strange 1982, Raya Rey et al. 2012). Sea Lions have also been recorded preying on Sea Turtles in Peru and northern Chile (Hückstädt pers. comm., Cárdenas-Alayza unpublished data).
Predators of South American Sea Lions include Killer Whales (Orcinus orca) (Grandi et al. 2012b), Sharks (Crespi Abril et al. 2004), and possibly Leopard Seals (Hydrurga leptonyx) and Puma (Puma concolor). Puma tracks have been observed on a rookery in Patagonia and remains of Sea Lions have been found in a cave used by a Puma in the area. At the well known rookery of Punta Norte at Península Valdés, Killer Whales are known to surf in on waves partially beaching themselves while grabbing predominantly young Sea Lions off the shoreline.