Population Abundance of the Endangered Galapagos Sea Lion Zalophus wollebaeki in the Southeastern Galapagos Archipelago

Abstract

There is great concern regarding the population status of the endangered Galapagos sea lion (GSL) because it has drastically decreased over the last 30 years. We determined the population size and growth trend of the GSL in the Galapagos southeastern region (SER) at three population levels based on the available census data: 1) SER (2011–2015), including 13 rookeries on the four islands San Cristóbal (SC), Española, Floreana, and Santa Fe, comprising 58% of the archipelago’s population; 2) SC (2011–2015), including five rookeries, comprising 52% of the SER population; and 3) El Malecón (2005–2015), the largest rookery on SC and in the SER (43% of the population on SC and 22% in the SER). We also analyzed the influence of environmental variability on pup abundance in these rookeries. The current GSL population size in the SER, after applying correction factors to the counts, is estimated at approximately 2300–4100 individuals and has declined at an average annual rate (ʎ) of 8.7% over the last five years. A similar trend was determined for SC but at ʎ = 1.4% during the same period. For El Malecón, a count-based population viability analysis using a diffusion approximation approach showed that the population increased from 2005 to 2015 at ʎ = 2%. The interannual variability in pup abundance was associated with anomalies in sea surface temperature linked to oceanographic-atmospheric events, which impact the abundance and availability of prey, and ultimately may determine the population’s reproductive success. Since rookeries in the SER had different population trends, management actions should be implemented based on their specific conditions, giving priority to rookeries such as El Malecón, which, despite showing a slightly increasing population trend, still faces a high risk of extinction due to anthropogenic disturbances and environmental variability that may affect its growth and survival.

Intraspecific variation in feeding strategies of Galapagos sea lions: A case of trophic specialization

“…Because of the otariids of the Galapagos Islands inhabit an ecosystem with fluctuating marine productivity which is strongly influenced by ocean currents and an upwelling pattern [ 27 , 28 ]; these species are continuously struggling to survive in an ecosystem with fluctuating marine productivity associated with oceanographic events characteristic of the region [ 25 , 26 ]. This battle to survive is reflected in their overall reduced and high variation in population size, which is magnified by anomalous oceanographic events, such as El Niño Southern Oscillation (ENSO), which increases mortality [ 46 , 47 ].…”

Influence of hunting strategy on foraging efficiency in Galapagos sea lions

“…1 ) from November–December 2012 (i.e., during a La Niña phase ( NOAA, 2020 )). The El Malecon colony is the largest colony of GSL and San Cristóbal Island hosts approximately 13% of the GSL population ( Riofrío-Lazo, Arreguín-Sánchez & Páez-Rosas, 2017 ). Adult females nursing pups 1–3 months old ( Riofrío-Lazo, Arreguín-Sánchez & Páez-Rosas, 2017 ) were selected opportunistically according to ease of capture access at the site.…”

“…The El Malecon colony is the largest colony of GSL and San Cristóbal Island hosts approximately 13% of the GSL population ( Riofrío-Lazo, Arreguín-Sánchez & Páez-Rosas, 2017 ). Adult females nursing pups 1–3 months old ( Riofrío-Lazo, Arreguín-Sánchez & Páez-Rosas, 2017 ) were selected opportunistically according to ease of capture access at the site. Individuals were captured using a modified hoop net before being tranquillised with a 0.8-1 mL intra-muscular injection of Telazol (teletamine/zolazepam HCl) at a 100mg/ml concentration.…”

Declines and recovery in endangered Galapagos pinnipeds during the El Niño event

“…However, pups are confined to land in breeding rookeries for the first few months of life, so their numbers are more constant 6 , 9 . Therefore, pups can be used as an indicator of abundance and a basis for estimating population trends 10 , 11 .…”