On the Challenge of Interpreting Census Data: Insights from a Study of an Endangered Pinniped

Trillmich, Fritz; Meise, Kristine; Kalberer, Stephanie; Mueller, Birte; Piedrahíta, Paolo; Pörschmann, Ulrich; Wolf, Jochen B. W.; Krüger, Oliver

doi:10.1371/journal.pone.0154588

Cited by 22 publications

(21 citation statements)

References 28 publications

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“…The average probabilities of observing individuals ashore during the counts, on which we based our correction factors for estimating the GSL population size in the SER, were calculated by Trillmich et al [28] using raw counts of tagged sea lions that were resighted and estimates of the population size at the Caamaño rookery while employing the Lincoln-Petersen capture-recapture estimator, which assumes a closed population. As explained by Trillmich et al [28], for juveniles, the Lincoln-Petersen estimates were close to the number of animals tagged. However, for adults, which haul out at places other than their home rookery during their foraging trips [41,43], there may be over- or underestimates of the number of individuals who belong in the rookery.…”

Section: Discussionmentioning

confidence: 99%

“…The GSL population size in each rookery and on each island in the SER was estimated by correcting the number of counted animals using correction factors derived from the probability of observing individuals of different age categories ashore during the counts [28]. By using the Lincoln-Petersen method, based on resights of marked animals, to estimate the population size of sea lions in the Caamaño rookery in the central archipelago over 13 years of counts, Trillmich et al [28] calculated that the probability of observing a given adult in the rookery ( P ob-r ) varied between 16% (95% confidence interval (CI) = 19%-12%) for the cold/reproductive season and 23% (95% CI = 28%-18%) for the warm/non-reproductive season.…”

Section: Methodsmentioning

confidence: 99%

“…By using the Lincoln-Petersen method, based on resights of marked animals, to estimate the population size of sea lions in the Caamaño rookery in the central archipelago over 13 years of counts, Trillmich et al [28] calculated that the probability of observing a given adult in the rookery ( P ob-r ) varied between 16% (95% confidence interval (CI) = 19%-12%) for the cold/reproductive season and 23% (95% CI = 28%-18%) for the warm/non-reproductive season. For juveniles, the probabilities were 35% (95% CI = 37%-34%) during the cold season and 50% (95% CI = 52%-46%) during the warm season.…”

Section: Methodsmentioning

confidence: 99%

“…For juveniles, the probabilities were 35% (95% CI = 37%-34%) during the cold season and 50% (95% CI = 52%-46%) during the warm season. According to Trillmich et al [28], the category of juveniles in the cold season included sea lions between 1 and 4–5 years of age, but in the warm season the pups (< 1 year of age) were also included in this category. Sea lions categorized as “indeterminate” in our censuses were considered juveniles, as categorization uncertainties mostly concerned the immature category [32].…”

Section: Methodsmentioning

confidence: 99%

“…In this study, we estimated the GSL population size in the southeastern region of the archipelago using correction factors for the census data derived from the probability of observing individuals of different age categories in their rookery during the counts [28]. Census data from 2005 to 2015 were used to estimate the average annual growth rate (ʎ) and population trend of the species in the El Malecón rookery using population viability analysis, which is a quantitative method used to predict the likely future status of a population [29].…”

Section: Introductionmentioning

confidence: 99%

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

2017

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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.

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Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

2017

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Minimal overlap between areas of high conservation priority for endangered Galapagos pinnipeds and the conservation zone of the Galapagos Marine Reserve

Ventura

Matthiopoulos

Jeglinski

2019

Aquatic Conservation

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Galapagos sea lions (Zalophus wollebaeki) and fur seals (Arctocephalus galapagoensis) are endangered pinnipeds that live in a highly variable marine environment, influenced by seasonal changes in productivity and by the unpredictable occurrence of El Niño. The Galapagos marine habitat is protected through the Galapagos Marine Reserve (GMR), which has recently undergone a redefinition of its zonation. However, the effectiveness of the GMR in protecting the habitat of Galapagos pinnipeds has never been investigated, and it is unclear how well their habitat use aligns with the newly proposed conservation zone. A spatial prioritization analysis framework was applied based on the complementary use of density estimation (kernel utilization distributions, KUDs) and habitat modelling (resource selection functions, RSFs). Using tracking data collected over two years and under three environmental scenarios (cold season, warm season, and El Niño) in the western archipelago, hot spots of pinniped habitat usage were identified (through KUDs), and predictions about the location of patches of good habitat were generated (through RSFs). The output of KUDs and RSFs was used for a spatial prioritization analysis to delineate areas of high conservation priority. The overlap between these areas and the GMR zonation was then calculated. Both species were found to use largely distinct habitat types: sea lions used the waters over the continental shelf, whereas fur seals used the offshore deep waters and showed a more heterogeneous space usage over time. The spatial prioritization analysis identified three key areas of high conservation priority for both species in the Western Galapagos. These areas were all within the boundaries of the GMR but the overlap with the conservation zone was only 8%. Hence, under the current proposition, the largest proportion of key pinniped foraging habitat in the western archipelago will not be protected from licensed activities in the sustainable use zone, particularly fishing and boat traffic.

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Reproductive performance of a tropical apex predator in an unpredictable habitat

Kalberer

Meise

Trillmich

et al. 2018

Behav Ecol Sociobiol

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On the Challenge of Interpreting Census Data: Insights from a Study of an Endangered Pinniped

Cited by 22 publications

References 28 publications

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

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

Minimal overlap between areas of high conservation priority for endangered Galapagos pinnipeds and the conservation zone of the Galapagos Marine Reserve

Reproductive performance of a tropical apex predator in an unpredictable habitat

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