ABSTRACT

Background and Aim: The Galapagos sea lion (Zalophus wollebaeki) and Galapagos fur seal (GFS; Arctocephalus galapagoensis) are endemic pinnipeds highly vulnerable to environmental fluctuations associated with the El Niño–Southern Oscillation (ENSO). This study characterized the morphometric, hematological, and biochemical profiles of Galapagos sea lion pups during the 2024 El Niño event and compared them with those of sympatric GFS pups to evaluate species-specific physiological responses to reduced marine productivity. 

Materials and Methods: Between March and April 2024, 40 apparently healthy Galapagos sea lion pups from 13 rookeries throughout the Galapagos archipelago were examined. Body weight, standard length, axillary girth, and body condition index were recorded. Blood samples were analyzed for serum biochemical variables, hematocrit, and leukocyte differential counts. Environmental conditions were characterized using Oceanic Niño Index values and satellite-derived chlorophyll-a concentrations. Sex-related and interspecific comparisons were performed using appropriate parametric and non-parametric statistical tests. 

Results: No significant sex-related differences were detected in morphometric measurements of Galapagos sea lion pups. Compared with GFS pups, Galapagos sea lion pups exhibited significantly higher body condition indices in both sexes. Female sea lion pups had higher globulin and total protein concentrations and lower albumin and blood urea nitrogen values than female fur seal pups, whereas male sea lion pups showed higher globulin and total protein concentrations but lower glucose concentrations. Most hematological variables overlapped between species, although male sea lion pups exhibited higher hematocrit and lower monocyte and eosinophil counts. During the study period, Oceanic Niño Index values indicated weak-to-moderate El Niño conditions, and chlorophyll-a concentrations were significantly lower than those recorded in 2023, indicating reduced marine productivity. 

Conclusion: Galapagos sea lion pups maintained favorable body condition and largely normal clinical profiles despite reduced productivity associated with the 2024 El Niño event. Distinct interspecific differences in body condition and serum proteins suggest species-specific physiological responses to environmental variability. These findings provide valuable baseline health data for long-term monitoring, early detection of nutritional stress, and conservation management of Galapagos pinnipeds under increasingly variable oceanographic conditions. 

Keywords: body condition, climate variability, El Niño–Southern Oscillation, Galapagos fur seal, Galapagos sea lion, hematology, marine mammal health, serum biochemistry.