INTRODUCTION

Rapid climate change is increasingly affecting marine ecosystems by altering ocean temperature, circulation patterns, and the frequency and intensity of extreme climatic events [1–3]. These changes influence primary productivity, trophic interactions, and prey availability, ultimately affecting the health and survival of upper trophic-level predators [2, 4–6]. Marine mammals are widely regarded as sentinel species because their physiological condition may reflect environmental variability, nutritional stress, disease exposure, and other ecological disturbances [7, 8].

Hematological, biochemical, and morphometric parameters provide objective indicators of physiological status in free-ranging wildlife. Serum biochemical analytes can provide information on metabolic, nutritional, and organ function status, whereas hematological variables, including hematocrit and leukocyte differentials, may help identify inflammatory responses, physiological stress, and changes in immune function [9–11]. Morphometric measurements are also valuable for evaluating growth and body condition. However, baseline clinical data remain limited for many pinniped species, particularly in remote island systems where repeated field sampling is logistically challenging [12, 13]. Expanding these datasets is essential for improving wildlife health assessments and understanding pinniped responses to environmental variability.

The Galapagos archipelago is strongly influenced by the El Niño–Southern Oscillation (ENSO), a major driver of interannual climate variability that interacts with broader global climate change processes [3, 14]. ENSO events modify sea surface temperature, ocean circulation, and nutrient upwelling, often reducing marine primary productivity and altering marine food-web dynamics [4, 15–19]. These changes are especially relevant for endemic otariids such as the Galapagos sea lion (GSL; _Zalophus _wollebaeki) and the Galapagos fur seal (GFS; _Arctocephalus _galapagoensis), whose reproductive success, pup survival, and population trends are closely linked to prey availability. Previous ENSO events have been associated with marked demographic impacts, including population declines and increased pup mortality in both species [20–22].

Recent studies have reported hematological and biochemical reference data for GFS pups under ENSO conditions [23]. However, comparable information for GSL pups remains scarce. This lack of species-specific clinical data limits veterinary interpretation during health assessments and restricts understanding of interspecific physiological responses to environmental stress. Generating such baseline information is particularly important during climatically anomalous periods, when nutritional and physiological challenges may be intensified.

Despite the recognized susceptibility of Galapagos pinnipeds to ENSO-driven environmental fluctuations, information regarding the health status of GSL pups during active ENSO events remains limited. Previous studies have primarily focused on population dynamics, reproductive success, and mortality patterns, whereas comprehensive evaluations that integrate morphometric, hematological, and biochemical parameters remain scarce [20–22]. Furthermore, although recent investigations have established clinical pathology reference data for GFS pups sampled under similar environmental conditions [23], direct interspecific comparisons between sympatric GSL and GFS pups during the same ENSO period have not been adequately investigated. Consequently, species-specific physiological responses to reduced marine productivity remain poorly understood, creating a critical knowledge gap that limits the early detection of nutritional stress and hinders the implementation of effective conservation-oriented health monitoring programs for Galapagos pinnipeds.

Therefore, the present study aimed to characterize the hematological, biochemical, and morphometric profiles of GSL pups during the 2024 El Niño event to assess their physiological status under anomalous oceanographic conditions. Specifically, the study evaluated biochemical indicators of metabolic and organ function, hematological parameters including hematocrit and differential leukocyte counts, and morphometric measurements related to body condition. In addition, the findings obtained for GSL pups were compared with previously reported data from sympatric GFS pups sampled during the same period [23].

By integrating clinical assessments with the prevailing environmental context, this study sought to improve understanding of species-specific physiological responses to reduced marine productivity during an active ENSO event. The findings provide clinically relevant baseline information that may facilitate long-term health monitoring, early detection of nutritional stress, and the development of evidence-based conservation strategies for Galapagos pinnipeds under increasingly variable oceanographic conditions.