RESULTS

Morphometric characteristics of GSL pups according to sex

No significant sex-related differences were detected in GSL pups with respect to body weight, standard length, axillary girth, or body condition (Table 2). Welch’s t-tests showed that all comparisons were non-significant (all p ≥ 0.89). Mean differences between males and females were minimal, effect sizes were negligible (Hedges’ g ranging from −0.03 to 0.05), and the 95% confidence intervals for all mean differences included zero. Overall, these findings indicate similar morphometric profiles between female and male GSL pups during the sampled postnatal period.

Variable | Female (n = 26) Mean ± SD | Male (n = 14) Mean ± SD | Welch's t | df | p -value | Mean difference (M−F) | 95% CI (M−F) | Hedges' g | 95% CI ( g )
Weight (kg) | 13.50 ± 2.49 | 13.57 ± 2.81 | 0.08 | 24.03 | 0.94 | 0.07 | −1.78 to 1.92 | 0.03 | −0.61 to 0.66
Standard length (cm) | 92.66 ± 6.37 | 92.86 ± 5.96 | 0.10 | 28.30 | 0.92 | 0.21 | −3.94 to 4.35 | 0.03 | −0.60 to 0.67
Axillary girth (cm) | 51.06 ± 4.29 | 50.95 ± 3.78 | −0.08 | 29.86 | 0.94 | −0.11 | −2.79 to 2.58 | −0.03 | −0.66 to 0.61
Body condition (kg/cm) | 0.14 ± 0.02 | 0.15 ± 0.02 | 0.14 | 23.01 | 0.89 | 0.00 | −0.01 to 0.02 | 0.05 | −0.59 to 0.69

Interspecific comparison of body condition

Body condition was significantly higher in GSL pups than in sympatric GFS pups in both sexes (Table 3). In females, mean body condition was higher in GSL than in GFS, and this difference was significant in both Welch’s t-test and Mann–Whitney U test (both p < 0.001), with a large effect size. A similar pattern was observed in males, with significantly higher body condition in GSL than in GFS (both p < 0.001) and a large effect size. These findings indicate consistently higher relative body condition in GSL pups than in GFS pups during the study period.

Sex | GSL Mean ± SD (n) | GFS a Mean ± SD (n) | Mean difference (GSL−GFS) | 95% CI | p-value (Welch's t -test) | p-value (Mann–Whitney U test) | Hedges' g
Females | 0.14 ± 0.02 (26) | 0.11 ± 0.01 (29) | 0.04 | 0.03 to 0.05 | 5.44 × 10⁻¹¹ | < 0.001 | 2.54
Males | 0.15 ± 0.02 (14) | 0.11 ± 0.02 (30) | 0.03 | 0.02 to 0.05 | 1.70 × 10⁻⁶ | < 0.001 | 1.96

Serum chemistry profiles of GSL and GFS pups

Sex-stratified serum chemistry values for GSL and GFS pups are summarized in Table 4. In females, significant interspecific differences were observed for ALB, GLOB, TP, and BUN. ALB concentrations were higher in GFS females than in GSL females, whereas GLOB and TP concentrations were higher in GSL females than in GFS females. BUN concentrations were also higher in GFS females. No significant interspecific differences were detected for ALT, ALP, TB, CRE, PHO, GLU, or Na⁺.

In males, GLOB and TP concentrations were higher in GSL than in GFS, whereas GLU concentrations were higher in GFS than in GSL. No other analytes differed significantly between species in males. Within GSL, females exhibited significantly higher GLOB and TP concentrations than males. No significant sex-related differences were observed within GFS for any serum chemistry analyte.

Analyte (unit) | GSL Female | GFSᵃ Female | p-value (species, F) | GSL Male | GFSᵃ Male | p-value (species, M) | p-value (sex, GSL) | p-value (sex, GFS)
ALT (U/L) | n = 26; 18.5 ± 6.4; median 19.0; range 8.0–34.0; 95% CI 16.0–21.1 | n = 29; 18.1 ± 11.6; median 15.0; range 8.0–64.0; 95% CI 13.7–22.5 | 0.851 | n = 14; 17.6 ± 7.0; median 17.5; range 9.0–36.0; 95% CI 13.6–21.7 | n = 30; 16.1 ± 6.8; median 14.0; range 6.0–34.0; 95% CI 13.6–18.7 | 0.509 | 0.695 | 0.441
ALP (U/L) | n = 10; 12.4 ± 9.3; median 10.0; range 2.0–30.0; 95% CI 5.8–19.0 | n = 8; 14.5 ± 15.8; median 10.0; range 2.0–51.0; 95% CI 1.3–27.7 | 0.745 | n = 3; 7.7 ± 6.0; median 7.0; range 2.0–14.0; 95% CI −7.3–22.6 | n = 16; 13.6 ± 11.7; median 8.5; range 2.0–36.0; 95% CI 7.4–19.8 | 0.246 | 0.344 | 0.884
ALB (g/L) | n = 26; 42.8 ± 6.4; median 43.5; range 26.0–56.0; 95% CI 40.3–45.4 | n = 29; 50.4 ± 6.0; median 50.0; range 31.0–60.0; 95% CI 48.1–52.7 | <0.001* | n = 14; 45.2 ± 3.5; median 44.5; range 39.0–52.0; 95% CI 43.2–47.3 | n = 30; 47.1 ± 7.6; median 49.5; range 23.0–57.0; 95% CI 44.2–49.9 | 0.275 | 0.138 | 0.065
GLOB (g/L) | n = 25; 21.6 ± 6.8; median 20.0; range 11.0–35.0; 95% CI 18.9–24.4 | n = 27; 12.0 ± 5.7; median 12.0; range 6.0–36.0; 95% CI 9.7–14.3 | <0.001* | n = 14; 17.9 ± 4.1; median 19.5; range 10.0–23.0; 95% CI 15.5–20.3 | n = 28; 13.8 ± 7.5; median 13.0; range 5.0–35.0; 95% CI 10.9–16.7 | 0.027* | 0.040* | 0.314
TP (g/L) | n = 26; 65.8 ± 5.4; median 65.0; range 57.0–79.0; 95% CI 63.6–68.0 | n = 29; 61.5 ± 6.4; median 61.0; range 38.0–73.0; 95% CI 59.0–63.9 | 0.009* | n = 14; 63.3 ± 2.5; median 63.5; range 57.0–68.0; 95% CI 61.8–64.7 | n = 30; 60.8 ± 5.0; median 60.5; range 51.0–69.0; 95% CI 58.9–62.6 | 0.032* | 0.049* | 0.635
TB (μmol/L) | n = 25; 6.6 ± 4.3; median 5.1; range 3.4–22.2; 95% CI 4.8–8.3 | n = 29; 5.3 ± 1.9; median 5.1; range 3.4–11.9; 95% CI 4.6–6.1 | 0.186 | n = 13; 5.26 ± 2.36; median 5.1; range 3.4–11.9; 95% CI 3.8–6.7 | n = 29; 4.9 ± 1.4; median 5.1; range 3.4–8.5; 95% CI 4.4–5.5 | 0.668 | 0.235 | 0.432
BUN (mmol/L) | n = 26; 7.5 ± 4.2; median 6.6; range 2.8–18.2; 95% CI 5.8–9.2 | n = 29; 10.4 ± 4.6; median 8.8; range 4.7–20.8; 95% CI 8.6–12.1 | 0.021* | n = 14; 8.7 ± 5.7; median 7.5; range 2.9–25.6; 95% CI 5.4–12.1 | n = 29; 9.0 ± 3.1; median 8.4; range 3.7–16.4; 95% CI 7.8–10.2 | 0.875 | 0.499 | 0.196
CRE (μmol/L) | n = 23; 51.5 ± 22.1; median 44.2; range 17.7–97.3; 95% CI 41.9–61.1 | n = 28; 63.5 ± 22.9; median 61.9; range 17.7–132.6; 95% CI 54.6–72.4 | 0.065 | n = 13; 48.9 ± 22.7; median 44.2; range 26.5–114.9; 95% CI 35.2–62.7 | n = 30; 60.1 ± 24.5; median 61.9; range 17.7–114.9; 95% CI 50.9–69.2 | 0.162 | 0.747 | 0.593
PHO (mmol/L) | n = 26; 1.6 ± 0.4; median 1.6; range 0.9–2.4; 95% CI 1.4–1.7 | n = 29; 1.45 ± 0.38; median 1.39; range 0.90–2.36; 95% CI 1.31–1.60 | 0.286 | n = 14; 1.5 ± 0.4; median 1.5; range 0.7–2.2; 95% CI 1.2–1.7 | n = 30; 1.6 ± 0.6; median 1.6; range 0.7–4.3; 95% CI 1.4–1.8 | 0.416 | 0.468 | 0.284
GLU (mmol/L) | n = 25; 6.1 ± 1.3; median 6.1; range 3.1–8.4; 95% CI 5.5–6.6 | n = 29; 6.70 ± 1.05; median 6.72; range 4.33–8.60; 95% CI 6.30–7.10 | 0.067 | n = 14; 5.6 ± 1.3; median 5.1; range 4.1–7.7; 95% CI 4.8–6.3 | n = 30; 7.2 ± 1.1; median 7.4; range 5.4–9.6; 95% CI 6.8–7.7 | <0.001* | 0.243 | 0.069
Na⁺ (mmol/L) | n = 26; 162 ± 4.0; median 162.0; range 154–173; 95% CI 160–164 | n = 29; 160 ± 5.0; median 160.0; range 150–170; 95% CI 158–161 | 0.055 | n = 14; 162 ± 7.0; median 162.0; range 151–176; 95% CI 158–166 | n = 30; 159 ± 4.0; median 159.0; range 152–167; 95% CI 158–161 | 0.178 | 0.969 | 0.669

Hematological characteristics of GSL and GFS pups

Hematological values are summarized in Table 5. In females, hematocrit, neutrophils, lymphocytes, monocytes, and eosinophils did not differ significantly between GSL and GFS, whereas basophil percentages were significantly lower in GSL females. In males, hematocrit values were significantly higher in GSL than in GFS, whereas monocyte and eosinophil percentages were significantly lower in GSL males. Neutrophil, lymphocyte, and basophil percentages did not differ significantly between species in males.

Variable (%) | GSL Female | GFSᵃ Female | p-value (species, F) | GSL Male | GFSᵃ Male | p-value (species, M)
Hematocrit | n = 23; 41.7 ± 7.3; range 21.0–49.0; 95% CI 38.5–44.8 | n = 28; 44.6 ± 6.1; range 32.0–59.0; 95% CI 42.2–46.9 | 0.133 | n = 13; 45.2 ± 3.6; range 40.0–51.0; 95% CI 43.0–47.4 | n = 29; 42.1 ± 6.3; range 23.0–49.0; 95% CI 39.7–44.5 | 0.049
Neutrophils | n = 26; 67.0 ± 9.6; range 46.0–86.0; 95% CI 63.2–70.9 | n = 29; 64.4 ± 6.0; range 46.0–73.0; 95% CI 62.1–66.7 | 0.238 | n = 14; 65.0 ± 10.2; range 51.0–85.0; 95% CI 59.1–70.9 | n = 29; 66.2 ± 5.3; range 54.0–77.0; 95% CI 64.2–68.2 | 0.692
Lymphocytes | n = 26; 29.2 ± 9.5; range 11.0–43.0; 95% CI 25.4–33.0 | n = 28; 28.9 ± 5.4; range 20.0–41.0; 95% CI 26.8–31.0 | 0.889 | n = 14; 31.4 ± 10.5; range 11.0–48.0; 95% CI 25.3–37.4 | n = 29; 26.6 ± 5.0; range 19.0–35.0; 95% CI 24.7–28.5 | 0.125
Monocytes | n = 26; 2.3 ± 1.0; range 1.0–5.0; 95% CI 2.0–2.7 | n = 28; 2.6 ± 2.5; range 0.0–9.0; 95% CI 1.6–3.5 | 0.658 | n = 14; 2.1 ± 0.9; range 1.0–4.0; 95% CI 1.5–2.6 | n = 29; 3.3 ± 2.4; range 0.0–10.0; 95% CI 2.4–4.2 | 0.022
Eosinophils | n = 26; 1.4 ± 2.9; range 0.0–11.0; 95% CI 0.2–2.6 | n = 28; 3.0 ± 3.3; range 0.0–11.0; 95% CI 1.7–4.3 | 0.064 | n = 14; 1.4 ± 2.0; range 0.0–6.0; 95% CI 0.3–2.6 | n = 29; 3.7 ± 3.5; range 0.0–11.0; 95% CI 2.3–5.0 | 0.012
Basophils | n = 26; 0.0 ± 0.0; range 0.0–0.0; 95% CI 0.0–0.0 | n = 28; 0.4 ± 0.9; range 0.0–4.0; 95% CI 0.1–0.8 | 0.016 | n = 14; 0.1 ± 0.5; range 0.0–2.0; 95% CI −0.2–0.5 | n = 29; 0.3 ± 0.7; range 0.0–2.0; 95% CI 0.0–0.6 | 0.396

Comparison with published data from other otariid pups

Published serum chemistry ranges for GSL and GFS pups broadly overlapped with those reported for other otariid species for most analytes. The most notable differences were observed for ALP, TB, and Na⁺. Both Galapagos species exhibited lower ALP and higher TB ranges than Antarctic fur seals, South American fur seals, and Steller sea lions, whereas Na⁺ concentrations were generally higher in Galapagos otariids. In contrast, ALT, ALB, TP, BUN, CRE, PHO, and GLU values generally fell within the broad ranges reported for other otariid pups (Supplementary Table S1).

Hematological profiles of GSL and GFS pups also broadly overlapped with published values reported for other otariids. Hematocrit, neutrophil, lymphocyte, monocyte, and eosinophil values were generally comparable across species, whereas basophil values were consistently low or were not reported in the available literature (Supplementary Table S2).

Environmental conditions during the study period

The 2024 sampling period coincided with a weak-to-moderate El Niño event (ONI: +0.8 to +1.3°C), whereas 2023 represented an ENSO-neutral baseline period (ONI < +0.5°C). Rookery-level chlorophyll-a concentrations were significantly lower in 2024 than in 2023 (Wilcoxon signed-rank test, V = 37, p = 0.005). Across rookeries, the median paired change indicated an overall decline in chlorophyll-a concentrations, although the magnitude of change varied among sites (Supplementary Table S3).

Despite this general reduction in marine productivity, some local areas retained relatively high chlorophyll-a concentrations, indicating spatial heterogeneity in ENSO-related productivity changes across the archipelago (Figures 1 and 2).

Figure 2: Distribution of rookery-level mean chlorophyll-a concentrations (mg m⁻³) in the Galapagos archipelago during 2023 (non-ENSO year) and 2024 (ENSO year). Violin plots illustrate the density distribution of chlorophyll-a values among pinniped rookeries, with individual rookeries represented by colored points and the archipelago-wide mean for each year indicated by a black diamond.