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The Mexican southwest Gulf of California represents an important winter area in the life cycle of blue whales, since it serves as a feeding, breeding and nursing area. Coastal developments are proposed in the future and an increase in marine traffic and whale-watching activities is expected in this area. Stress and reproductive hormones have proved useful in defining endocrine status of terrestrial and marine mammals, revealing the mechanisms by which individuals respond to environmental challenges. The objective of this study was to determine baseline concentrations of glucocorticoids (cortisol, corticosterone) and reproductive status for mature, lactating and pregnant female blue whales in feces. A total of 31 fecal samples were collected (24 adult females, 1 adult male and 6 individuals of unknown-sex) during the 2009-2012 period. The samples were dried after collection and stored until analysis. Steroid hormones were extracted and radioimmunoassays were validated for cortisol, corticosterone and progesterone, through accuracy, parallelism and high performance liquid chromatography. Pregnancy was assessed through progesterone concentrations (Mean±SE: 1363.10±389.12 ng/g), which were at least 100 times higher (X²=17.105, p ? 0.001) than mature non-pregnant (W=72, p ? 0.001; 13.25±3.32 ng/g) and lactating females (W=56, p ? 0.001; 22.98±5.40 ng/g). Pregnant females also showed higher concentrations of corticosterone (X²=17.10, p ? 0.001; 45.71±8.70 ng/g) when compared to mature (W=66, p=0.002; 17.40±1.90 ng/g) and lactating females (W=36, p=0.005; 16.84±2.84 ng/g). In contrast, cortisol did not show significant differences between three different reproductive states: pregnant (X²=3.631, p=0.3041; 178.71±15.60 ng/g), mature (233.00±37.30 ng/g) and lactating (114.43±30.00 ng/g). These results indicate an association between corticosterone and progesterone, similar to other mammals. The baseline concentrations of corticosterone (8.2–80 ng/g), for female blue whales in their current conditions, were successfully established and are now validated bioindicators to assess the effects of coastal development on blue whales in the área.

Palabras clave: blue whales; steroid hormones; reference point

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