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¹ Instituto Politécnico Nacional, Centro Interdisciplinario de Ciencias Marinas

Abstract Introduction This is the first worldwide systematic and quantitative study to count and identify helminth parasites from 100 blue and 44 fin whale fecal samples collected in the Gulf of California during winter (1993–2014). Results Blue and fin whale feces had similar prevalence of adult acanthocephalans (Bolbosoma sp.) in feces (18.2% and 14.6%, respectively), but blue whales had significantly higher helminth egg prevalence in feces (100%) and mean intensity (443 ± 318 eggs/g) compared to fin whales (61%, 252 ± 327 eggs/g). Diphyllobothrium sp. eggs were identified in blue whale feces and Diphyllobothridae, Ogmogaster sp. and Crassicauda sp. eggs were identified in fin whale feces. We tested the hypothesis that egg intensity in blue whale’s feces varies as a function of age class, reproductive status, sex, preserva- tion and sampling years using a Generalized Linear Model. This model explained 61% of the variance in the helminth egg intensity, but it was not significant. Eighteen blue whale individuals were resampled over time without significant difference between consecutive samples. Conclusions Thus, all individual blue whales that migrate to the Gulf of California during winter are permanently parasitized with helminths, while the resident fin whales showed lower prevalence and intensity. This helminth load difference is likely due to their different diets duringsummer–fall, when blue whales feed on other krill species in the California Current System and fin whales shift to school fish prey types in the Gulf of California.

Palabras clave: Balaenoptera musculus; Balaenoptera physalus; Gulf of California

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