Geographic variation in Risso's dolphin echolocation click spectra.

Area of Science:

• Marine Biology
• Bioacoustics
• Cetacean Ecology

Background:

• Passive acoustic monitoring is crucial for studying cetacean ecology.
• Risso's dolphins (Grampus griseus) possess unique echolocation click spectra off southern California.
• The acoustic distinctiveness of Risso's dolphin populations across wider geographic ranges remains largely uncharacterized.

Purpose of the Study:

• To determine if Risso's dolphin echolocation clicks exhibit species-specific spectral characteristics across diverse geographic locations.
• To investigate whether Risso's dolphin populations from different ocean basins are acoustically distinguishable.
• To characterize the spectral structure of Risso's dolphin echolocation clicks from the North Atlantic, Gulf of Mexico, North Pacific, and Eastern Tropical Pacific.

Main Methods:

• Analysis of spectral structure of Risso's dolphin echolocation clicks.
• Recording clicks across wide-ranging geographic regions including U.S. waters (North Atlantic, Gulf of Mexico, North Pacific) and international waters (Eastern Tropical Pacific).
• Statistical evaluation of spectral peak frequencies and notches to identify variations.

Main Results:

• All recorded Risso's dolphin clicks displayed the characteristic spectral peak and notch pattern, confirming its utility for species discrimination.
• Significant geographic variations in click spectral peak frequencies were observed, with higher frequencies noted off Hawaii and the southeastern U.S. compared to California and the Gulf of Mexico, respectively.
• A notable latitudinal cline in click frequencies was identified within the North Atlantic population.

Conclusions:

• The spectral peak and notch pattern in echolocation clicks serves as a reliable indicator for Risso's dolphin species identification.
• Geographic location is a significant factor influencing the acoustic properties of Risso's dolphin echolocation clicks, suggesting potential population-level acoustic differentiation.
• Further research is warranted to explore the drivers of acoustic variation within and among Risso's dolphin populations.