Accuracy, precision, and error in age estimation of Florida manatees using growth layer groups in earbones

Area of Science:

• Marine Mammal Biology
• Conservation Science
• Age Estimation Techniques

Background:

• Florida manatees (Trichechus manatus latirostris) age estimation relies on counting annual growth layer groups (GLGs) in earbones.
• A large archive of over 8,700 manatee earbones exists, collected between 1989 and 2017.
• Changes in processing protocols and personnel over time necessitate a comprehensive evaluation of aging techniques.

Purpose of the Study:

• To comprehensively evaluate and establish new standards for estimating Florida manatee ages from earbones.
• To quantify the accuracy, precision, and error associated with current age estimation methods.
• To understand how age estimation uncertainty affects the utility of manatee population models.

Main Methods:

• Developed new standards for age estimation involving two independent readers.
• Measured within- and between-reader precision using coefficients of variation (CV).
• Quantified accuracy and error using earbones from manatees with approximately known ages (KAC) and minimum known ages (MKA).

Main Results:

• Within-reader precision (CV: 2.4-8.5%) was higher than between-reader precision (CV: 13.1-13.3%).
• Age estimates were within the true age range for 63.1% of KAC and at least the sighting duration for 75.0% of MKA.
• Accuracy decreased with age, particularly for manatees over 15 years, with ages often underestimated due to bone resorption.

Conclusions:

• Established new standards for manatee age estimation, improving precision but highlighting age-related accuracy limitations.
• Bone resorption in older manatees leads to underestimation of age, impacting population model reliability.
• Routine standardization and review of age estimation protocols are crucial for consistent and reliable manatee demographic data.