Photogrammetric Estimates of Size and Mass in Hawaiian Monk Seals (Monachus schauinslandi)

Abstract: A non-invasive photographic technique was developed to estimate the body mass of Hawaiian monk seals (Monachus schauinslandi). Newly weaned monk seal pups (n = 31) were photographed and measured at Kure Atoll in the northwestern Hawaiian Islands. Length, side area, anterior/posterior area, and perimeter were measured from photographs to establish predictive relationships with body mass using regression analyses. Photographs were ranked subjectively in terms of quality, based on the degree to which the seal’s body position deviated from the ideal position used to obtain standardized photographs. Results indicated that deviations in body positioning (e.g., a seal rolled on its side) did not significantly alter photogrammetric (surface area or perimeter) values compared to those obtained in a standard position. Although the most reliable models (based on information criterion analysis and 95% CIs) were based on directly measured morphological variables, models using only photogrammetric variables also yielded practical and reliable models with 95% CI, ranging from ± 4.95 to 9.12 kg and R2 values from 0.93 to 0.77. This finding indicated that the use of photogrammetry alone to assess body condition is suitable to estimate body mass in 10- to 120-kg weaned Hawaiian monk seal pups.

Key Words: Hawaiian monk seals, Monachus schauinslandi, photogrammetry, morphometric, mass, surface area, northwest Hawaiian Islands

Document Type: Research article

DOI: 10.1578/AM.32.1.2006.31

Page Numbers: 31-40

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