Photogrammetric Estimates of Size and Mass in Hawaiian Monk Seals (<I>Monachus schauinslandi</I>)

Abstract

A non-invasive photographic technique was devel- oped 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/pos- terior area, and perimeter were measured from pho- tographs to establish predictive relationships with body mass using regression analyses. Photographs were subjectively ranked 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 photogram- metric (surface area or perimeter) values compared to those obtained in a standardized position. Although the most reliable models (based on information criterion analysis and 95% CIs) were based on directly measured morphological vari- ables, models using only photogrammetric vari- ables also yielded practical and reliable models with 95% CI, ranging from ± 4.95 to 9.12 kg and R 2 values from 0.93 to 0.77. This finding indicates that the use of photogrammetry alone to assess body condition is suitable to estimate body mass in 10- to 120-kg weaned pups.