Estimating Body Mass and Condition of Leopard Seals by Allometrics

Abstract

Leopard seals (Hydrurga leptonyx) are formidable marine predators and require sedation before scientific examination. Mass‐specific drug dosage for leopard seals has usually been determined from generic allometric equations or visual estimates. However, the leopard seal is a slender phocid and generic equations are likely to return inaccurate mass estimates, which may lead to fatal overdoses of drugs. We used published and unpublished morphometric data to construct allometric models for estimating leopard seal body mass. The model using volume (Vol), which combined measures of snout‐tail length (STL) and the square of girth (G2), provided our best estimate of mass (r2 = 0.97). The model using STL alone was sample‐site specific (each r2 ≥ 0.85), highlighting G as an important measure to obtain where possible. The confidence and prediction intervals associated with each model broadened with increased seal size and decreased sample size, suggesting the use of extra caution when estimating drug dosage for larger seals to avoid over‐ or under‐dosing. We also developed a seal body condition index that can assist wildlife management when deciding if rehabilitation of vagrant seals is warranted. Body condition may also affect the induction, duration and recovery times of anaesthetized leopard seals.

An accurate and adaptable photogrammetric approach for estimating the mass and body condition of pinnipeds using an unmanned aerial system

“…The straight line measurement accuracy in this study (~2%) was as, or more, accurate than previous pinniped photogrammetry reports. Further, leopard seals are large pinnipeds [ 76 ], which has been an historically challenging size class for mass-estimation models, and their streamlined body shape [ 31 ] contains elements of both phocid (e.g., spindle-shaped body) and otariid (e.g., long neck, developed fore-flippers) dimensions. Therefore, a successful photogrammetric technique for leopard seals may be applicable to other pinnipeds.…”

“…Previously published leopard seal mass-estimation models [ 28 , 31 ] were also run using ground-based leopard seal measurements to evaluate their performance. The level of significance used for all tests was P < 0.05.…”

“…If it is impractical to weigh an animal due to its size or position, reliable mass estimates can be derived using multiple girth and blubber thickness measurements [ 25 , 26 ]. Alternatively, mass can be estimated using only length and girth [ 27 – 31 ], which reduces capture times. However, the accuracy of such models varies widely, particularly for large (>400 kg) pinnipeds [ 28 , 30 , 31 ].…”

“…Alternatively, mass can be estimated using only length and girth [ 27 – 31 ], which reduces capture times. However, the accuracy of such models varies widely, particularly for large (>400 kg) pinnipeds [ 28 , 30 , 31 ]. Moreover, manual techniques for collecting morphological data necessarily involve physical captures, which are time- and resource-intensive disturbance events that carry substantial risk for both animals and researchers.…”

“…Experienced field personnel can often visually estimate mass within pharmaceutical safety margins. However, for problematic (e.g., [ 31 ]) or endangered species a more accurate, non-invasive approach may be needed.…”