Development and Application of a Mass‐estimation Method for Weddell Seals

Abstract

Traditional methods of acquiring mass data limit the ability to collect large samples from across populations of some pinnipeds, or to sample without great disturbance to the animals. In order to collect substantial samples of mass data from the Weddell seal (Leptonychotes weddellii) population in Erebus Bay, Antarctica, we developed the equipment and methods for estimating the mass of Weddell seals using digital photographs. Resulting regression models predict the mass of adult female seals to within ±13.8% of estimated mass, and ±25.9% of estimated mass for pups. We show the protocols developed are repeatable and efficient enough to be applied to a large number of animals in a relatively short period of time and may be useful for studies of other marine mammals. We caution that prediction intervals exist around mass estimates and must be accounted for when estimates are applied to biological questions. In a limited application of the method, differences in mass transfer between experienced and inexperienced maternal females and their pups were detected when prediction error variance around mass estimates was explicitly included. Similar mass-estimation methods may therefore be useful in consideration of biological questions requiring large samples of mass previously unattainable.

Morphometric relationships of franciscana dolphin, Pontoporia blainvillei (Cetacea), off Rio Grande do Sul coast, southern Brazil

“…It also is the most fundamental character of an individual in limiting its morphological aspects and structural dimensions (Lindstedt and Swain, 1988;Begon et al, 1990;Smith et al, 2001). Body mass, as a measurement of body size, is often hard to estimate in marine mammals because of the inherent difficulty in handling large wild animals as well as other logistical constraints (Haley et al, 1991;Bell et al, 1997;Ireland et al, 2006). This is especially true for cetaceans due to their limited availability often resulting in reduced sample sizes.…”

“…Steller sea lions, Eumetopias jubatus, Castellini and Calkins (1993); northern elephant seals, Mirounga angustirotris, Haley et al (1991); southern elephant seals, Mirounga leonina, Bell et al (1997); Weddell seals, Leptonychotes weddellii, Ireland et al (2006)) and cetaceans (fin and sei whales, Balaenoptera physalus and B. borealis, Lockyer and Waters (1986); bottlenose dolphins, Tursiops truncatus, Read et al (1993); harbour porpoises, Phocoena phocoena, Lockyer (1995); Read and Tolley (1997); killer whales, Orcinus orca, Clark et al (2000); estuarine dolphins, Sotalia guianensis, Rosas et al (2003)). In the case of the franciscana, mass estimation models can be found in the literature, but consist solely of simple regressions of mass versus total length (Kasuya and Brownell, 1979;Rosas, 2000;Rodriguez et al, 2002;Botta, 2005) and do not account for seasonal or developmental variations in body mass.…”

How to weigh an elephant seal with one finger: a simple three-dimensional photogrammetric application

“…The photographer is required to know the exact distance between the camera and the seal and scaling measure. More recently, Ireland et al (2006) and Waite et al (2007) made significant advances using new technology to estimate the masses of Weddell seals Leptonychotes weddellii and Steller sea lions Eumetopias jubatus, respectively. These methods have increased the accuracy of mass estimation for the particular species, but introduced (or maintained) various constraining field procedures, restricting their use in the field.…”

“…These methods have increased the accuracy of mass estimation for the particular species, but introduced (or maintained) various constraining field procedures, restricting their use in the field. The Ireland et al (2006) method requires customised photographic equipment that is bulky and impractical in situations where the only method of traversing large distances between study subjects is by walking. Proffitt et al (2008) successfully improved the photogrammetric mass estimation and confidence of the Ireland et al (2006) procedure, by post hoc body form analysis using elliptical Fourier decomposition.…”

Prediction Equation of Body Weight of Amazonian Manatee (Trichechus inunguis) Calves in Captivity Using Biometry

“…Body length/weight ratio is widely used to monitor the growth and condition factor of several animal species [20][21][22]. Furthermore, the use of other biometrics such as curvilinear length and circumference of the body are also important tools in the estimation of body weight in predictive models, such as the seals [23][24][25].…”