ROBERT A. GARROTT

Professor        Fish and Wildlife Management

Ecology Department--Montana State University

Office Phone:  (406) 994-2270        FAX:  (406) 994-3190        e-mail:  rgarrott@montana.edu

 

 

 

 

 

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Mass Dynamics of Weddell Seals, Leptonychotes weddellii, In Eastern McMurdo Sound, Antarctica

Kelly M. Proffitt and Robert Garrott

 

For the past thirty years, the population of Weddell seals in Erebus Bay has been studied using mark-recapture techniques. Annual variations in population growth rate and recruitment have been observed, but causes of the observed variation are not understood. Inter-annual variations in seal body mass may reflect variations in resource availability, and can be used as a surrogate measure of resource availability. The purpose of this project is to investigate the body mass dynamics of Weddell seals and to relate seal mass dynamics to environmental conditions.

Figure 1

Figure 1. At parturition, female seals weigh approximately 400-500kg and pups weigh 28-35kg.

 

Objective 1: To investigate inter-annual variation in Weddell seal parturition mass and relate parturition mass to maternal characteristics and environmental variability

Over the next three field seasons, the parturition mass of a large sample of adult females (200/year) will be estimated using photogrammic techniques developed by Ireland and Garrott (2004). Breeding colonies will be surveyed every three days, and mass will be estimated 24 to 72 hours after parturition. A digital overhead and sideview photograph of the animal will be taken and scaled, the photograph will be measured using specialized computer software, and morphological measurements will be input to a regression equation predicting animal mass. Inter-annual variation in female parturition mass will be related to maternal characteristics such as age and breeding experience and to environmental characteristics such as sea ice extent and sea ice area. To assess variation in Weddell seal parturition masses, I will compare a priori hypotheses representing the potential effects of the maternal and environmental characteristics.

 

Objective 2: To investigate environmental influences and maternal effects on Weddell seal pup parturition mass, growth rate, and weaning mass

Figure 2

Figures 2 and 3. Body mass is estimated based in a series of morphometric measurements calculated from scaled digital photographs (see Darren Ireland’s project for details). Two photographs of the animal are taken. One side view photograph and one overhead photograph are taken with specially designed photographic equipment.

Figure 3

Pup parturition mass (n=200/yr) will be estimated by rolling the animal into a net, lifting the animal, and measuring mass using a spring scale. To assess inter-annual variation in Weddell seal pup parturition masses, I will compare a priori hypotheses representing the potential effects of maternal characteristics such as parturition mass, age, and breeding experience, and environmental characteristics such as sea ice extent and sea ice area.

 

Approximately 35 days after parturition, pup weaning mass (n=200/yr) will be estimated using photographic techniques. A scaled, digital overhead photograph of the animal will be taken, the surface area of the animal will be measured using specialized computer software, and measurements will be input to a regression equation predicting animal mass. Unique regression equations will be utilized for adult and pup seals. Pup weaning mass will be related to maternal and environmental characteristics.

 

Ten pups at Hutton Cliffs, Turks Head, Big Razorback and North Base colonies will be randomly identified each year and their mass will be estimated every five days throughout the lactation period. Pup growth rate will be calculated as the amount of mass gained divided by the number of days. Inter-annual variation in pup growth rate will be related to maternal and environmental characteristics.

 

Objective 3: To investigate maternal care and lactation strategies of Weddell seals

Supplemental feeding may be a maternal investment strategy among Weddell seals, particularly in years when resources are insufficient for females to store energy to last throughout the full lactation period. Ten mother- pup pairs at Hutton Cliffs, Turks Head, Big Razorback and North Base will be randomly identified and their mass will be estimated every five days throughout the lactation period. Conversion indexes will be calculated for early, mid and late lactation, and over the entire lactation period. In collaboration with visiting scientists, a sub-sample of these females will be equipped with time-depth recorders and diving behavior will be monitored. Mother-pup pairs’ conversion index will be related to maternal characteristics such as age, parturition mass, diving behavior, and breeding experience, and to environmental characteristics such as sea ice. Conversion indices combined with diving records will provide insight into the lactation strategies employed by Weddell seals, and will be used to infer if animals are feeding during the lactation period.

Sea ice edge

Figure 4. The extent, concentration, and persistence of sea ice vary annually in the Ross Sea. Sea ice conditions may influence marine productivity and be reflected by seal body mass.

 

Funding:  National Science Foundation