In mid-August I began my study on the foraging ecology of adult female fur seals. My research objectives are: (1) to determine the area-at-sea (or pelagic habitat) used by fur seals during the breeding season on a finer spatial scale than is currently available, and (2) to assess the degree of spatial overlap between fur seal foraging areas and commercial fishing activity in the eastern Bering Sea. 

In order to collect fine-scale foraging behavior data, I have attached harmless tracking devices to the backs of 15 lactating fur seals (that is, females who are nursing pups). I attach three types of instruments to each female: a dead-reckoner, an Argos satellite tag (which determines location at sea) and a VHF radio transmitter (which determines the animal’s presence or absence on land). 

The dead-reckoner is the heart and soul of my research project. It is a 12-channel data logger that records compass headings (in three-dimensions), the animal’s tilt angles and body position (belly up or belly down), as well as depth, temperature, light and speed. By using the heading and swim speed of these animals, I can recalculate their entire route at sea. These instruments were programmed to collect data every two seconds – making it possible to visualize their fine-scale movements underwater, and to distinguish between different behaviors such as sleeping, traveling and foraging. 

This map shows the satellite locations collected from the Argos satellite tags. It illustrates the vast area of the Bering
Sea covered by ten fur seals over a one-week period (one foraging trip).

It takes a certain finesse to capture these relatively small (compared to Steller sea lions), yet lively animals. We approach the target animals (females nursing pups) by a slow crawl on the rookery, and capture them with hoop nets. I was amazed by how close I could get to these curious animals just by keeping a low profile and by avoiding any sudden movements. 

Once an animal is caught in the net she is moved to the edge of the rookery, to minimize disturbance to the surrounding seals, and held in place using a neck and flipper restraint. We handle every animal with the utmost care and for the shortest possible time in order to minimize stress.

Before gluing instruments onto the fur I record the girth and weight of each animal, and if the opportunity arises I also collect a sample of scat (feces). Analyzing scat can tell me what the female ate in the last few days of her foraging trip. 

Females are tracked for either one or two foraging trips, with an average trip of about seven days at sea. When the females return to shore to nurse their pups, I have a 1-2 day window to retrieve my tracking instruments. We recapture the females on land and gently remove the devices. Data from the dead-reckoners are downloaded to a laptop computer and then reprogrammed for use on different animals.

Right now I am sitting, waiting, and hoping for my tagged females to return with valuable dive data! Keep an eye out for my next update – at which time the field component of my study should be complete.

Pamela

25 September 2006

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To see the previous and next installments from "Summer Field Season 2006
Field Notes from the Pribilof Islands", click below:

update #1 | update #2 | update #4 | update #5