Weddell Seal Foraging Behavior
What kind of data are we obtaining through the VDAP deployments?
The VDAP performs two primary functions: video images are recorded from the head mounted imaging sensor. This will essentially give us a glimpse of what the seal can see when it is diving. In fact, we can see more than the seal can: the camera is sensitive to infrared light. In order to be able to record images at the great depths these seals dive to, where it is nearly completely dark, we illuminate the water in front of the seal with infrared LEDs. The seals cannot see this kind of light, and so their behavior is not altered, but we can see what is in their immediate environment.
In addition to recording these video images, the VDAP records an extensive data set during deployments. Primary data consists of depth (via a pressure sensor), swimspeed (via a velocity sensor), and magnetic heading or compass direction (via a digital flux-gate sensor). Using these data, we can compute the three-dimensional dive path the animal has followed using dead-reckoning calculations. The measured path (relative path, also called "course steered" in nautical terms) has to be corrected for current and drift to obtain the absolute path (also called "course made good" in nautical terms).
In the image below, you can see a 3-D dive path reconstructed by Lee Fuiman from data collected during the previous project. Shown is one complete dive originating and ending at the same breathing hole (our experimental hole). Swimspeeds vary and are color coded in this graph. The numbers along the track indicate elapsed time in minutes, during the dive. The dive is just over 10 minutes long. At the location indicated by the star, the seal encountered and tried to catch an antarctic cod (Dissostychus mawsoni), but failed.
The area in the blue box is expanded in the graph below:
The above graphics were prepared by Lee Fuiman.
In addition to this data, the VDAP also records the stroking pattern of the seal, using a tail-mounted accelerometer sensor. The sensor is glued to the dorsal fur of the seal at the base of the tail flippers:
This sensor detects the lateral motion of the the hind flippers, which are the main swimming flippers of phocid seals. In the figure below, you can observe the stroking pattern during a short segment of a weddell seal dive. Tail motion is represented by the red line, swimspeed by the blue line, during a 2.5 minute segment of a 10-minute dive. The seal performs what is called burst-and-glide type swimming. Several strokes of the tail flippers are recognized by the alternating accelerometer data. Simultaneously, the seal accelerates during these bursts of stroking, and then decelerates during the intermittent glide periods, through the effects of hydrodynamic drag.
The next image shows a slightly expanded section of the preceeding figure:
The following figure shows a complete dive, and how stoke patterns vary even during a dive. This seal exhibited three distinct stroking phases during the dive: burst and glide during the first descent phase until a depth of approximately 125 meters. From that depth until the maximum depth of close to 600m, the seal glided without stroking. This was possible since due to lung collapse at just over 100m the seal became negatively buoyant. During the ascent, the seal strokes performed constant stroking. This triple stroking mode during deep dives is most likely a cost-saving behavior, enabling seals to cover a given horizontal distance at a lesser cost than swimming the same distance at a shallow depth using constant or burst-and-glide stroking.
More on our findings soon!
