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March, 24th 2017
Methods for measuring prey ingestion
A simple method that tells us when a sea lion is trying to catch fish is head-mounted accelerometry. Most marine mammals are raptorial feeders, meaning that they capture prey with their jaws by quickly lunging forward with their head, neck or entire body and seizing the prey with their jaws or sucking it into their mouths. In this sequence of two photos you can see that as this sea lion is about to seize the fish, she strikes out with her head.
To work out a method of recording the head-striking of Steller sea lions, we initially thought we would have to compare the acceleration of the body to the head, “differential acceleration”, to find a signal that was unique to feeding. To do this we built and attached accelerometry dataloggers to the head and the back, as illustrated in this photo of Sugar, one of our Alaska SeaLife Center Steller sea lions, chasing a rainbow trout in the Steller sea lion habitat.
As it turns out, the acceleration signal of the head alone is sufficiently unique during prey capture attempts by Steller sea lions that we can rely on just the head accelerometer, as seen in this slow-motion video.
The accelerometer technique is cheap and easy to apply to many sea lions, but it only tells us when sea lions are chasing fish, not when they successfully catch and swallow them. For that, we use stomach temperature telemetry, which involves a small electronic “pill” that transmits temperature from inside the stomach to a receiver in a backpack that is glued to the fur of the sea lion’s back (which falls off every fall when they molt). The backpack device includes a satellite transmitter to send us the data back to our office. Stomach temperature telemetry works because Steller sea lions are endothermic, or “warm-blooded”, but the fish they eat are ectothermic, or “cold-blooded”, i.e. fish are the same temperature as the cold seawater here in Alaska, so every ingested fish makes the stomach temperature decrease briefly.
The magnitude of the temperature drop and the time it takes for stomach temperature to return to normal is related to the size of the fish, and we work out that relationship by feeding our Alaska SeaLife Center Steller sea lions known amounts of fish while recording the stomach temperature changes. This gives us a rough idea of how much sea lions eat, but it doesn’t tell us which species of fish were eaten, nor the exact size of individual fish. To do that we are working out which methods of head-mounted video cameras work best to give us an exact picture of feeding behavior. In this video clip, you can see side by side synchronized video images. On the left side is the view from the head-mounted video camera, and on the right side is the view from outside the habitat.
That was an early camera system and as you can see it was not very hydrodynamic. By measuring how fast the sea lions slowed down during long underwater glides, we determined that this blocky camera increased the drag on the sea lion by about 4%. To decrease that potentially adverse effect we’ve been investigating more streamlined models, such as the VDAP system developed by our collaborators Bill Hagey and Randy Davis.