Visualizing the underwater behavior of humpback whales

Abstract

U ntil recently, scientists knew little about what whales did underwater. Studying the underwater behavior of marine mammals is difficult; light doesn't travel far through water, and divers can't keep within visual range of an animal capable of sustained speeds of 5 knots. Scientists can use sonar technologies to image marine mammals underwater, but sonar records provide only occasional brief glimpses of whales underwater.A new collaboration between visualization experts, engineers, and marine biologists has changed this. For the first time, we can see and study the foraging behavior of humpback whales. This is important not just for purely scientific reasons. Whales are dying because of ship collisions and entanglements with fishing gear in ever-increasing numbers. Understanding their behavior could lead to changes in shipping regulations or in the nature and deployment of fishing apparati.Our study's primary objective was furthering the science of marine mammal ethology. We also had a second objective: field testing GeoZui4D, an innovative testbench for investigate effective ways of navigating through time-varying geospatial data. The study involved two expeditions, one in 2004 and another in 2005, in which we tagged whales and recorded their movements. Recording whale behaviorThe digital recording acoustic tag, 1 shown in Figure 1, is the key technology allowing us to see underwater whale behavior. DTAG is a recording device containing several instruments. Three axis accelerometers provide information about the gravity vector's direction, three axis magnetometers measure the direction of the earth's magnetic field, a pressure sensor provides depth information, and a hydrophone continuously records sound. Except for sound, which is recorded at 64 kHz, all instruments record at 50 Hz. DTAG merges and compresses data records to be stored on 3 gigabytes of flash memory. DTAG is about 10 centimeters long, excluding the antenna, and is attached to a whale's back using suction cups. After some preset interval, typically 10 to 22 hours, DTAG automatically releases suction and floats to the surface. Once it's on the surface, a ship can locate it by its radio beacon. The ship picks up the DTAG, and scientists download the data.To place the tag, scientists attach it loosely to the tip of a 45-foot carbon fiber pole, mounted (as Figure 1 shows) with a gimbal on a rigid hull inflatable boat's (RHIB) bow. Placing the tag on a whale isn't easy given that a whale usually only surfaces for a few seconds at a time. The operation requires careful timing and coordination between the boat driver and pole handler.Scientists analyze the DTAG data to produce a pseudotrack. This is only an approximate estimate of the animal's path because DTAG doesn't give information about speed through the water. Therefore, scientists must derive the pseudotrack using a form of dead reckoning. The cross-product of the magnetometer vector with the gravity vector (from the accelerometers) provides heading information. Together, heading and gravity pr...

Diel changes in humpback whale Megaptera novaeangliae feeding behavior in response to sand lance Ammodytes spp. behavior and distribution

“…We have identified and quantified consistently patterned feeding behaviors, and related these to a suite of environmental variables and prey metrics in this and companion studies (e.g. Ware et al 2006, Stimpert et al 2007, Hazen et al 2009, this Theme Section) to test specific hypotheses regarding feeding mechanics and prey behavior. We have used proven fisheries acoustic methods in a dynamic way to link the behavior and movement patterns of prey with those of their predators in the ecological context of predatorprey interaction.…”

“…Nowacek et al 2004, Johnson & Tyack 2003, Ware et al 2006. This technology has the ability to measure the underwater orientation of whales and track changes in their behavior over time, which, in some cases, can be corroborated with observations of whales at the surface.…”

“…We used Trackplot (Ware et al 2006) to visualize and analyze the movement patterns of whales from DTAG sensors (Fig. 1).…”

“…Trackplot is a visualization tool that represents the continuous movement and orientation of the whale as a ribbon surface. Behaviors such as rolling are indicated by twists around the along-track ribbon (Ware et al 2006). The ribbon uses a sampling rate of 1 Hz and includes not only pitch, roll, and heading, but also depth and time.…”

“…Ware et al (2006) use this information to code the ribbon plots in Trackplot to highlight any off axis roll of > 40°. Thus, we similarly processed the dive records, recording the time, duration, and depth of apparent bottom feeding behaviors as indicated by repeated rolling behavior at or near the bottom (Fig.…”