Is it possible the "shark cafe" is an area where the earth's magnetic field is stronger?
Posted March 8th, 2008Question from Aaron McQueen:
I listened to Quirks and Quarks on CBC radio today where Dr. Barbara Block spoke about the "shark cafe" as a point about halfway between California and Hawaii where great white sharks congregate. She said they dive down very deep and it's not known for sure what attracts them to this isolated spot. I remember hearing on another episode of Quirks and Quarks about a study that showed that crabs can navigate their way to their "home" using receptors that are sensitive to the earth's magnetic field. My question is: has it been considered or investigated geophysically if there's an anomoly in the Earth's magnetic field at the "shark cafe"? (ie. if there's a geophysical structure there that creates an increase in the earth's magnetic field)? Thanks, Aaron McQueen Fort McMurray, Alberta Canada
Answered by KevinWengHi Aaron, I looked at this question as part of my PhD thesis. In the image below, you see a magnetic anomaly map of the eastern North Pacific. While there are patterns at the shark cafe, there is nothing that is obviously different from other regions. Having said that, many animals can navigate across the planet to and from specific places -- they need both compass sense and a map to do this. It is possible that patterns in the Earth's magnetic field can form such a map.
The following text is from the supplement to our marine biology paper:
"Variations in magnetic intensity in the region of the migration corridor exceed variations measured by Klimley (1993), which were correlated with directed movements of hammerhead sharks (Sphyrna lewini), suggesting that magnetic navigation is possible. Klimley (1993) hypothesized that vertical movements of hammerhead sharks would allow them to sense altitudinal variations in the earth’s magnetic field, and migrating sharks did undertake such movements, though not in a regular pattern. Magnetic inclination and declination vary gradually across the region (Peddie 1993), and could provide direction but no obvious landmarks. The track of shark 18-F, for which geolocations were obtained almost daily during the migration, and magnetic anomaly fields of the region, show little correlation; however, it remains possible that the heterogeneous magnetic anomaly patterns could be used as a map (a). The winter 95% density contour, showing the migration corridor for all sharks, was in agreement with the track of shark 18-F, forming an envelope around it. The pattern of movement evident from both 18-F’s track and the 95% density contour show oblique movement across the magnetic anomaly lineations created by the Cenozoic spreading center of the east Pacific, which are oriented approximately north-south in the eastern portion of the migration corridor. Further west the magnetic anomaly data become increasingly sparse, but lineations appear to be oriented northwest-southeast, again meaning that shark movements are oblique. The final movements of shark 18-F to the Main Hawaiian Islands were beyond the extent of the magnetic anomaly data. The offshore focal area lies over the Molokai Fracture Zone, and has a complex pattern of magnetic lineations in different orientations as well as zones without lineations."
Weng K, Boustany A, Pyle P, Anderson S, Brown A, Block B (2007) Migration and Habitat of White Sharks (Carcharodon carcharias) in the Eastern Pacific Ocean. Marine Biology 152: 877-894
