August 2007    Download the Entire Issue (PDF)

Vol. 22, No. 8     What's this?

Shark Sign Language

from the August, 2007 issue of Undercurrent  

If you’re going to jump into the sea with sharks, it’s good to have some understanding of how they behave. A study of possible threat displays in 23 shark species could make it easier to read their signs. Aidan Martin, a recently deceased shark expert at the University of British Columbia, described 29 different components to threat displays by sharks in his study published last spring in Marine and Freshwater Behavior and Physiology. It’s the first detailed record of behavior for two-thirds of all species studied.

The most common display – and one that every diver should know about – is downward pointing of the pectoral fin, seen in all of the 23 species Martin studied. The most obvious display is the “hunch,” signaling stress. “In great whites, the hunch lasts only three or four seconds compared with a blatant 30- to 40-second signal of the gray reef shark,” says Martin’s widow, fish biologist Anne Martin.

Most of the signals reported by Martin were recorded after he or another diver had rapidly approached a shark without leaving it an escape route, or had pursued it. (Observers left without waiting to see if the shark would carry out its threat.) Fewer displays were recorded when sharks were feeding, suggesting signals are more to do with self-protection rather than defending its resources, but the difficulty of observing sharks in the wild made it difficult for Martin to draw a conclusion. Jurg Brunnschweiler, a shark ecologist from the University of Zurich in Switzerland, is preparing a PADI course that will cover the threat displays reported by Martin. They include:
The hunch: Nose up, pectoral fins down, back hunched; signals a high degree of stress and common to many species, including great whites.
Pectoral fins down: Nearly universal. Body shiver: Shark appears to shudder and stall in the water; only found in silvertips. Jaw gaping: Like a yawn, displaying teeth; seen in many species including tiger, great white and bull sharks.
Flank displaying: Turning sideways to target, slowing swimming; seen in many species, including great white and tiger sharks.
Tail popping: Shotgun-like sounds from exaggerated tail beats; a neutral display in sandtiger sharks.
Laterally exaggerated swimming: Eel-like swimming, folding almost in half; seen in a few species, including Galapagos sharks.
Give way: Swims straight at target but turns away at last moment; typical of great whites.
Gill-pouch bellowing: Seen in Galapagos sharks and sometimes great whites.

Excerpted from the article “This Shark Is Telling You Something” in the magazine New Scientist.