Cutting off a shark's tail

So I’m watching Shark Week on Discovery, and I’m amazed at how big the tail fins of some of these sharks are. The Great White sharks they’re showing on this particular show, in particular, have some really gigantic tail fins. So it got me to thinking.

Clearly most, if not all, of the shark’s ability to propel itself through the water comes from the tail fin and the way it is swished back and forth through the water, right? So let’s say you had something capable of cutting through the shark’s skin and muscle and cartilage – some kind of Niven-esque molecular monofilament or shadow square wire or something. And you’re swimming across the ocean, minding your own business, when J. Random Shark comes up and attacks you. If you could somehow cut its tail off[sup]1[/sup], would the shark still be a threat to you? Or would it just … I don’t know … sink slowly away while looking at you with a sad and pathetic “What did you do that for?” look on its face?

Can they still outswim a human even with no tail, or are they pretty much helpless?

[sup]1[/sup]Note: I’m not saying this would be remotely possible in real life, mind you, nor am I advocating the cutting off of sharks’ tails as something you should do unless you are in mortal danger, since it seems pretty mean.

It’s done (illegally) commercially as the cartilage is worth quite a lot. Basically the fins in the tail and the one that sticks up is cut off with a knife, the shark is thrown back into the water. Unable to propel itself and needing to swim to maintain boyancy it sinks and dies.

Nearly all of a shark’s propulsive power comes from its tail. The front fins are used to steer, and have little ability to move the animal through the water. So yeah, a tailless shark would essentially be crippled.

Fishermen often will “tail” a shark if they catch one in their nets.

Adding to my above statement:

Actually, as the Wiki article says it is most commonly the pectoral fins (front side fins) and dorsal fins (the fin on the top of the body) that are cut off to make shark fin soup, rather than the tail. Without the pectoral fins to maintain “lift” when moving forward the shark will sink even if the tail is intact.

Expounding on this:
A defining characteristic of sharks is that they lack swim bladders, and instead rely on oil-filled livers for bouyancy. The liver is insufficient to provide positive- or even neutral-bouyancy, so (as kanicbird mentioned) they must constantly swim to keep from sinking.

What would happen if you removed the tail from a swim-bladder-equipped (and thus neutrally- or positively-bouyant) marine animal, I cannot say.

The tail fin in some sharks (e.g., the leopard shark) also helps provide lift, due to its heterocercal shape (the upper lobe is larger than the other, with the spine extending into the upper lobe[sup]*[/sup]). Thus, aside from a loss of propulsion, there is also a further loss of lift, which certainly complicates matters for a tailless shark. Not that the loss of lift would probably be even a primary concern for the beast at that point…

[sup]*[/sup] Extant fishes exhibit either homocercal (both fin lobes approx. equal in size) or heterocercal. The aquatic reptiles, ichthyosaurs, along with some early fish, exhibited what is known as a “reverse heterocercal” tail, in which the upper lobe remains larger, but the spinal column extends into the lower lobe. This is thought to have possibly provided the reverse effect: providing a downward force from the tail. Yet another tail variation is the “hypocercal” tail, in which the lower lobe is larger than the upper lobe.

I can’t believe you just made me feel sympathy for a shark.