I am trying to find a weapon that I could use to fish with.  The only catch is that it has to plausibly fit in a 10 foot kayak.  I've tried just throwing a sharpened stick, but it didn't work as well as I'd hoped.  I've thought about trying the atlatl, or a blowgun.  Any ideas?

I have tried bowfishing from a kayak, you need a short bow like the Bear Supermag 48 or the Kodiak Magnum.  With a 60 inch recurve, the bottom limb goes in the water on close shots.  Instead of an atlatl with a flexable dart, the traditional system is a non flexable throwing board with a relativly short stiff harpon.  It works well for short downward shots, not so well for far shots.  Oh, yes, very important, have a strong cord securely attached to your dart, you can sink your dart really deep in a log on the bottom of the river, you want to be able to pull the log up so you can dig the point out with your knife.  And when you are pulling, never forget that kayaks tip over easily.  Bill

If you are just looking for a weapon to fish with, try a hand grenade. You will get plenty of fish.

Hinge gun ... think Hawaiian sling with a trigger
http://rocknfish.com/HingeGun.html
 
For one handed operation after being cocked.

Quote from Galvanicfish on Aug 17^th, 2011, 11:38pm:

mount a laser pointer in/on the handle and just put the red dot on the fish. saw this done by a bow fisherman.
 
 
Here is another view of a hinge gun http://www.reocities.com/Pipeline/curb/5893/images/hingegun.jpg

Kind of like a sling. There are no dimensions; just make something that fits.
 
I'd probably start with a block of wood 8 or 9 inches long.
Maximum pull for propelling rubber .. probably in the neighbor hood of 2 to 2.5 feet
So how long to cut the rubber? : http://slingshotforum.com/topic/255-band-or-tube-length-how-to-calculate/page__h l__%2Bcalculate+%2Bband+%2Blength__fromsearch__1
 
Basically stretch and try till it feels right then cut a couple of bands/tubes

You need to check out Paleoarts fish spear.

it would have to be attached to your boat, or you, because if not, the fish wil get hit and swom away, with your dart, and worse,an agonizing death.