So: blowgun fishing is, apparently, a thing. This instructable shows a process by which the most commonly used darts can be modified so that they are much less likely to fail when retrieving a fish that has been darted.
Step 1: Get Your Materials Sorted Out
Presumably, if you're interested in this instructable, you've already got the blowgun itself, so I haven't included that in the materials list... This is based on the darts for use in the Cold Steel "Big Bore" (= .625 cal) blowgun, namely, the razor darts. You will also need some very small split-rings (I used Size 1), some very small barrel swivels (I used Size 8), Gorilla Glue (although, you could alternatively use epoxy), a 3/32" drill bit (and, naturally, a drill- not pictured), a hammer (and anvil or vise, not shown here), and some fishing line. As for this lattermost item, you would as likely as not have your line spooled onto a reel, but that is not, strictly speaking, necessary. You might do without a reel if you attached the free end of the line to your blowgun, but it's probably a better idea to devote an old/cheap reel (not shown) to this project. This is especially true if you use braided line (as shown), because you don't want to be retrieving that stuff by hand! It'll cut through your fingers like a hot knife through butter.
Step 2: Take the Dart Apart
Yeah, just like that- carefully take the cone off the end of the dart. And do be careful, because these cones are not made of the stoutest stuff. Set the main shaft of the dart aside and get to work on that cone...
Step 3: Drill Through the Cone
With your 3/32" bit, drill all the way through the hole where the main shaft was before you took the thing apart. Easy peasy.
Step 4: Split-ring, Meet Barrel Swivel...
Attach a barrel swivel to a split-ring. The tiny size of these makes this just about the hardest part of the whole procedure! I have no good tips on making it easier to attach the two- apart from patience.
Step 5: Put'em All Together...
Now, put the split ring on the shaft of the dart, and slide the now-fully-perforated cone onto and up the length of the dart shaft, leaving a couple of inches of the rear end of the shaft exposed. Be sure you've got both components in place before you go on to the next step!
Step 6: Hammer Time!
Now take that hammer and flatten out (or rather, flare out) the rear end of the dart shaft. A few blows ought to do it- no need to beat the devil out of it.
Step 7: Glue It
And now you'll slide the cone down to the rear end of the shaft and apply a little dab of glue to give it a little extra bit of robustness. I like to use Gorilla glue, as it adheres nicely to both plastic and metal, and it's waterproof once cured.
Step 8: Attach Your Fishing Line
Once the glue is dry, you can attach your fishing line to the 'empty' ring of the barrel swivel. I won't give any advice on what kind of knot to use- I'll let you make that decision based on what kind of line you're using.
Now, you're good to go! Be mindful that you have to feed the dart into the 'muzzle' end of the blowgun with the fishing line trailing behind (i.e., trailing out the muzzle end). I know, that might sound obvious, but it never hurts to overcommunicate...