Hmm, I'd think a gig rig would be a better survival accouterment. In fact, you could use nails you normally take along for survival (10p come to mind) cut the heads off and barb the tips using the file on your leatherman tool, and catch any number of morsels, both above and below. I like multi taskers, and cheap as well.

Now, if you are considering a harpoon as more of a defensive tool, well, I would think a knife would be a sufficient, albeit shorter range, substitute.

I've made gig rigs using stainless steel antenna rod (one of the fringe benefits of working in the radio business), that were scary sharp and held against a solid 40 lb resistance before tearing out. I would think that to be sufficient for most uses.