I never thought about this much until the topic was raised on another forum. I grew up in an area where frog gigging was known, but gigging for herring was common. a hooked spear (technically a gaff, I believe, but called a "gig") is used to pull the herring out of the runs by their gills. At that time at least, nets were illegal during the run and spawning herring won't take a hook (or so I'm told)) I always assumed that it was a "gig" because the musician was using his performance to "hook" in money (tips). Odd that none of the frog/fish "gig" definitions seem to have made the books... at least from the above examples, though I can see one definition that enters the realm...