I've done Gyotaku before.
Also play the ukulele........
I don't know what the start up costs are for Etsy and going from hobby to sales but your already invested in the time and materials as a hobby. Quality paper, quality inks/paints, attention to detail. Check online for sales of other Gyotaku works. How fast to they seem to sell. You might be limited by the kinds of fish you kind find in your local, or the fish in your local might not be regular Gyotaku subjects so something newer to genre.
Spouses cousin started photographing birds. He sells the photos as a side business. Some awards won. By all accounts he's doing well and making money at it.
I started leather working some years back. Made some investments in tools, time, learning. Sold some items. I found that going to the next level and actually making money at it was going to take a lot larger investment of time, tools, learning. I was able to pay for my materials from what I sold. I still make things for myself and sell something now and then but it's all pretty basic utilitarian useful to me kind of items--sheaths, pouches, arm guards, quivers , fixing older leather goods, plans for a quiver in the works.....
I say do some investigative work and if it looks even a little promising give it a shot. Maybe you'll earn enough for that hunting trip, or family vacation, or new bow, or new ukulele, or the kids college fund.......