I'm not an expert but I would expect a fishing spear to have a smaller blade and a lot more and more effective barbs.

The main effective 'barbs' here are the two on the blade itself and look like they are designed to make it difficult for a victim to withdraw it. I would think it is from a long shafted throwing spear. I think a thrusting spear with short shaft, or not, would more likely be designed for easier withdrawal by the user, as he would want to keep it handy for the next use.

The wooden 'barbs' on the shaft are also interesting, and have some at right angles to the majority in line with the blade.

Not sure which South Pacific culture but it looks like something from the Solomon Islands, Bouganville, New Guinea region. These people are Melanesian are not Polynesian peoples. But I don't know.

Maybe a 'cannibal' spear.

I am willing to be corrected of course.