The easiest way to spot an octopus is to see it swimming (top photo). They’re not large creatures but they’re quite distinctive when they swim.
If they’re not swimming, one thing to look for is certain fish, such as goatfishes and jacks, just hanging around in a spot for no apparent reason. When these fish are hunting alone, they tend to be more active in probing the rocks and trying to disturb prey. But when they’re hunting with an octopus, they seem more content to let the octopus do the work and snapping up whatever emerges. I’ve found that goatfishes are particularly helpful as an octopus indicator.
A while back, there were videos online of an octopus apparently punching a goatfish. I wasn’t surprised by this. The octopuses I’ve seen don’t seem best pleased by the presence of goatfishes. Part of this might be down to feeling that the goatfishes are not pulling their weight in the hunt. But another factor might be that if goatfishes give away their position, for the octopus that can be fatal.
Octopus is a popular food in Hawaii and has long been so. If I’ve learned to look for goatfish as an indicator of their presence, then no doubt spear fishermen have too. These days, if I see an octopus when anyone’s spear fishing nearby, I don’t do anything to draw attention to it.