Does not seem so in the latest 0.15 -- my fast inserters are standing still, and don't care whether there is any fish below them (even when inserting to a chest, thus an implicit filter shouldn't be a problem)

No, they can't :-P

It's a secret/easter-egg homage to davemcw (https://forums.factorio.com/memberlist.php?mode=viewprofile&u=1868) who wrote the blueprint-string mod (https://mods.factorio.com/mods/DaveMcW) and has done lots of other good things for factorio.

He uses an inserter in this fashion on his forum avatar.

It would be nice to clarify this as it "could" be very misleading. I did spend quite a bit of time trying to grab the fish with inserters...lol.

OK, so if they can't be collected with inserters (as per the screenshot), how do I get them into the barrels ?