Maybe this only works on certain fish, but if you stop the first long cut just short of the tail you can flip the fillet over and it stays connected to the body, making it easier to hold while you cut the fillet away from the skin.

Could this be a panametric fan?

If this was filmed in the late sixties using an older orthicon camera it might be an artifact of the way that the image is produced.

I’m just going from memory, but I believe the tubes used a brightness-amplifying screen kept charged with electrons that, when struck by light, would result in a brighter image that could be scanned by a beam. The downside of this technique is that a very bright area would suck up electrons from around it faster than they could recharge, resulting in a dark halo.

I think I remember some of the oldest classic Doctor Who episodes has this visual artifact, as well as some old Beatles TV recordings.