I think, techinically, its "Weight Relative."

Essentially, there's some averaged data of weight of a fish compared to it's length, and that average is assumed to be for a healthy fish.

So, against that standard, you compare the length and weight of your fish as a gauge as to how they're doing in your pond.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."