body condition
A measurement that can be used to infer the health status of an organism. This is typically calculated by comparing a measurement like length to another like weight, for which a healthy ratio is known.
The human body-mass-index (BMI) can be considered a measurement of body condition, as this is derived from a person's height and the range of weights that are considered to be healthy for that height.
A fish with a high body condition is likely to be on the plump side with excess energy stored as a buffer for times of low food intake or high energy use, while a fish with low body condition would be skinny for its length and have low energy reserves.