I created Five fish species for Euro Fishing (Carp, Bream, Tench, Roach and Catfish).
My task was to design and develop a fully randomized fish creation system. I created modular assets and materials for the task, this was done in UE4. Every fish caught (that isn't a 'boss fish') is unique.

Here is an example of the values I set up and exposed via Blueprint. The engineers then create a system that will randomize them based on preset values that I provided. Some of the blendshapes directly effect the weight of the fish. When a fish is created, it picks a weight first, and then decides what blendshapes to use from that, never going above the decided weight. (The engineers created this part of the system). 
It will then decide a base Diffuse texture (Usually i create half a dozen for a fish) and then it will apply a color saturation/desaturation for more variance. It applies the tiling scales, and decides on what size to have them (This is directly linked to the scale of the actual fish, but allows for a slight bit of variance either side). I use masks to add color to the tips of this fish as observed in reference imagery, I also add a shadow to the root of the scales which can be adjusted here.