I am attempting to display a creek with multiple species of fish. I have one layer that contains the species of fish and I want to symbolize each species as its own color and line.

[CooperSmith]

[XanderBakker]

The screenshot you included represents what you want to obtain or this is not what you want? In case it is not the desired result, what should be different?

It seems that for every specie in the symbology of the line a different offset is applied.

Result when using 5 lines with thickness 1.2 and offset 5,  2.5,  0,  -2.5 and -5