I made this for a co-worker's birthday. It's all buttercream frosting. I'm not very experienced with cake-making, yet. I imagine one could get better results if using gum paste for the fins, but I just carved the frozen, layered cake into the shape of the trout and painted stripes of very thinned-out black and red buttercream and used an artist's brush dipped in vanilla extract to merge the colors together.