Through his calling.
When did "God" design and built this fish?
He probably designed it before time began. He built it, according to the bible, on the fifth day:
Gen 1:20 And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven.
By what methods, processes and/or mechanisms was this fish designed and built? How have you determined that the fish was "designed and built"?
I can only speculate about that because God only knows.