Fish hydrolysate, in its simplest form, is ground up fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 carcasses. After the usable portions are removed for human

Human

Humans are the only living species in the Homo genus...

 consumption, the remaining fish body, which means the guts, bone

Bone

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

s, cartilage

Cartilage

Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

, scales

Scale (zoology)

In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...

, meat

Meat

Meat is animal flesh that is used as food. Most often, this means the skeletal muscle and associated fat and other tissues, but it may also describe other edible tissues such as organs and offal...

, etc., is put into water and ground up. Some fish hydrolysate is ground more finely than others so more bone material is able to remain suspended. Enzymes may also be used to solubilize bones, scale and meat.