The introduction to the “Fish” chapter in a hundred-year-old cookbook explains why we should eat fish:
If more fish and less meat were used in the daily meals, it would help to reduce the cost of living. Fish provides the same nutrients as meat at a much smaller cost, and furnishes a food that is not only palatable but easily digested.
Whitefish, haddock, halibut, cod, flounder, smelts, perch, pickerel, sunfish and crappies belong to the white-fleshed family. Salmon, shad, lake trout, butterfish, and herring being to the red-fleshed family.
As the white-fleshed fish is considered more easy of digestion than the red-fleshed, it should be selected for invalids, convalescents or those suffering from weak digestion.
Fish should be eaten while fresh and in season; then it provides a delicate protein food. Stale fish is poisonous, so great care should be used in its selection. Fish contains albumen, and as albumen (which is like the white of egg) coagulates at a low temperature, it should be cooked at a temperature below the boiling point for water.
The Housewife’s Cook Book by Lilla Frich (1917)
I find the last sentence about cooking fish at a temperature below the boiling point of water a bit befuddling. Have you ever heard that it was important to cook fish at low temperatures? I often put fish in the oven at 400° F. or sear on top of the stove using high temperatures.