Salt water contains a greater concentration of minerals (particularly salt) than does fresh water and therefore has a higher specific density. Thus, a fish has greater buoyancy in salt water. This added buoyancy allows a saltwater fish to have a heavier bone structure. When some of the oceangoing fishes made the switch to a freshwater home eons ago, they evolved a lighter, thinner, more delicate bone structure to compensate for the diminished buoyancy. All well and good for a fish, but what about the millions of diners who dread choking on tiny and spiny bones?