The ocean is not as full of fish as most people believe, because the shallower regions, where most commercial fishing is done, have plentiful fish, but farther from shore, the numbers are harder to estimate. These regions are less explored and less understood, and Kurlansky uses the near extinction of the orange roughy to illustrate how our lack of knowledge and understanding has devastating consequences for the species in these ecosystems.

Most fishing is done within 200 miles of land. An important aspect of these ecosystems is that sunlight can penetrate much of the water, leading to a dense population of plankton. Plankton comes in two varieties: phytoplankton, which are microscopic plants that float through the water and produce energy through photosynthesis, and zooplankton, which eat phytoplankton and range in size from microscopic to eight inches. Zooplankton and phytoplankton form the base of the food chain, and seas rich in zooplankton provide food for krill, which other sea life depends upon as a food source.

Because the water gets increasingly deeper the further one ventures from shore, the same density of fish is not found throughout the ocean. There is still lots of life and lots of fish, but many of them live in great, unexplored depths.