Food web and trophic level
The web of who eats whom; a fish's trophic level is how many steps it sits above plants and algae.
Also: trophic cascade, food chain
Algae and plankton are at the base of the food web, level one. Animals that eat them, such as sardines and silver carp, are at level two, fish that eat those fish at level three, and so on up to tuna and sharks at four or more. Energy is lost at each step, so a sea holds far more anchovies than tuna. When fishing removes the big predators first and then moves down to smaller fish, the average trophic level of the catch falls, a pattern the scientist Daniel Pauly called fishing down marine food webs.