Each step in a food chain occupies a trophic level defined by how an organism obtains its energy. Primary producers form the first level; herbivores the second; carnivores that eat herbivores the third; and so on. Few marine food chains exceed five or six levels because energy is lost at each transfer.

In practice, many species feed at more than one level, so trophic position is often fractional when measured by stable isotopes or diet analysis. Understanding trophic levels helps quantify the efficiency of energy transfer, the potential yield of fisheries, and the consequences of removing top predators or basal producers.

Human fishing typically targets high trophic levels first, then moves down the web as larger predators decline—a pattern described as fishing down the food web. Maintaining a balance of trophic levels is a goal of ecosystem-based fisheries management. Models that incorporate trophic structure are used to explore management scenarios and climate impacts.

• Position in a food chain based on energy source
• Producers at level 1, then successive consumers
• Energy decreases at each transfer
• Central to ecosystem and fisheries analysis