Trophic Level

Trophic level is an organism’s position in a food web, set by what it eats and counted from primary producers at level 1. Herbivores and detritivores sit near level 2, small forage fish near 3, and large predators such as tuna and billfish reach 4 to 4.5. The metric is continuous, not integer, because most fish feed across several levels. The mean trophic level of landings is a fisheries indicator: a sustained decline signals fishing down the food web, where fleets shift from depleted top predators to lower levels. Trophic level scales the energy a stock receives and its vulnerability to top-down effects.