In "The Seagull" the peak buildup of DDT is shown as an example of biomagnification. Biomagnification is the buildup of DDT, which is a pesticide that does not break down and loves fat, because the body cannot metabolize and excrete the pesticide readily. As one goes up the food chain, the more DDT is accumulated in the body.
In the food chain of the aquatic ecosystem, the Seagull is the final consumer of the food chain and is located at the top of the food chain. The Seagull preys on the fish and other sea creatures that have already accumulated the DDT from the lower levels of the food chain.