An aquatic food chain can go as
Phytoplankton (primary producers) ⇒ Zooplankton (primary consumer) ⇒ Small fish (secondary consumer) ⇒ Large fish (sharks etc, tertiary consumer)

Pesticides are not degradable and progressively accumulated at each trophic level.

Pesticides are used by farmers to protect their crops. These chemicals are absorbed by the plants along with water and minerals from the soil and are taken by aquatic plants and animals as well.

And since these chemicals are non-biodegradable, they will remain in the food chain. Tertiary are the final consumers of this food chain hence, all of this chemical gets in their body when they feed on organisms or plants of the lower trophic level. Its level will increase as we move from bottom to top of a food chain. In this way, these pesticides can cause harm to us. Hence they will be maximum in a tertiary aquatic organism like large fishes.