The scientific reason that sardines make good bait for saltwater fishing is associated with the concept of predators and prey. Sardines are the universal prey of both larger and smaller saltwater fish therefore they make good bait. Large saltwater fish such as salmon are predators of sardines and will seek out live specimens. This makes sardines a favorite bait of deep sea fishermen. One drawback is that smaller saltwater fish will also feed on sardines, which destroys the live bait.
Freshwater fisherman use frozen, dead sardines to augment their lures. Although the sardines are not direct prey for freshwater fish, they provide a fishy scent to the lures used by these fisherman. The smell attracts fish thus adding another dimension to the plugs. Often these fishermen will use only pieces of the smaller fish or they will make a brine out of them.