Which of the fins on a perch is the largest?

The largest fin on a perch is the leading dorsal fin. It's located on the back of the fish, before the second soft-rayed dorsal fin.

Perch typically have two dorsal fins: a leading dorsal fin and a second dorsal fin. The leading dorsal fin is more rigid and usually contains six to 15 spines. The second dorsal fin is soft-rayed and does not contain spines. The two fins can be separate or joined together.

Perch belong to the largest and most diverse order of fish called Order Perciformes in the subclass Actinapterygii. Fish in this category commonly have fins supported by spines or jointed rays.