A modular fly fishing reel includes a frame with a side wall, a generally cylindrical axle cartridge removably mounted in a mounting socket of the frame side wall and also having an outer rotatable axle tube, and a spool having a hub removably secured on the axle tube by a quick release mechanism. The axle cartridge includes a drag assembly adjustable by a drag nut positioned adjacent the frame side wall and acting on the rotatable axle tube and spool. The quick release mechanism includes a detent that is spring biased through the spool hub to engage the axle tube, and releasable by a slide button or push button mounted on a spoke of the spool. The spool has a rim defining a line pocket and the side walls and bottom wall of the line pocket are convex for high strength and low weight.