Mystically, the fried fish is Christ Who suffered. And He is the bread that came down from heaven. To Him the Church is united to His body for participation of eternal bliss. Wherefore He says, Bring of the fishes which ye have now caught; to signify that all of us who have this hope, and are in that septenary number of disciples, which represents the universal Church here, partake of this great sacrament, and are admitted to this bliss.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Tract. cxxiii. 2