What you catch is still dependant on rod enchants and the RNG and other standard fishing-system variables, it just happens more frequently. Every 16 fish up to 64 will halve the current delay range for the bite, after that it will reduce by %25 per additional 32. [numbers given might be highly unrealistic due to spawning mechanics]
You can also toss out bits of food to chum the waters. Meats (other than rotten flesh) will attract dolphins directly but not fish. Rotten flesh will attract zombies and drowned. Everything else attracts more fish. Every 32 fish present will create a cumulative 10% chance of attracting dolphins (10% for 32, 20% for 64, etc), which scares the fish away (random despawns over time, similar to hostile mobs when too far away).