The relaxing yet lucrative pastime of fishing will be seeing significant changes in the upcoming version update, particularly in allowing anglers to more accurately gauge their actions. New messages, allowing for more accurate determination of what has been caught, as well as a new "critical bite" system, will be among the changes made.

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From the official Final Fantasy XI forums:

Fishing Improvements on the Way
The forthcoming version update will see two refinements aimed at anglers of Vana'diel: more informative fishing messages and the introduction of a "critical bite" system.

• New fishing messages
  Messages displayed upon getting a bite will indicate the size and type of your potential catch as follows:

  Small fish: "Something caught the hook!"
  Large fish: "Something caught the hook!!!"
  Non-fish targets: "You feel something pulling at your line."
  Monsters: "Something clamps onto your line ferociously!"

• Critical bites
  Players with fishing skill surpassing the rank of their target will register a critical bite, and be rewarded with advance knowledge as to the identity of their prey.
  
  Ex.: (When scoring a critical bite against a black sole)

  "Something caught the hook!
  Your keen angler's senses tell you that this is the pull of a black sole!"

  Scoring a critical bite will also slow the target's stamina recovery in the event of improper reeling, allowing players to reel in lower-level fish with greater ease.

We will also be implementing the next stage of efficiency tweaks and refinements to the fishing system code itself, a process which began last patch with the goal of allowing for a greater range of adjustments in the future. We will continue to monitor game balance and adjust the system as necessary.

Furthermore, we intend to preserve the timing-dependent minigame nature of the fishing system, and make balance tweaks and adjustments accordingly. We are also aware of the significant outstanding issues with RMT abuse and difficulty imbalances related to players’ system environments, and will continue working towards a viable solution. Thank you for your patience and understanding.