Here are some things I've figured out by experimenting with fishing over time.
I've been using this info for a while and I just figured I'd drop this here.
The amount of bonding shard obtained is fairly minor until you get the sturdy rod (which you get from Timera's shop shortly after ch 13-14)
You can then get extreme level of bonding shard once you get the supreme rod which you can get after chapter 20 (you can get a lot with sturdy rod but its much more painful)
I'd recommend banking paralogues until you finish ch 20 if you're on maddening.
Why would you want to optimize fishing?
I mean, how good could the rewards be for such a large amount of work. It really depends on how much you like trying new emblem setups or engravings.
When you just get fishing, you can get a piddly amount of bonding shard so it might not seem to be that worth it.
The fun starts with sturdy rod (although it can be painful to get really high amount out of it) and it gets really easy to get 10.000+ bond shards per battle after obtaining supreme rod.
And if you play on maddening and have been saving up your paralogues for when you get supreme rod it pretty much means you'll be getting an insane amount of potential shards.
Fishing Count per battle
The number of fish per battle depends on the best rod you have available. 3 for initial rod, 6 for sturdy and 10 for supreme.
You can use any of your catch with any rod. You can use all your catches on sturdy or all on supreme if you wish.
If you let a fish go after the reeling part of the minigame starts you will not lose a chance to fish so you can let got of any fish until you get the one you want. This ties in to the next section.
Fish RNG
I thought it'd be important to mention this since it can be tricky to notice sometimes but fish RNG is not fixed. Every cast gives you a chance of catching a different fish. Be aware however that not all fishes seem to be as likely to be caught (ex: chromatic trout seems a lot rarer than its other large fish in the morning time slot)
Fish Shadow Size
Fish shadows comes in 3 sizes : small, large and giant. Some might have already noticed but you can tell which shadow you'll obtain by looking at the size of the ripples and how fast it expands. You can get good enough at telling them to get a reliable rate of catching the correct size. (I average about 90% rate for example)
Knowing this is important for what comes next.
Bonus fish mechanic
As you probably noticed, there is a mechanic that doubles the amount of shard a given fish will give that is shown once you talk to the fishing npc.
You can change the bonus fish by simply going to sleep in your bed.
There is a further thing to note. The bonus fish WILL NOT CHANGE if you don't look up the current bonus fish before switching.
Example :
Start at afternoon and you're trying to cycle the next afternoon to a different fish. If you sleep alll the way to the next afternoon, the bonus fish will not change.
However, if you were to stop anywhere along the line during sleeping to next afternoon (say, morning) and then go see the current bonus fish and THEN sleep one more time the bonus fish will then be able to change once you get to the afternoon. (Naturally you can just be unlucky and the next cycle could keep the same fish, rng being rng)
This way of how the bonus fish doesn't change unless you look at its current state can be exploited to keep the same bonus fish at all time.
After coming back from a battle, if you're in the same time slot as your previous fishing session the bonus fish will stay the same.
If the time period has changed, do not look at the current bonus fish and cycle all the way to the time period of your bonus fish and it will still be the same as when you left for battle.
Important fishes
(The amount of shard shown below are base shards, they double with bonus)
For sturdy rod :
Chromatic Trout - Morning - Large Shadow : 500 shard
Tipa Salmon - Evening/Night - Giant Shadow : 150 shard
For sturdy rods those are what I believe are the two best options. Chromatic Trout gives the best absolute amount of shards but will make you want to die inside due to the amount of save scumming you will have to do to get 6 per battle. Tipa Salmon is much more reliable since it is the only giant shadow at this point so no need to save scum.
For supreme rod :
Enchan Tuna - Afternoon - Giant Shadow : 1000 shard
Bisot Tuna - Afternoon - Giant Shadow : 500 shard
This is where the easy farming REALLY starts. Both of those fishes are valid catches for a healthy amount of shards and if you get bonus on Bisot Tuna it essentially guarantees 10,000 shard per battle.
Personally I prefer to set the bonus to Bisot and catch either Bisot and Enchan for a safe 10,000 shard every battle. You need to get more than 3 Enchan for Enchan bonus to surpass Bisot Bonus.
Different fishes behave differently
If you tried catching the above 2 giant fishes for supreme rod you'll probably have noticed there's a 3rd giant fish in that same time slot.
As the title of this section mentions, not all fishes behave the same once the reeling minigame happens.
I cannot tell the different between chromatic trout and its other large counterpart but there is an easy tell between the 3 giant fishes in the afternoon slot for supreme rod.
The bottom 3rd fish (which only gives a piddly 250 shard) is much much easier to catch. Each "hit" you give it will take out a sizable chunk of its "hp". You'll definitely notice the difference between it and the other 2 fishes in same time slot. It also moves around a lot more.
You can also tell the difference between Enchan Tuna and the other 2 by the fact it takes up almost the entire length of the lighter quadrant of the fishing area.
Turbo Controller
Lastly, if you have a turbo controller, most fishes can be no brained fish with spamming turbo (as in not even trying to "stun" them).
This does not, however, apply to all fishes; Especially the giant ones.
Although the turbo controller will still be able to remove as much hp as possible for those you can't no brain
Rare
