Harvest moon: It's a Wonderful Life, SE - Fishing Guide

You’ll be able to buy your fishing rod from Van, the earliest opportunity would be the 3rd day of Spring and I recommend you do so immediately. Some people think that fishing isn’t much of a money maker but I’m not sure what else they’re doing in their spare time that’s better than fishing. See the chart below for the kinds of fish you could be catching to see just how much you could be making.

To actually fish you need to take yourself over to a body of water such as the river, (you cannot fish in the ocean), and withdraw your fishing rod from your inventory. You’ll then be able to cast your line into your water. Now, you will wait. You’ll get a few nibbles but you should ignore them, as the nibbles become more frequent you’ll know a fish is about to bite. When there’s a very large splash you should wait 1 second then start to draw your line in. if you press X too soon you won’t get the fish, so always wait a heartbeat before attempting it. I usually put the sound up quite loud so I can hear the loud, telltale plop of the fish biting and, if you have vibration on, your controller will vibrate as a further indicator but it will slow your character down when you jump the fence, due to a bug.

It’s fairly likely you’ll be catching little rubbishy fish like Snelt and Colombos, but that’s not going to be a huge problem. For every fish that is worth below 150g you can boost their value by turning them into sashimi; a recipe that uses 1 raw fish and which can be sold to Van for a haggled price of about 170g each. So, rather than sell 10 tiny Colombo’s for a mere 100g you can be selling 10, tiny Colombo’s turned sashimi, for a decent 1700g. On top of that, every little fish you catch you can also give to people as gifts; namely Daryl, Carter and Flora. Apparently it's the intellectuals who like fish the most.
Now, the best fishing location is a much debated topic. A lot of people say to fish near the waterfall and down by the sea. Apparently, fishing right down by where the river meets the ocean is the best way to land a Sharshark, however, I’ve caught several huge Sharshark’s right by where the river passes by the farm. Minimal travelling, doesn’t seem to make a difference. Of course in the chart below you can see where location changes the type of fish received, so fish accordingly. Fishing by the waterfall is also a viable option as there are several, fairly decent fish up there and it’s also right next to Carter’s dig site. This is useful if you’re waiting to go in or just coming out after a long day scratching about in the dirt.

Tip: Eat a plant or something before you go fishing, if your stomach rumbles it will interrupt the fishing process and you could lose out on a great catch.

Fish:	Prices:	Location:
Huge Arna Big Arna Arna Tiny Arna	1500G → 1800G 800G 400G →  480G 300G → 360G	Spring Swamp Swamp Swamp
Huge Colombo Big Colombo Colombo Tiny Colombo	100G 40G 20G 10G	Downstream Spring, Downstream Swamp, Downstream Downstream
Huge Nyamame Big Nyamame Nyamame Tiny Nyamame	200G → 240G 130G → 156G 60G 50G	Upstream Upstream Upstream Upstream
Huge Rainbob Big Rainbob Rainbob Tiny Rainbob	500G → 600G 150G → 180G 80G 70G	Downstream Spring, Downstream Downstream Downstream
Huge Sharshark Big Sharshark Sharshark Tiny Sharshark	2500G → 3000G 1200G → 1440G 600G → 720G 500G → 600G	Downstream Downstream Downstream Downstream
Huge Snelt Big Snelt Snelt Tiny Snelt	100G 60G 30G 20G	Downstream Spring, Downstream Downstream Downstream
Huge Yamame Big Yamame Yamame Tiny Yamame	2000G → 2400G 1000G → 1400G 500G → 600G 300G → 360G	Swamp, Upstream Upstream Upstream Upstream
Huge Huchep Big Huchep Huchep Tiny Huchep	250G - 300G 170G - 204G 90G 60G	Upstream Upstream Upstream Upstream

Upstream = Right by the waterfall
Downstream = Bridge, near the sea or near the farm 

Note: By watching the ‘dating show’ on the television it seems as though whether it says you will be lucky/unlucky and so on depends on how well you will succeed at fishing. This luck thing may also extend into how much you get from the dig site, this is all speculation, but it does ring true now and then.