The fishing spot identified for the quest is the old hot spot and just gives the adjusted new fishing table (after 17 June Patch). The Piranha now appear to mainly come from one of the new fish spawns (jumping fish like at sea but without the gulls) There happens to be one such spawn location to the SW corner of the island in front of Splashing Point; where a small sandy beach is found.

The fish spawn can be close to the beach or mid stream which is hard to fish but reachable with an energy cast (extra distance) or waist high in the water on the rocks of the opposite bank. Be aware that currently these spawns appear bugged and despawn rapidly after a minute. You may stil catch the Piranha even when despawned for a few casts though it may not.

The spawn is of Piranha the 15 times I have fished it so should give the quest target. Also the Crawfish can be caught there also so worth fishing that spot to wait for the fish spawn to arrive. The fish count wherever you catch them.

You can't bring fish before the quest as it counts those caught after the quest is taken however.

It seems that Piranha can also be caught in open water but the drop rate for yellow quality fish seems low so the spawn offers the best option.

Anyone who is still struggling with this, the river hotspots appear randomly throughout the river and look like this:
http://i.imgur.com/eWVc60m.png
These will drop piranha. Keep casting on them even if the graphic disappears because they will usually still drop yellows for a while after.

The crawfish are a standard white drop from any spot in the river.