Today I won my 4th Extravaganza out of the about 9 or 10 contests I have done.
So I'd figure I'd post some strategies here.
I started about 10 minutes before the start of the contest to find a good place. I decided to begin fairly north on the east-coast where it wasn't fairly busy.
As Blizzard made winning the contest an achievement, the contest was more busy than before.
As a general strategy throughout the contest I tried avoiding the directions where everyone was going. I figured if the group, say, all headed south and fished theirselfs enough Stranglekelp and whatever they find, there would be quite a high rate of spawning in the north. This seemed to work allright.
If you know where a big group of people is and you are moving in front of them in the same direction, another thing to do is to make "decoy" pools. With this I mean, you catch 4 fish from one school, and when you have done that, you automatically know that there is not much fish left in the pool (if it didn't disappear already). Though, the competition will not know that those pools are almost empty, and in that way can be used to slow down whole groups of moving fishers.
I think this, with most basic strategies as not fishing in pools with more than 2 or 3 people already fishing there and after a pool is empty, watch where other guys go, and go in a different direction will give you fairly good chance of winning the contest.