Since there were 300 fish-eater birds in the lake on the first day, the number doubled to 2*300 = <<300*2=600>>600 birds on the second day.
The total number of fish that Cohen saw in the lake in the two days is 600+300 = <<600+300=900>>900
On the third day, the number of fish in the lake reduced to 600-200 = <<600-200=400>>400 birds
In the three days, Cohen saw 400+900 = <<400+900=1300>>1300 fish-eater birds at the lake.
#### 1300