When fishers caught 3/4 of the fish that swam westward, their total number reduced by 3/4 * 1800 fish = <<3/4*1800=1350>>1350
The total number of fish that remained from the total number that swam eastward is 1800 fish - 1350 fish = <<1800-1350=450>>450 fish
If fishers caught 2/5 of the fish that swam eastward, their total number reduced by 2/5 * 3200 fish = <<2/5*3200=1280>>1280 fish
3200 fish - 1280 fish = <<3200-1280=1920>>1920 fish remained in the sea from the number that swam westward.
The total number of fish in the sea after fishers caught some is 500 of the number that went north + 450 of the fish that swam westward + 1920 of the fish that swam eastward = <<500+450+1920=2870>>2870 fish
#### 2870