Last all 3 minutes in Time Attack
Make use of the bubblefish pick-ups, avoid hitting boomers and toxic barrels and try to kill as many fish as possible.
Boomers, toxic barrels and letting fish leave the screen all increase your pollution meter.
If you are struggling even with the help of bubblefish then use the wave method listed in "Loner."
When you last the full three minutes and this will unlock.