Ocean wars

I had a similar idea to this a while ago, except it was about salt levels/temperature in the water and whether or not fish are racist against other fish with different salt tolerance.

Migrant fish being profiled at the borders.

https://qph.ec.quoracdn.net/main-qimg-9fea9c6a58af552c2505412053b445b4-c
They are different. They don’t mix so much because of density differences caused by tempurature and salinity