River and pond fish together

[Red Cat]

Yesterday, I caught a carp in a river that I made and then a few seconds later I caught a dorado in the same body of water. I was doing some terraforming, so I thought the carp may have been a leftover pond fish. Today I caught a catfish in a river and then a few seconds later I caught a crucian carp in the same river. I was not doing any terraforming today. I caught a few more fish in the river and they were all river fish. The river is very close to another pond, so I wonder if that's somehow causing pond fish to occasionally spawn in the river. The river is just a long rectangular body of water not connected to any waterfall or outlet.

Has anyone else seen pond fish in a river or vice versa? I don't mind it happening, but I think it's interesting that it can happen and I'd like to figure out what causes it.

 

[Uffe]

The only time I've seen something like this is when I go to the mouth of the river and go fishing, only to catch a sea bass instead of some river fish.