These are carp genetically modified to be sterile and will eventually die off.
It's good that the City learned their lesson. Previously I forget which year, they also released into Town Lake 11k carp which, un-surprisingly, ate every green thing in the lake.
I've been paddle boarding Town Lake since 2007. I remember soon after their release that the lake was absolutely barren of any vegetation. A few years after the release, I remember seeing schools of 30+ inch carp swim around the lake. I don't imagine they were 30 inches when they were released so those fuckers ate well, too well. I knew those carp died off a couple of years ago when the vegetation started coming back. A good sign is to watch for vegetation in Barton Creek from the bridge to the mouth of Town lake.
I also remember the enormous backlash from bass fisherman about Lake Austin. Lake Austin was a well known lake for producing big bass. After the carp release, the vegetation disappered and so did the bass fishing.