Snow hike

We just got back from a short hike outside. The trails were still a bit snowy in places and the ground was damp, but we were eager to get outside again! During our run earlier in the day, we'd seen a blue jay and a beautiful finch, so we had high expectations for what we'd see on our hike.
We saw barely any wildlife; just a few robins, but we heard frogs and birds -- spring is upon us!

At the end, we were relaxing by a pond when we saw a strange orange glow coming from under the ice still on the pond:
Upon closer inspection, we realized that this orange glow was comprised of hundreds of goldfish!
We hiked here last year and never saw them; also, they were all relatively small. I'm so curious as to how they got here. Did one person dump a lot of them? Or did it start with a relatively small number and they bred themselves into this giant school?
In any case, even though they were beautiful to see, it made me a bit sad to see this invasive species in the pond.