Koi (Japanese word for carp) were developed in Japan in the 1820's, bred from common Carp for decorative purposes. They remain popular today in domestic tanks and garden ponds as well as larger ponds and lakes. Where these particular individuals came from is a matter of supposition, but Malcolm and I think they must have been introduced to the lake from someones garden pond.
The small, black specs on the water surface are flies and mosquitoes, perfect fish food! No doubt one of the reasons why the Koi have grown so large.