During the fishing scene, the shots of Andy and Opie show the lake's surface being riffled by the wind. In the shots of Floyd and Goober, the lake is flat calm.
In the first shot of the lake, there are no trees as you look from Floyd & Goober to Howard and then on to Andy & Opie. Although no one changed positions, from other camera angles there are clearly trees to the right of Goober and all around Howard.
It is generally accepted that silver carp were imported to North America, from Asian lakes and rivers, in the 1970s to control algae growth in aquaculture and municipal water treatment facilities. When they escaped containment, it was into the Mississippi and Missouri watersheds that they spread, and not on the Eastern seaboard waterways. Additionally, mention is made that 'Old Sam' had eluded the fishermen of Mayberry for years. As this episode takes place in 1966 and in North Carolina, it would not be likely that 'Old Sam', as a silver carp, would inhabit the local waters.