|Son photographing grebe|
"Although most grebes are nocturnal foragers, in a large population it is possible to encounter a few birds foraging in daylight. In 1994/1995 I observed over 2,000 dives by grebes foraging during the day in Saanich Inlet, but fish were returned to the surface in only 5 of these dives. In all occasions the prey were slender silvery fish. It is likely that grebes are able to swallow most fish underwater."
The article also indicates that 80% of their dives are search dives where no prey is located. That conforms to what my son observed where the grebe brought prey to the surface in only one of every five dives. Additionally, if the grebe is foraging at night or swallowing the gambusias underwater it would indicate an even higher rate of success. I concluded that at a minimum it was having no trouble staying healthy. So I'll find something else to worry about.