Yes there is a natural mortality (Morts) rate and it is very high. Obviously most BG are eaten while still very small (fry) and this effects the rate. With a life span of only a few years many die early. Were all the dead fish in good condition ? I often see a few dead BG just after the first major spawn but they usually look rough (not like the pic above). I will check but IIRC second year morts are in the 15 to 20% range.