As noted by Greg some studies show that based on predation dynamics it is not unususl for a particulsr spawn (sometimes the first of the year or earliest spawned BG fry or sometimes a later spawn)to be almost entirely consumed. As a result you may see 2 clear sizes of yoy BG , even if there were 3 spawns. This could also occur if there were 1 extended spawn and the hatchlings in the mid-time spawn period were the unlucky ones to hatch and swim up during the max. predator feeding point for that size prey. What you observe is important even if it does not clearly answer the question as it adds data.

More info from others would help. Even if you are not sure ,it would still help to know what you observed , even if no conclusions can be drawn.

Dave you posted while I was writing but I note between early fast growers ( which are often noted in BG studies) and mid-term yoy BG being consumed you could easily have one rolling spawn with 2 or 3 size BG yoy by fall.