A while ago there was an article in Bassmaster touching on this. According to some research when you hook a fish it releases some scent into the water that warns other fish about dnager. Think about this along the same line as land animal screaming when attacked. However the scent dissipates quickly and on top of it other fish in the area might even ignore it (I guess it depends on how hungry they are). I cannot remeber if this was true for all fish or just certain spiecies. My thinkin is that stressed fish will continue to "stink up the water" after being released therefore your catch rate is lower. Either that or simply the comotion in the water simply turns fish off.