Answer: 20,064 fish.
As only 3/4 of the over 50s claimed 16 fish, this averages out to 12 fish each.
Similarly only 1/8 of the under 50s claimed 96 fish, this again averages out to be 12 fish each.
Therefore the total number of fish was 1,672 x 12 = 20,064.
It doesn't matter how many over/under 50s there are.