The 2.5% mortality rate only applies to fish that survived the prior year. Im not very good with words, but this table should help:
To make the numbers easier lets say 1000 fish were stocked.
Year # Fish at end of year
1 975
2 951
3 927
4 904
5 881
6 859
7 838
8 817
9 796
10 776
11 757
12 738
13 720
14 702
15 684
16 667
17 650
18 634
19 618
20 603
So over 20 years you would expect just under 40% of the fish to die.
I added the situation with all the fish surviving to show an exception to the rule for a VERY unlucky pond owner
. These numbers are for diploid fish though (thanks for filling in that gap Dr. Willis). You should try to find out if your grass carp are diploids or triploids to determine if these numbers even apply.
Pic of me with a grass carp electrofished from an oxbow lake in Mississippi. He was 47" and likely a state record by weight, but we didn't have a scale to verify. The state record at the time was under 50 lbs. and we estimated this one somewhere between 50 and 60.
