1
2
CASE STUDY
3
4
5
6
7
8
9
Deaths in farmed abalone at “ Happy Abs ”
Abalone are dying on an onshore abalone farm “ Happy Abs ” 3 . The farm has 1000 tanks , and predominantly grows out stock but also has broodstock and a hatchery . The initial signs are sick and dying abalone and the cause was believed to be poor water quality . Only two tanks of broodstock are initially affected with increased mortality .
The expected level of mortality in the broodstock is very low , usually less than 1 per cent per month ( this is an approximation of monthly cumulative incidence ). We have two tanks of broodstock with 156 in one tank and 97 in the other on 11 June . Table 1 records the number of dead abalone found every day in these two tanks . Note the change in population of the tank as the dead abalone are removed from the “ at risk ” group ( total abalone ). Most of the other tanks on the farm recorded zero mortalities during this period .
This is only five days of daily cumulative incidence data for two tanks but comparing this with the average monthly cumulative incidence data of less than 1 per cent , we can see that the deaths in the each of the two tanks is clearly well beyond the expected normal disease incidence for the farm .
10
3 This is a fictitious example .
12