Even though WSSV infections detected in summer and autumn
did not show any significant difference, related shrimp production
varied by WSSV infection load (Table 4). In autumn, shrimp
production with infection loads with fewer than 103 WSSV copies/
ng DNA was 4.53 mt/ha, which was highly significantly different
(P=0.003b0.01) from those with loads featuring more than 103
WSSV copies/ng DNA (i.e., 0.42 mt/ha). The result suggests that WSSV
infections with more than 103 copies/ng DNA can strongly impact
shrimp productivity. In comparison, shrimp productivity showed
no significant difference based on the same range of infection
detected in summer (P=0.78N0.05). This is possibly due to the
long interval between the time of viral detection (July 23–August 12)
and shrimp harvest (October). During this period, the viral load can
change due to a number of factors, such as farm management and
weather conditions. Results of WSSV infection and shrimp production
testing on the four farms sampled in both summer and autumn also
support this point: Farms TA-1 and SN-1 maintained low WSSV
infection loads, resulting in good harvests; GC and YK-1 collapsed
just before harvest in autumn, due to a dramatic increase in WSSV
load from summer (809.3 and 338.9) to autumn (2.1×106 and
1.7×105) (Tables 2 and 3). These results indicate that the summer
viral infections were not sufficient to predict shrimp production
during cultivation. Lo et al. (1998) suggest weekly monitoring of
WSSV by nested PCR during cultivation, to provide ample opportunity
to reduce any potential stressors in a shrimp farm and thereby
prevent a transition from low to high infection, allowing shrimp to
grow to full size prior to harvest.