Results

The results so far are fascinating!

Weather conditions this summer have been hot and dry unlike last summer which was cool and wet. We do not yet know whether the local climate is a factor, but the outbreak of POMS this summer is milder than last summer.  Not all our spat have died, and overall mortality rates so far are lower than in the summer of 2011-2012. The outbreak began at different times on the three different leases (Sites A, B, C), and was confirmed using qPCR. We notified the NSW Chief Veterinary Officer as soon as we observed the onset of the latest outbreak.

Although we are only part way through the period when POMS is likely to be active, we can already see the same trend as last summer for oysters grown in trays: it is beneficial to set oysters at a height considerably above normal growing height. The results are illustrated below in Figure 1. The mortality rate is far lower in the adult oysters in the high trays than in the standard (low) trays.

Figure 1. Oysters grown in trays. Total mortality over time since the start of the experiment at Site A. Spat and adult oysters were grown in trays at standard height (low) or 300mm higher (high).

This trend has been seen in the trays at each of the three leases in Woolooware Bay, and also in spat grown in hanging baskets (Figure 2). The lowest mortality rate is in the baskets at the highest height (HH).

Unfortunately there has been high mortality in spat in floating baskets (Figure 2). This is completely different to the pattern we saw in summer 2011-2012 when floating baskets seemed to be beneficial for spat. We were very keen to run this experiment again in 2012-2013 to determine whether these promising results with floating baskets were repeatable – clearly they are not, and the natural variability in POMS expression probably explains the difference.

Figure 2. Oysters grown in baskets. Total mortality over time since the start of the experiment at Site A. Spat were grown in floating baskets or in BST baskets on long lines at the same height as the high trays (H) or 300mm higher (HH).

We are monitoring all of our oysters weekly to fortnightly.