Experimental design overview

Our new field trial started on 19th October 2012 with the deployment of 18.000 oysters among three sites (A, B, C – see map below) and different cultivation systems:

Trays at low and high heights:

The aim of this experiment is to confirm the results obtained during the trial in 2011/2012, i.e. a significant increase of survival for adult oysters placed at high height (=standard growing height + 300mm) likely due to a lower immersion time leading to a lower exposure to viral particles (for more details see “Results experiment 1 – 2011”). Indeed, as last summer was cool and rainy it is essential to confirm these results during a typical hot Australian summer. Consequently, this experiment is being repeated in 2012/2013 using the same conditions as in the previous experiment: 4000 spat (4 month old) and 4000 adult oysters (12 month old) in plastic trays placed on wood racks at low (H1) and high (H2 = H1+300mm) heights at 3 different sites.

(Click on the pictures to enlarge them)

Trays placed at two different heights: low (standard) and high (standard + 300mm)

Trays placed at different heights: low (standard) and high (standard + 300mm). The low trays are immersed longer than high trays (there is a difference of 3h per tide on average).

Long lines and floating baskets:

The long line cultivation system will be employed to assess the effect of different heights on growth and mortality of juvenile oysters (7 month old) supplied by Shellfish Culture Tasmania. As lifting oysters during the POMS exposure period leads to a better survival rate (for more details see “Results experiment 1 – 2011”), the aim of this experiment is to determine the optimal height to farm oysters in terms of growth and survival throughout the summer, which is the most sensitive period with respect to a POMS outbreak. It is unknown how much time Pacific oyster spat or juveniles/sub-adults can be kept out of water in each tide cycle during summer without causing morbidity or mortality. On the 19th October 2012 10,000 oysters were placed in BST baskets and floating baskets at three different heights:

• H0: floating baskets set up on a floating line; they will stay immersed all times

• H2: BST baskets set up on a long line at high height (H1 + 300mm)

• H3: BST baskets set up on a long line at an even higher height (H2 + 300mm)

Floating baskets placed on a floating line. Oysters stay immersed all the time.

BST baskets placed on long lines fixed on Woodshield posts. Oysters placed at H2 and H3 experience different immersion times.

The advantage of using the long line cultivation system is the possibility to be flexible and quickly raise or lower the oysters according to the window of POMS infection, a heat wave or an environmental event that could affect oyster survival. The floating baskets will be used to confirm the results obtained during the trial in 2011/2012: all spat placed in intertidal trays died regardless of the site or height while a substantial proportion of spat survived in floating baskets (for more details see “Results experiment 2 -2011“).

In all cultivation systems (trays, long lines, floating), the mortality rate will be recorded fortnightly before the expected POMS outbreak, and then at appropriate intervals during and after the outbreak. Additionally, the spatial pattern of mortality observed between and within trays/baskets last summer (2011/2012) gave some insight into the virus distribution in the water column. Therefore, detailed location and orientation of trays and baskets (individually numbered) within the lease will be recorded at the beginning of the trial and will be conserved throughout the experiment in order to detect directional transmission/progression of POMS disease. Such a rigorous approach should help our understanding of POMS virus behaviour in natural aquatic systems. Growth rate will be measured fortnightly by assessing length, width, depth and total weight of randomly selected oysters. POMS virus will be detected and quantified by qPCR to assess the disease intensity and prevalence among treatments in each site at different intervals before/during/after the outbreak.

Detailed location and orientation of trays and baskets within the three sites (A, B and C)

Trays, long lines and floating baskets at site C

Location of our three sites (A, B, C) in Woolooware Bay. Image courtesy of NSW I&I.

Acknowledgement

We would like to thank the following people and organisations who were involved in the set up and funding of this new trial and without whom it would not have been possible:

–       Our oyster grower partners John Stubbs, Bruce Alford and Rob Moxham (Broken Bay Oysters); Len, Ted and Bob Drake (Endeavour Oysters Pty Ltd); Bob Hill and Keith Duggan (Endeavour Oysters Pty Ltd). We sincerely thank them for having spent so much of their own time and energy to set up the long line systems in Woolooware Bay, to provide floating baskets and the adult oysters required for this experiment, and for helping us each day in the field.

–       Our sponsors: the Fisheries Research and Development Corporation (FRDC), the University of Sydney and the Sydney Metropolitan Catchment Management Authority are funding this project. Ashley Turner at BST Oysters Pty Ltd, Ashley Davidson at Woodshield, and Scott Parkinson at Shellfish Culture Limited very generously donated the materials and triploid spat required for this experiment (for more details see “our sponsors”).

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