We utilised 10-month old oysters, spawned under the breeding program at the Port Stephens Fisheries Institute (PSFI) in January 2015 and subsequently grown out on nearby oyster leases, to establish three phenotypic treatments with respect to size. The phenotypes were Small (mean size ± SE; 18.88 ± 0.17 mm shell length), Medium (32.23 ±0.21 mm shell length) and Large (39.04 ± 0.32 mm shell length), with each phenotype receiving at least 100 oysters from each of the same 16 family lines to produce treatments of comparable genetic diversity. 

Oysters were manipulated on concrete tiles that were deployed at a mid-intertidal elevation along the eastern shoreline of Chowder Bay, Sydney Harbour, Australia (33°50’19.80”S, 151°15’16.50”E).

Oyster phenotypic diversity (3 levels; 1, 2 or 3 size classes/tile) and predator access (2 levels: small mesh cages and large mesh cages) were manipulated in a fully orthogonal experiment on concrete tiles, measuring 30cm (length) x 30cm (width) x 4cm (height). The large mesh was 2.5 cm in diameter. The small mesh was 1.5 cm in diameter. In total we had 14 treatments: small and large mesh cages containing small, medium and large oysters on their own, all pairwise combinations, and all three phenotypes together. There were five replicates per treatment, giving a total of 70 tiles.

Oysters were glued to concrete tiles using a 2-part epoxy adhesive (Megapoxy HT, Permatech), with the various body sizes haphazardly interspersed on tiles assigned to receive multiple phenotypes.  There was a 3 cm margin with no oysters around each plate. No oysters died from this process.

Prior to caging of tiles, each was photographed (with a scale bar included) to determine the initial size of all oysters and the position on each tile of the various size classes.  We used maximum shell length along the anterior–posterior axis as our measure of body size

Tiles were then enclosed within a box cage consisting of stainless steel mesh of the assigned size. The tiles were not affixed to the mesh cage but sat on the bottom of it. The mesh extended 10 cm above the surface of the tile. Mesh was secured on the underside of the tile with cable ties.

Tiles were placed in Chowder Bay at the mid intertidal elevation at which oysters naturally occur on 17th November 2015. Tiles were interspersed with respect to treatment, separated by at least 0.5 m and wedged between boulders to minimise flipping by waves. The tiles were sampled at 1 week and after 8 weeks (15 Jan 2016). At 1 week, three tiles were flipped by wave action but no damage to oysters or cages were observed, so these were righted and secured in new positions between boulders. No other flipping or damage to tiles was recorded at the end of the experiment.