Natural attachment and size of cultured oysters limit mortality from a non‐indigenous predator.

Oyster recruitment to spat collectors can result in the clumping of two or more individuals, whereby they naturally cement to each other's shells. On‐bottom oyster farmers typically separate these oysters and re‐deploy them on the seafloor, potentially exposing them to predators. This raises the question of whether refraining from separating these oysters might provide an advantage against predation. We addressed this question using eastern oysters (Crassostrea virginica) and the non‐indigenous European green crab (Carcinus maenas) from part of their invaded range in Atlantic Canada. We assessed the influence of oyster arrangement (single vs. paired) and oyster size (small‐ vs. mid‐size) on mortality caused by two sizes of this invasive predator. Mortality rates were assessed with 24 h trials in which 30 oysters of a given size and arrangement were exposed to green crab predation. We found a significant influence of oyster arrangement (paired<single) and size (mid<small size) on mortality rates, without a significant interaction between these factors. A consistent predation tactic was used by the green crabs to attack and feed on the oysters, but the natural pairing of oysters seemed to delay or limit the ability of the crabs to break the shells. Our results suggest avoiding or delaying the separation of oysters until they reach a refuge size, particularly in oyster‐suitable habitats invaded or likely to be invaded by green crabs. [ABSTRACT FROM AUTHOR]