Predators affect community structure by influencing prey density and traits, but the importance of these effects often is difficult to predict. We measured the strength of blue crab predator effects on mud crab prey consumption of juvenile oysters across a flow gradient that inflicts both physical and sensory stress to determine how the relative importance of top predator density-mediated indirect effects (DMIEs) and trait-mediated indirect effects (TMIEs) change within systems. Overall, TMIEs dominated in relatively benign flow conditions where blue crab predator cues increased oyster survivorship by reducing mud crab oyster consumption. Blue crab DMIEs became more important in high sensory stress conditions, which impaired mud crab perception of blue crab chemical cues. At high physical stress, the environment benefitted oyster survival by physically constraining mud crabs. Thus, factors that structure communities may be predicted based on an understanding of how physical and sensory performances change across environmental stress gradients.

Blue crab predator effect trials were performed in enclosures (1.25 m x 1.25 m x 0.3 m; l x w x h) that contained an oyster reef in the center as a refuge for mud crabs. Oyster shells were glued together to create artificial oyster clusters that were used to manipulate the placement of oyster spat within the enclosure. Four oyster spat were epoxied to the artificial oyster clusters and 4 clusters were placed within the constructed oyster reef and 4 clusters outside the reef. In total, each cage contained 32 oyster spat with 16 spat each inside and outside the reef.  Fifteen mud crabs were added to the oyster reef in the enclosure. One of four blue crab predator treatments (control/cull/predation-risk/lethal) were added to enclosures. Control treatments contained no blue crab predator. Cull treatments simulated blue crab predation without the presence of blue crab cues by removing 5 mud crabs at 24 hours. Predation-risk blue crab treatments conatined one blue crab that was mobile but with chelipeds clamped shut with heat-shrink tubing covered by duct tape and cinched down by a cable tie to prevent blue crabs from attacking mud crabs. Lethal blue crab treatments consisted of one mobile and unrestrained blue crab. 

Enclosures were deployed on intertidal mudflats at either Priest Landing or Skidaway Narrows during either mean tide or spring tide. The number of mud crabs recovered, oysters alive, and oysters dead were counted after 48 hours. Blue crab predator effect size on oyster survival was calculated using ratio-based indices. The formulas used were as followed:

Density-mediated indirect effect = (oyster survival with lethal blue crab/ oyster survival with predation-risk blue crab) - 1     

Trait-mediated indirect effect = (oyster survival with predation-risk blue crab/ oyster survival with no blue crab) - 1

Total predator indirect effect = (oyster survival with lethal blue crab/ oyster survival with no blue crab) - 1

The numerator was the number of surviving oysters in a single replicate for the stated treatment and the denominator was the average number of surviving oysters of the stated treatment for a given trial block. Flow measurements during each trial block were estimated using the predictive relationship between tidal range and flow conditions (regression equations derived by Pruett & Weissburg (2018)).