Habitat Structure and Fish Foraging: Effects of Eelgrass Epibiont

Structural complexity plays an important role in mediating predator-prey interactions. Structural elements can limit predator mobility and reduce predator efficiency, increasing prey survival. Seagrasses are marine plants that provide refuge for many fishes and invertebrates by adding structural complexity to the seafloor. Though many studies have found positive correlations between prey survival and seagrass biomass or shoot density, seagrass is frequently colonized by epibiotic organisms, including algae and colonial invertebrates like bryozoans. Epibionts may fundamentally alter the structural complexity in seagrass habitat, but their effects on predator-prey interactions rarely have been investigated. The goal of this study was to determine whether epibionts of eelgrass (Zostera marina) alter predator-prey interactions by mediating predator and prey behaviors and prey survival probability. In San Diego Bay, Thalamoporella californica is a common rigid branching bryozoan that creates a unique 3-dimensional structure. I conducted (i) a laboratory experiment to determine whether predator (juvenile giant kelpfish Heterostichus rostratus) foraging behaviors and prey (grass shrimp Hippolyte californiensis) escape behavior in eelgrass are altered by colonization by T. californica, (ii) a field experiment to determine whether prey survival is correlated with the degree of bryozoan encrustation of eelgrass habitat, and (iii) a habitat selection experiment to determine if the presence of epibionts deter fish from foraging in eelgrass habitat. I found that increasing eelgrass structural complexity altered kelpfish foraging behaviors, reducing their foraging efficiency but the presence of bryozoans did not. In contrast, intermediate levels of structural complexity optimized the ability of shrimp to evade oncoming predators. Despite clear effects of structural complexity on fish foraging in the lab experiment, my field experiment in San Diego Bay eelgrass suggested that eelgrass habitat structure did not influence prey survival. Finally, I found that kelpfish strongly avoided foraging in bryozoan-encrusted eelgrass compared to uncolonized eelgrass. Overall, bryozoan structure influenced prey behavior in the lab but did not affect prey survival or prey density in the field, indicating that bryozoan structure in the field does not increased prey refuge. Understanding how fish interact with their habitat is crucial when determining how to effectively conserve and restore critical habitats like eelgrass beds.