Structural Complexity, Seascape Patchiness, and Body Size Interactively Mediate Seagrass Habitat Value for a Fish Mesopredato

Seagrasses form important coastal habitats that promote the foraging and survival of mesopredators. Variation in seagrass habitat structure at local and seascape scales mediates foraging success and survival, but the interactive effects of structure at these scales rarely is quantified when evaluating nursery habitat function. For my thesis, I tested how the interactions of multiscale habitat structural variation on juvenile fish body size mediates the value of seagrass habitat through survival and foraging success.;In Chapter 1, I tested the hypothesis that in eelgrass (Zostera marina) optimal structural complexity (SC) for juvenile giant kelpfish (Heterostichus rostratus) changes through ontogeny. I found that habitat selection differed with kelpfish size: small and large fish selected high and low SC respectively. Smaller kelpfish experienced lower predation risk and higher foraging in high SC, suggesting high SC is selected by these fish because it minimizes risk and maximizes growth potential. Larger kelpfish experienced lower predation risk and higher foraging in high and low SC respectively, suggesting they select low SC to maximize foraging efficiency. My study highlights that trade-offs between predation risk and foraging can occur within a single habitat type, that studies should consider how habitat value changes through ontogeny, and that seagrass nursery habitat value may be maximal when within-patch variability in SC is high.;In Chapter 2, I used a spatially explicit individual-based model to examine how seagrass fragmentation influences foraging and survival of a mesopredator, and how these relationships are influenced by SC, body size, and mesopredator and prey densities. I found that mesopredator survival and foraging dropped beyond threshold levels of habitat area (60 and 30% respectively) and depended on level of SC in the seascape. The relationship between habitat area and foraging did not depend on SC or body size, but did depend on organismal densities: when mesopredators and prey densities increased with decreasing habitat area, foraging was highest in highly fragmented seascapes. My results suggest that small- and large-scale habitat structure jointly dictate the value of a nursery habitat, and the effects of survival and foraging should consider interactions with habitat structure at multiple scales.