Predator-prey interactions are strongly influenced by habitat structure, particularly in coastal marine habitats such as seagrasses in which structural complexity may vary over small spatial scales. For seagrass mesopredators such as juvenile fishes, nursery habitat value may be dictated by the level of structural complexity that exists within a seagrass bed. Habitat value has been tested via optimality models which predict that fitness will be maximized at levels of structural complexity that enhance foraging but minimize predation risk, both of which are functions of body size. Additionally, optimality models have been shown to accurately predict habitat use patterns in nature. Past work measuring distinct ontogenetic habitat shifts has often assumed each habitat to have an equal value of predation risk to growth. However, it is well known that one habitat can have variation in habitat structure which can mediate predator-prey relationships. We tested the hypothesis that in eelgrass (Zostera marina) habitat, optimal structural complexity for juvenile giant kelpfish (Heterostichus rostratus), an abundant eelgrass mesopredator in southern California, changes through ontogeny. To do this, we quantified the eelgrass structural complexity effects on habitat associations, relative predation risk, and foraging efficiency for three size classes of juvenile giant kelpfish.
We found that small kelpfish experienced the lowest relative predation risk in dense eelgrass but also had higher foraging efficiency, suggesting that dense eelgrass is selected by these fish because it minimizes risk and maximizes potential for growth. Surprisingly, larger kelpfish did not experience lower predation risk compared to small kelpfish. However, larger kelpfish experienced higher foraging efficiency in sparse eelgrass vs. dense eelgrass, suggesting that they select sparse eelgrass to maximize foraging efficiency. Additional lab experiments and field surveys revealed that juvenile kelpfish preferred to inhabit areas of eelgrass that conferred the best combination of refuge and foraging opportunities, confirming that optimality models accurately predict habitat use patterns of structural complexity changes within one habitat. Our 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 habitat value may be maximal when within-patch variability in structural complexity is high.