Impacts of an invasive alga on recruitment of a temperate reef fish

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The invasive brown macroalga, Sargassum horneri (devil weed), has spread along the southern California coast, replacing the native habitat-forming macroalga, Macrocystis pyifera (giant kelp). M. pyrifera is a critical habitat for macroalgal-associated fishes, especially for the arrival and survival of juvenile fish that constitutes recruitment success. It is unclear whether critical processes in recruitment may be altered if S. horneri replaces M. pyrifera as the dominant biogenic macroalgal habitat on rocky reefs. Through a series of laboratory experiments, we investigated the effects of macroalgal identity on post-settlement processes contributing to recruitment success for a common macrophyte-associated fish, Heterostichus rostratus (giant kelpfish). To evaluate the potential impacts of S. horneri vs. M. pyrifera in their effects on juvenile H. rostratus, we explored whether there were differences for H. rostratus in their (1) habitat choice in the presence of both macroalgae, (2) prey choice employing the mysid shrimp, Holmesimysis costata, that were fed on either of the two macroalgae, and (3) predator-mediated mortality by Paralabrax clathratus (kelp bass), a common piscivore in both macroalgal habitats. The results of these experiments suggest that for the post-settlement processes evaluated here, macroalgal identity does not appear to have a significant impact on giant kelpfish, and recruitment success in S. horneri habitat may be equivalent to that observed in M. pyrifera. It is possible, however, that differential settlement to these macroalgae, species-specific habitat requirements, or other post-settlement factors may play a role in influencing recruitment success within each of these two habitats. Conducting further studies that include ambient invertebrate prey densities of H. rostratus in each macroalgal habitat, particularly in a field setting, will potentially provide greater insight into these processes.