Limited recruitment of an ecologically and economically important fish, Paralabrax clathratus, to an invasive alga

Some invasive species have devastating ecological impacts on native species, whereas others have little effect. The macroalga Sargassum horneri recently invaded the waters of California (USA) and Mexico, and in some places has largely replaced an important native macroalga, the giant kelp Macrocystis pyrifera. To explore the potential impacts of the invasive alga on populations of an ecologically and economically important fish, the kelp bass Paralabrax clathratus, we tested whether recruitment of kelp bass onto macroalgae differed between the invasive S. horneri and native M. pyrifera. Field surveys by scuba divers on naturally occurring S. horneri and M pyrifera at Santa Catalina Island revealed that kelp bass recruits were -30 times more abundant on individual M. pyrifera than on individual S. horneri. To assess the cause of this pattern, we conducted a field experiment in which plots containing 3 treatments (M. pyrifera only, S. horneri only, and M. pyrifera and S. horneri together) were isolated over a sandy bottom, which limited post-settlement movement of recently settled kelp bass among macroalgae. Kelp bass recruitment was similar for the 2 treatments containing M. pyrifera, but no recruits were observed on S. horneri, regardless of whether the alga was alone or paired with M. pyrifera. Additionally, we found a strong spatial gradient in recruitment among the experimental plots that was explained by the abundance of larger, cannibalistic conspecifics. This study presents the first evidence that invasive S. horneri may negatively impact an important reef fish.