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MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps13321

Limited effects of Sargassum horneri, an invasive alga, on temperate reef fish assemblages

Samuel C. Ginther*, Mark A. Steele

*Corresponding author:

ABSTRACT: Biological invasions can dramatically affect the ecology of invaded regions, and globally have resulted in economic damages that total billions of dollars annually. In recent years, an invasive alga, Sargassum horneri, has become established and spread along the coast of southern California. On temperate reefs in southern California where S. horneri has become prolific, we explored how this non-native alga influences the structure of fish assemblages using field observations and a field experiment. Fish and algal assemblages were quantified along transects on rocky reefs at depths of 3 and 6 m at 6-8 study sites spanning 5 km on 4 occasions over 1.5 yr. Spatiotemporal variation in the fish assemblage was not strongly correlated with the abundance of invasive S. horneri over this period, though it became less variable as native giant kelp (Macrocystis pyrifera) disappeared from the study sites due to a warm-water event, during which the invasive S. horneri became more dominant. An experiment removing a total of 4.25 metric tons of S. horneri from fourteen 6×6-m plots revealed that the invasive alga did not affect fish abundance, species richness, species diversity (H'), or multivariate assemblage structure over a 5-month-long period. Overall, our study found little evidence of negative effects of S. horneri on fishes despite it drastically changing the underwater landscape. Nevertheless, we advise cautionary management actions to limit the movement of this invasive alga because its effects on other community members, such as other algal species, may be detrimental, and longer-term effects on fishes might develop.