A little can be enough. Native fish from the Western Mediterranean Sea can act as a control agent for the invasive alga Caulerpa cylindracea

In the Mediterranean Sea, different organisms can feed on invasive algae species, yet, how these species provide biotic resistance against algal invasions remains unclear. In this study, we analyzed fish stomach contents to determine which fish species feed on Caulerpa cylindracea and we performed an exclusion experiment to experimentally test how this grazing activity may limit invasive algae abundance and spread. Our results show that several fish species, many of them not considered strictly herbivores, feed on the invasive alga; however, the Ivlev´s Index suggests that its consumption was accidental except for Sarpa salpa. Additionally, the exclusion experiment demonstrated that fish species can limit C. cylindracea coverage at 10m but not at 30m deep; which is likely linked to the higher abundance and activity of these fish species at depths above 25m. These results are in agreement with the current distribution of C. cylindracea, which is much more abundant at depths from 25 to 50m. In this study we show that fish herbivory is a form of biotic resistance against C. cylindracea at shallow depths, not being able to completely remove it, but controlling its abundance.