Nicole McGee

Comparing fish populations at sites dominated by native and invasive algae species in the Southern California Bight

Mentor: Jonathan Williams, Biology Department

Major: Biology

Funding: Virginia Reid Moore '37 Summer Research Fellowship


Macrocystis pyrifera, or giant kelp, is a canopy-forming species native to California that forms underwater forests, providing food and shelter to numerous reef fish and invertebrates. These giant kelp forests in Southern California are some of the most productive and diverse ecosystems on the planet. Sargassum horneri, an invasive species of macroalgae from the warm waters of Japan and Korea, was first reported in California in 2003. The two species became seasonal competitors as the density of the invasive algae increased throughout southern California. We aimed to identify areas dominated by Macrocystis pyrifera or Sargassum horneri and describe differences in their associated fish communities. Over 100 rocky reef sites were surveyed from 2011-2019 to estimate density of Macrocystis pyrifera, Sargassum horneri, and all species of fish. We categorized all sites by their dominant algae, then compared the fish communities between sites dominated by Macrocystis or Sargassum. We examined and visualized these communities by using two-dimensional, non-metric multidimensional scaling (nMDS) and found significantly different reef fish communities at sites dominated by Macrocystis compared to those dominated by Sargassum. We identified five species of fish that, combined, contributed to more than 50% of the difference in community structure. Blacksmith, Bluebanded Gobies, and Rock Wrasse were found to reside more frequently in Sargassum dominated sites while Señorita and Opaleye appeared more frequently in Macrocystis dominated sites. While the fish communities are different, the results do not indicate any specific negative relationship among species of concern, including those of commercial or recreational importance.

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