By Gail Gallessich

Overfishing presents a much greater risk to the kelp forest ecosystems that span the West Coast from Alaska to Mexico’s Baja Peninsula than do the effects of run-off from fertilizers or sewage from the shore, say scientists at UC Santa Barbara.

In an article published in an issue of Science last month, researchers describe the first study to compare the top-down versus bottom-up human influences on the food chain of the kelp forest ecosystems.

Scientists at UCSB’s National Center for Ecological Analysis and Synthesis conducted the study. “This study shows that California is on the right track by limiting fishing in certain areas in an effort to comply with the Marine Life Protection Act,” said Ben Halpern, project director at NCEAS.

Kelp forests are giant brown algae that reach up to 120 feet in height and support diverse ecosystems. They provide beautiful settings for scuba diving, and are rich areas for commercial and recreational fishing.

Halpern explained that until now studies of kelp forests looked at either overfishing or increased nutrients. This is the first study to put both variables together.

The research team took data from four years of marine life surveys by the National Park Service. The park service regularly checks 16 different sites around the northern Channel Islands. They maintain data on 46 different species.

Next, the scientists matched park service data with ocean nutrient levels data provided by SeaWiFs, a satellite-monitoring project that photographs and analyzes ocean color for information about ocean life.

Coastal run-off from fertilizers and sewage overflows increase the amount of organic material in the near-shore ocean. According to the study, differences in the amount of organic material do not have much effect on the delicate food chain of the kelp ecosystem, except at extreme levels. However, removal of the fish at the top of the food chain has a profound effect.

When the predators, such as rockfish, at the top of the food chain are removed, the species that they normally eat, such as snails and barnacles, increase in number. Many of these are herbivores that eat kelp. When their numbers increase, they decrease the amount of kelp, in turn changing both how kelp forests look and the species that live in the kelp.

“Kelp forests are so sensitive,” said Halpern. “If you remove some of the predators, then you can have an effect on the entire kelp forest ecosystem.”