Warm Waters Impact Kelp Forests

Unusually red abalone feeding behavior in Mendicino County (image credit K. Joe, CDFW)
Contributed by CA DFW Environmental Scientist Cynthia Catton (lead photo credit: K. Joe, CDFW)

Many important nearshore fisheries in northern California rely on healthy kelp forests, either as a food source or important habitat.  These fisheries are threatened because of a dramatic decline of the state’s kelp forests north of San Francisco.  Aerial surveys of the kelp in 2008 and 2014 by the California Department of Fish and Wildlife (CDFW), revealed a 93% decline of canopy-forming kelp species in this region.
Two consecutive years of warm water conditions and unprecedented numbers of purple urchins on the north coast have contributed to the lack of kelp in 2014 and 2015.  Subsurface temperatures (at 30-feet) in the kelp forests in Mendocino County hit a record high in the Fall of 2014, and saw comparable temperatures in 2015.  These spikes in temperature were caused by the “Warm Blob” in 2014, coupled with the strong El Niño conditions  in 2015.  Warm water, and diminished nutrients associated with warm water, stresses the kelp and makes it more vulnerable to urchin grazing.

Impact to Fisheries
The recent loss of kelp has led to starvation conditions for both abalone and urchins.  Large abalone are now more commonly observed climbing bare kelp stalks, and small abalone have abandoned the protection of crevices to search for food. Recent observations of abalone caught by recreational fishermen reveal signs of starvation-induced shrinking of the foot muscle. These conditions are also impacting the commercially important red urchin by limiting gonad production  (“uni”).  The north coast urchin fishery declined by 66% in 2015.  Other invertebrate and fish species that depend on kelp forest ecosystems for food and protection (such as rockfish) are also being affected. 

Kelp cover at four important abalone fishery sites in 2008 and 2014.
Green indicates kelp canopy observed. Maps created from annual aerial
surveys of the coast, conducted by CDFW. (Data: M. Fredle)
Assessing Change
Areas dominated by purple urchins (often called “urchin barrens”) have been observed in California before.  Once established, these barrens can become a stable state of the ecosystem making it very difficult for kelp forests to re-establish in these areas.  In the past, urchin barrens have occurred at much smaller spatial scales than is currently being observed in Northern California.  The bull kelp forest (Nereocystis luetkeana), common on the north coast, is particularly vulnerable to multiple years of intensive urchin grazing because it is an annual species that requires successful re-establishment of populations each year to persist.  Spores must fall from established blades onto suitable rocky habitat, settle, and grow to the surface each spring.  Given this particular vulnerability of the north coast kelp forest, and the associated nearshore fisheries, the California Department of Fish and Wildlife (CDFW) is leading research activities to assess the recovery potential of the kelp and the impacts to the red urchin and red abalone fisheries. 

From Left: Purple urchins grazing a desolate kelp forest, Fort Ross (credit A. Weltz); Abalone feeding (credit A. Weltz); Starving abalone with shrunken foot muscle (credit S. Holmes)