By Reef Check California Director Dr. Jan Freiwald
In late August dead red abalone and other invertebrates started washing up on beaches along the Sonoma coast of California. At the same time, Reef Check divers started reporting many dead or dying abalone on the shallow rocky reefs at many of our favorite dive sites. This sudden and unprecedented die-off was not limited to abalone — gumboot chitons, sea urchins and sea stars were also affected. This event coincided with one of the most intense algal blooms (red tides) seen in this region.
Plankton blooms are caused by the large numbers of microscopic planktonic algae (protists) which reach such high densities (15,000 per ml) that the water appears red.
As these events unfolded, Reef Check California teams, in collaboration with PISCO, were in Sonoma County to survey 32 reef sites as part of the baseline MPA monitoring for the MPAs that were established in 2010. In addition, a team of RC staff and volunteers went to the Sonoma coast for a long weekend of surveying to document the effects of the die-off on invertebrate populations in the shallow coves that we have been monitoring for six years now. Despite rough conditions, we were able to survey several sites where we recorded many fresh, empty abalone shells and our data from one of the hardest hit sites at Gerstle Cove showed a decline in abalone density since our last survey on September 24th 2011. In the fall of 2007, we observed a severe reduction of red abalone densities at this site as well. This reduction was accompanied by observations of empty abalone shells, not unlike what has been reported recently along the Sonoma coast.
No one yet knows what has caused the invertebrate die-offs in Sonoma. Some past localized events we recorded have been attributed to local oxygen depletion in the shallow and protected coves along this stretch of coast. When high densities of phytoplankton die, bacteria feeding on them can severely reduce oxygen levels low enough to kill invertebrates, including abalone. This year’s event was more extensive, affecting reefs along a stretch of over 50 miles of coastline inside and outside of coves and at all depths that have been surveyed. There is no pattern in these deaths that would suggest local oxygen depletions.
Researchers are testing abalone for the toxic compounds produced by some of the species found in this bloom. One of the species identified so far is the dinoflagellate, Gonyaulax spinifera. Rita Horner at the University of Washington and David Crane at Fish and Game’s Office of Spill Prevention and Response determined that this was the most abundant phytoplankton species and it releases a toxin called a Yessotoxin. But only minute amounts were detected in dead abalone. According to Dr. Raphael Kundela at the University of California Santa Cruz it is unlikely that the die-off was caused by this toxin alone. The concentrations found in the water and abalone tissue were too low. They are also well below any levels that are considered detrimental to human health. Kundela’s laboratory is researching other potential causes for the invertebrate death. These include unknown or little understood toxins and the possibility of a bacterial or viral infection stimulated by the red tide that caused the fatalities. If toxins are responsible for the death it is also not clear how they were ingested by species such as abalone and urchins that are not directly feeding on plankton by filtering it from the water. Abalone feed on larger seaweeds and kelps. Studies in other regions of the world have suggested that invertebrates can take in toxins from algae blooms because they coat the seaweeds that they feed on. As the research into the mystery die-off continues, the Department of Fish and Game has closed the recreational abalone fishery in Sonoma County for the remainder of this year’s fishing season. Reef Check teams will be surveying the region again over the next couple of months to investigate the effects of the die-off and provide data to California Department of Fish and Game and other researchers to inform decision making on abalone fishery management. Pet CPR
While many divers were distraught after seeing hundreds of dead abalone, one beneficial side effect of the algal blooms was an amazing light show of bioluminescence. Some of the phytoplankton emit light when disturbed and this bioluminescence causes breaking waves to glow green or blue at night. Jerry Rudy, owner of Stillwater Cove Ranch and long-time resident of the area, reported nightly blue bioluminescence (like these photos from San Diego) along the coast at an intensity that he had never seen before.
|Abalone recorded by Reef Check at Gerstle Cove, 2006-2011|
For more information see the following sites:
Harmful Algal Blooms: http://www.mbari.org/staff/conn/botany/dinos/paper.htm and http://www.whoi.edu/redtide/page.do?pid=12777