Carnivorous conchs to blame for oyster decline

New research from North­eastern Uni­ver­sity marine and envi­ron­mental sci­ences pro­fessor David Kimbro and grad­uate stu­dent Hanna Gar­land, pub­lished in PLOS ONE, links the dete­ri­o­ra­tion of oyster reefs in Florida's Matanzas River Estuary (MRE) to a pop­u­la­tion out­break of car­niv­o­rous conchs and high water salinity--or saltiness--caused by a pro­longed regional drought.

This isn't just bad news for oyster lovers.

"Coastal ecosys­tems around the world depend greatly on the ser­vices pro­vided by oys­ters," Kimbro said. "They are impor­tant for the sta­bi­liza­tion of shore­lines, fil­tra­tion of coastal water, pro­tec­tion of impor­tant eco­nom­i­cally valu­able fishes and inver­te­brates, and the removal of excess nitrogen."

As a result of degra­da­tion, over­har­vesting, and human activity, the global abun­dance of this habitat has declined by 85 per­cent, according to the Nature Con­ser­vancy. Today, most of the world's remaining reefs are con­cen­trated in only six eco-regions--four in the United States.

"Luckily, there are gov­ern­ment and non-government-led efforts that will begin to restore this habitat in 15 dif­ferent states," Kimbro said. "But if an area to be restored con­tains or is likely to develop an out­break of conchs like the one in Matanzas, then the restora­tion effort will fail, regard­less of the expen­di­ture of effort or expense, unless the salinity and conch problem is first solved."

When one of these eco-regions expe­ri­ences an envi­ron­mental stress, like that seen in the Floridian estuary, the impact can be felt across industry and ecosystems.

"Envi­ron­mental change and con­sumer pressure--the conchs being the consumer--can impact foun­da­tion species like oys­ters on their own," Kimbro said. "But we have a case here where it is the inter­ac­tion between the two stres­sors that is causing the greatest impact on the decline of the oysters."

The team found in this case that conchs repro­duce better in water with a high salinity. Because of the deficit of fresh­water and increase in salinity from the drought, conch larvae pro­lif­er­ated, resulting in an abun­dance of the conch, which then led to a greater con­sump­tion of oys­ters on the reef.

While the team deter­mined the conch pop­u­la­tion out­break to be the prox­imal cause of oyster loss, it is the salinity of the water spurred by the multi-year drought that is the ulti­mate cause, because that is what led to a spike in the car­niv­o­rous conch population.

Kimbro says there is opti­mism the reef could recover if the high salin­iza­tion sub­sides. And a nor­mal­ized conch pop­u­la­tion can actu­ally be ben­e­fi­cial to oyster reproduction--after conchs pry open the oyster valves to con­sume the tissue inside, they leave behind a clean internal cavity, which oyster larvae can then use for its own development.

This research, along with addi­tional studies on the conch-oyster dynamic in this eco-region could prove vital to oyster reef con­ser­va­tion efforts.