Global oyster habitats on decline – Blame anthropogenic activity

A new study by scientists at Northwestern University has found that estuaries suffering the negative effects of drought are becoming saltier, opening the door to a calamitous decline globally of oyster reefs.

Marine and envi­ron­mental sci­ences pro­fessor David Kimbro, along with graduate student Hanna Gar­land, linked the deterioration of oyster reefs in Florida’s Matanzas River Estuary (MRE) to a pop­u­la­tion out­break of car­niv­o­rous conchs, and increased salinity caused by a regional drought, says Science News Online.

But the study is not just about not having oysters on the local restaurant menus. “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.”

The saltier water actually encourages the proliferation of conch larvae, and these larvae feed on oysters. The study found that under controlled circumstances, oysters could thrive regardless of the salinity of the water, as long as carnivorous conchs were absent. But the researchers found that the conchs do poorly in water with low salinity, but reproduced better in water with higher salinity.

These findings led to the conclusion that starting in 2006, when salinity levels in the southernmost parts of the estuary increased because of reduced inputs of freshwater, this led to an increasing population of carnivorous conchs, drawn by the increased saltiness of the waters.

Looking at the bigger picture, over-harvesting, degradation and pollution have led to an 85 percent decrease in global habitats, according to the nature Conservancy. Of the six ecoregions with oyster reefs left in the world today, five of them are in the United States, according to the study.

“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.”

Kimbro says that while a declining oyster population loss hurts the industry and the ecosystems, it is ultimately the increasing salinity of the water caused by multi-year droughts that is to blame, and not the conchs.With droughts becoming increasingly common with the changing climate, the overabundant conch populations may become a long-term problem.

This study was published in the journal PLOS ONE on August 14, 2015, entitled: “Drought Increases Consumer Pressure on Oyster Reefs in Florida, USA.”