July 11, 2018

KEVIN WADLOW/Free Press
A dead needlefish floats amid a sargassum mat off Key Largo on July 4. More than two dozen fish kills throughout the Florida Keys have been reported since June 12. The most likely cause is depleted oxygen levels in the water, state experts say.

KEVIN WADLOW/Free Press A dead needlefish floats amid a sargassum mat off Key Largo on July 4. More than two dozen fish kills throughout the Florida Keys have been reported since June 12. The most likely cause is depleted oxygen levels in the water, state experts say.

FLORIDA KEYS — Dead fish began surfacing in Florida Keys waters on June 12, launching a streak of 28 fish kills reported to the state through July 2.

Sites of the local incidents reported to the state’s Fish Kill Database range from Summerland Key near mile marker 25 to the Ocean Reef Club at the northern tip of Key Largo, a coastline distance of about 90 miles.

“Monroe County normally is almost never on that daily list” of fish kills compiled by the Florida Fish and Wildlife Conservation Commission, said Keys FWC Officer Bobby Dube. “The last few weeks, it seems like it’s been on there almost every single day.”

Sizes of the floating fish cover the gamut from baitfish to tarpon, sharks and lobster. Thirteen of the reports came in a two-day span, June 21 and 22.

A dozen water samples taken between Key Largo and Summerland found no evidence of an algae bloom or red tide, state Fish and Wildlife Research Institute information officer Michelle Kerr said Friday.

The most likely culprit appears to be warm temperatures that suck oxygen from the water.

“Low dissolved oxygen can lead to localized fish kills,” Kerr said. “In reports from Plantation Key and Big Pine Key, they found very low dissolved oxygen levels.”

Processing of other water samples is ongoing. Kerr noted that not all of the citizen reports have been confirmed, and some may be different reports from the same location.

Many of the recent Keys fish kills took place in canals, a common environment for dissolved-oxygen problems.

“Many factors can contribute to a fish kill. Weather-related factors are common causes,” Kerr said. “Sudden temperature fluctuations or extreme temperatures can result in fish kills any time of the year.”

“In Florida, most [dissolved oxygen]-related fish kills occur in the warmer months from May through September,” she said. “Once a kill starts, there is nothing that can be done.”

“Warm water holds less dissolved oxygen than cold water, so summer is the time when fish can have a hard time getting enough oxygen,” says a general U.S. Geological Service summary of fish kill causes.

Aquatic plants produce oxygen during the day, but nights and cloudy days, or excessive plant growth, can cause marine organisms to drain the available oxygen out of the water. That essentially causes the fish to suffocate.

To report a fish kill, log onto the website myfwc.com/Fish Kill, or call the FWC Fish Kill Hotline at 1-800-636-0511. Mobile devices can use the “FWC Reporter” app, available for iOS or Android.

kwadlow@keysnews.com