A widespread fish kill is underway across Mobile Bay. It appears to be affecting primarily filter-feeding fish such as menhaden, sardines, alewives and shad.

Dead fish are present in the shallows and on beaches on both sides of the bay, from Point Clear to Daphne on the eastern shore and from Arlington Point south to Fowl River on the western shore. Dead and dying fish also dot the surface of the bay, from one side to the other.

The kill does not appear to be related to a red tide bloom occurring in the Gulf of Mexico and around Dauphin Island. Instead, the bloom in the upper bay appears to be another species of algae with a similar neurotoxic effect on fish.

Fish affected by the algae swim in a markedly erratic fashion, zipping straight ahead for a time, then falling into lazy circles, often swimming on their side or even upside down.

AL.com collected samples of sick fish and the water where they were found during trips across the bay, from Fairhope to the mouth of Fowl River, on both Tuesday and Wednesday. Those fish were provided to the Dauphin Island Sea Lab for analysis. In addition, AL.com measured oxygen levels and salinity where the samples were collected using a YSI meter, a standard tool for environmental sampling.

“There does not appear to be a correlation with oxygen or salinity,” said Andy Depaola, a microbiologist with the U.S. Food and Drug Administration lab on Dauphin Island. Depaola participated in the AL.com sampling trip. “Those oxygen levels were not low enough to kill fish.”

Alison Robertson, who specializes in harmful algal blooms with the Dauphin Island Sea Lab, said she had ruled out red tide, which is caused by an organism called Karenia brevis, in the bay. The red tide algae prefers the high salinity found in the Gulf of Mexico. It does not do well in lower salinity, particularly the low levels of salt measured in the AL.com sampling.

Robertson said she had received numerous reports of dead fish from locations all around the bay. She is working to isolate the algae causing the problem.

“Along the Gulf coast, Destin to Dauphin Island, we have had many fish kills over the past 7 weeks that we have shown were associated with Karenia (red tide) blooms offshore,” Robertson said.The toxins associated with red tide were found in the guts and flesh of fish from those kills.

“The fish kills being seen in and around Mobile Bay at the moment are inconsistent with that, and there are in many cases none or at least very low numbers of Karenia in the Bay waters that we have tested alongside affected fish,” Robertson said. “Lots of small menhaden are being reported in many areas — from Weeks Bay to Fairhope, to Fowl River — exhibiting spiraled swimming or swimming sideways. We tested dissolved oxygen in over 26 sites over the past 2 days and everything looks great, levels between 7-10 mg/L which is exactly what we would expect based on the water temperature.”

Robertson said her lab was investigating whether there was another phytoplankton species affecting the fish. She said that other most other algal species do not pose a human health concern.

“We are following those leads right now,” Robertson said. “A few fish collected were dissected and examined under microscope but there are no overt signs of problems and the gills look healthy.”

She noted that all of the samples provided to her by AL.com were small juveniles.

“While this is concerning and we will continue to look at the cause of the kills, we are really only seeing the very small planktivores being affected at the moment,” Robertson said. “And fish such as menhaden are particularly sensitive to changes in their environment.”

Dead or dying fish were visible at all times during the AL.com trips across the bay. Pelicans were seen swimming around on the surface of the bay scooping the dead fish out of the water. Likewise, pods of dolphins were seen gorging on the stricken fish.

Algal blooms in the bay typically peter out after a few days. This bloom has been particularly long-lived, and is already into its second week.

While only filter feeding fish have been affected, fishermen around the bay have reported that both speckled trout and redfish appear to have vacated the areas where the sick fish are visible.