EPSCoR researcher says menhaden die-offs raise questions

This summer, in early June, Rhode Island NSF EPSCoR and NIH INBRE researcher David Taylor was conducting his weekly seine sampling in the Seekonk River when he saw scores of dead menhaden, a fish in the herring family.

“While I did not do an exact count, it was obvious there were dead fish everywhere,” said Taylor, an associate professor at Roger Williams University. “I’d put the number of dead menhaden in the hundreds, maybe thousands.”

After seine sampling for his RI EPSCoR research on a mid-June day, RWU Associate Professor David Taylor stops on his way back to campus after spotting a school of menhaden. Taylor was hoping to catch either bluefish or striped bass, which prey on the menhaden, for his INBRE research on mercury contamination in local fisheries. Below, a seagull swoops down over the menhaden and grabs a bite to eat.[/caption]

Taylor’s EPSCoR research regularly takes him up the Seekonk and Taunton Rivers as part of his investigation into how climate change is affecting the range expansion of more southern species, like juvenile summer flounder and blue crabs, and the impact of their presence on native Rhode Island fishes.

More recently, on July 29, the RI Department of Environmental Management reported a fish kill that took place in the upper Seekonk River in the evening or early morning hours of July 17-18. A DEM press release stated:

“Approximately 100 dead menhaden were found during the shoreline count at Bishop’s Cove and the Pawtucket boat ramp and pier area. Dead fish were also strewn along inaccessible shoreline areas, and so the estimated total count was in the low hundreds.”

Taylor, who has been sampling in the Seekonk River for seven years, said he had never seen a fish kill in the area until this summer. And, as far as he knew, there were two incidences this year.

Typically, as he crosses Narragansett Bay to or from sampling in the Seekonk, Taylor will see patches of swirling water with an active school of menhaden, pushed up near the surface by predator bluefish or striped bass below. He will idle the boat, grab a fishing rod and catch menhaden for bait to lure the larger fish, which he also studies as part of his INBRE research on mercury contamination in local fisheries.

In some respect, the fish kills are alarming, Taylor said, in that they are a new occurrence for him in the Seekonk River. But, until DEM scientists determine the cause, it remains unknown whether the source is natural or anthropogenic, meaning caused by human activity.

“It’s hard to determine what constitutes a ‘large-scale’ fish kill,” Taylor said. “A challenge that is made more difficult because we can only observe the fish that are visibly dead, either on the surface or washed up on the shoreline. There could be others out of view.”

Low oxygen events — when the percent of concentration drops below 2 milligrams of oxygen per liter of water — that potentially lead to fish kills do occur naturally.

“The situation becomes alarming when the frequency, duration, and severity of those events increase,” Taylor explained.

Low oxygen during the dog days of summer, when the water temperature warms, the water holds less oxygen, the nighttime grows still with little or no wind, and the Bay doesn’t turn over with atmospheric oxygen, doesn’t necessarily signal something wrong.

I’m definitely paying attention given that this is my seventh year and I’ve never seen a fish kill in the Seekonk, and now there’s two in one year. It leads me to believe that the low oxygen events are caused by recent human activities.

So, in that sense, the July fish kill wasn’t overly surprising. Taylor said the weather was hot and the wind was low. But, those conditions did not apply to the larger die-off in early June, he said, when temperatures were cooler and the water should hold more dissolved oxygen.

The other intriguing factor is that when there is a large fish kill, the deaths usually extend to other species. Although, Taylor said, organisms differ in their capacity to deal with decreased oxygen levels.

“Organisms like worms and clams are relatively tolerant to low oxygen,” he said. “Fish are more sensitive. But, the advantage of being a fish is that they can often swim to a better place. Sessile organisms cannot.”

Both the June and July fish kills seemed to be exclusively menhaden from what Taylor observed. And, he said oxygen level readings he took in June during his seine sampling of shallow areas were normal, which suggests the low oxygen was limited to the deeper channels of the Seekonk River.

Additionally, he noted, the large impact area of the July fish kill meant that the menhaden could not reach healthier levels of oxygen in time to survive.

“I’m definitely paying attention given that this is my seventh year and I’ve never seen a fish kill in the Seekonk, and now there’s two in one year,” Taylor said. “It leads me to believe that the low oxygen events are caused by recent human activities.”

In its July 29 press release, the DEM said it would continue to collect information on the mid-July low-oxygen event:

“Large algae blooms often result in low oxygen levels that are lethal to fish. Major wastewater treatment facilities (WWTFs) in Rhode Island remove a large percent of the nutrients through tertiary treatment before releasing their treated effluent into the state’s waters. However, other sources of nutrients such as fertilizers in storm water runoff and discharges from WWTFs that do not provide tertiary treatment are significant sources of nutrients, especially on the Blackstone River.”

DEM officials said they expected additional fish kills would occur while large schools of menhaden continue to congregate in the Providence and Seekonk River areas, until weather patterns change, bringing in a strong wind or a cooling pattern.

Taylor reiterated that the key issue would be whether the frequency, duration and severity of the low oxygen events increased. If they do keep happening in greater numbers and to larger extents, he said, affected areas of the Narragansett Bay could see a community shift where the ecosystem is dominated by species that can tolerate lower oxygen levels.

At this point, though, the fish kills that have occurred are not enough to radically alter the dynamics. Still, said Taylor:

“I do find it alarming; perhaps recent anthropogenic activities have caused an increased pulse of nutrients into the water.”

Story and photos by Amy Dunkle