When lethal red tides sweep in, salmon perish
Story Mark Hume
Late one day a few years ago an evening shift worker at a salmon farm near Cypress Island, in Puget Sound, noticed the fish were acting jumpy and ran what had become a routine test for toxic algae.
Fish farmers around the world are on alert for harmful algae blooms, or HAB’s, because when they drift in to an area they can wipe out thousands of fish overnight.
Earlier in the day tests at the Cypress farm had shown no signs of algae, but when the later results came back there were 150,000 cells of Heterosigma per litre of sea water.
A dangerous algae bloom was exploding.
According to a chronology of events contained in a technical report by Dr. Jack Rensel, a research scientist and consultant based in Washington State, the fish farm immediately cancelled plans to feed the salmon that night, hoping to reduce the amount of algae ingested.
The next morning Kevin Bright, the site manager for the farm, did an overflight to get a fix on just how big the HAB was. He saw the tell-tale red streaks left by clouds of Heterosigma around Cypress Island, Guemes Channel and north of Sinclair Island.
It was spreading fast and intensifying.
Cell counts start out low in the morning hours but continually climb throughout the day. Peak counts are 1.1 million cells per liter at 3:45 PM, wrote Dr. Rensel.
The next day a flight showed the HAB had spread dramatically, apparently pushed by the freshwater plume pouring out of the Fraser River, in British Columbia, just north of the Canada-U.S. border, where it is believed the bloom began..
Counts peaked at 1.8 million cells per liter around 5:00 PM which was toxic to the fish, Dr. Rensel reported.
By the next morning four days after the HAB had exploded from a count of zero cells per liter – salmon in the net pens began to die.
Fish were visibly stressed throughout the day, with fish gasping and rolling over in the corners of the pens. Mortality increased significantly in the afternoon with increasing cell counts recorded during the beginning of flood tide, wrote Dr. Rensel.
At one point, tests showed four million cells of algae per litre.
Over the next few days the cell count dropped, and then, almost as quickly as it had appeared, the bloom was over.
Four fish farms in the area lost 364,000 subadult Atlantic salmon to that toxic algae bloom, which hit in 2006.
That wasn’t the biggest kill.
In 1990, 649,544 subadult Atlantics died in three farms when an algal bloom swept through Central Puget Sound. And there have been several incidents with smaller death tolls.
Nobody knows how those blooms affected wild salmon and trout, but there have been reports of small numbers of cutthroat, coho, chum and chinooks being found dead near shore after blooms.
Dr. Rensel says that fish usually sink when they die in cold water, and that could explain why there have been no reports of massive wild salmon kills linked to HABs.
But what he witnessed in the Cypress Island fish farm, may well be what takes place over a much larger area with wild salmon.
Heterosigma blooms in Puget Sound may be doing much more damage than killing farmed fish, writes Dr. Rensel. Wild salmon and other fish as well as invertebrates and plankton may be adversely affected by sublethal effects or killed. The most vulnerable are juvenile fishes, invertebrates and plankton that are restricted to the surface and near-surface waters where the blooms persist. Near-surface migrating fishes such as sockeye salmon that pass through the surface waters of the area in potential bloom periods may also be affected.
Dr. Rensel reports HABs can cover huge areas. One bloom in southern Georgia Strait and Puget Sound spread over more than 30,000 square kilometres before it disappeared, about two weeks after it began.
Testifying in August, 2011, before the Cohen Commission of Inquiry, in Vancouver, Dr. Rensel said it’s not known how the algae kills fish. It might kill them directly, it might affect the functioning of their gills, it might weaken them, leaving them open to death from other disease.
What he does know, based on 20 years of research, is that when there are big blooms in Georgia Strait, two years later there are poor returns of salmon.
There was a massive bloom in Georgia Strait in 2007. In 2009, only about one million sockeye returned to the Fraser River when more than 10 million had been anticipated. The collapse of that run convinced Prime Minister Stephen Harper to appoint British Columbia Supreme Court Justice, Bruce Cohen, to hold an inquiry into what happened. That $26 million inquiry is due to report next year.
Dr. Rensel told the Commission that algae is a prime suspect in the mystery of what happened to all those salmon in 2009.
Dr. Rensel didn’t say that algae blooms killed 9 million sockeye. But the incident at the Cypress Island fish farm certainly indicates that is possible. Sockeye swim close to the surface, and an algae bloom spread of thousands of kilometers would be impossible for them to avoid.
Toxic algae blooms have become an increasing problem in oceans around the world, from Chile to China, and from Scotland to the East Coast of North America.
The blooms are triggered by a combination of factors including nutrient levels in the water, temperatures, salinity and the amount of sunlight. It is thought that climate change and warming oceans together with increased nutrient loading from pollution can only exacerbate the problem.
There is speculation that nutrients released by sewage plants and by fish farms are contributing factors, but the science on that is not conclusive, and blooms have occurred in the Pacific Northwest long before salmon farms were introduced.
A lot remains unknown.
There is a need to investigate wild fish kills involving HABs more completely and expeditiously and to document accompanying hydrographic and meteorological conditions. These investigations can form a basis for understanding HAB dynamics, trends and possible connections with land and riverine management practices. Nitrogen loading trends for streams, rivers and municipal discharges should be examined for nutrient-sensitive areas, Dr. Rensel writes.
In the U.S. some serious research efforts are underway.
But in Canada, where toxic blooms erupt with some regularity in southern Georgia Strait, the Department of Fisheries and Oceans currently has no one investigating HABs, or the impact toxi algal blooms have on salmon.
Formerly DFO had a harmful algal bloom research program and researchers at University of B.C. and Simon Fraser University were involved in basic research, but DFO terminated the program about 6 years ago and the academics either retired or moved on, Dr. Rensel states in an affidavit filed with the Cohen Commission.
The Cohen Commission won’t hand down its report until next year. But based on Dr. Rensel’s testimony, it is probably a safe bet to say that it will call for an intensive study of HABs. It could well be that the red tides which until now have been thought only to affect shellfish, could be killing salmon by the millions.