What causes massive fish die-offs?

On Monday morning, November 14, 2016, residents of Hampton Bays in eastern Long Island, New York gathered at the Shinnecock Canal to stare at the tragic sight of thousands and thousands of dead fish. As you might expect, many locals voiced concerns about pollution, but experts provided a much more likely explanation for the die-off: the fish suffocated.

This is pretty disturbing...this was the Shinnecock canal this morning pic.twitter.com/02pvd8lCKy — Rude Gyal (@JustineAiko) November 14, 2016

When fish die in large numbers, it's officially called a "fish kill". These events can have a variety of causes, but among the most common is a lack of oxygen. Fish don't breathe air, but they still take in oxygen dissolved in the water around them, and if those oxygen levels drop too low, it can spell disaster. As the USGS Water website points out: "That's why it's important to have an aeration device, a bubbler, in your home aquarium."

When the locks in Shinnecock Canal were closed at 3:00am on Monday, they locked in a large school of Atlantic menhaden, or bunker fish. According to the Department of Environmental Conservation, these fish probably fled into the canal to escape predators, and then became trapped. Local government and university experts have pointed out that with so many fish in a small, sealed-off area, oxygen levels would have dropped quickly. By the time the locks were opened at 10:00am, many fish had already died.

This event may be dramatic, but it isn't unique. A similar fish kill situation occurred in California in 2011, where thousands of sardines apparently crammed (you know, like sardines) into a harbour to escape rough waters, and became stuck there as oxygen in the water depleted.

In fact, fish kills are a lot more common than you may think. The Southwest Regional Office of the Florida Fish & Wildlife Conservation Commission reports that, across just 13 counties in Florida, between 100 and 150 fish kills are recorded each year, most due to low oxygen. This tends to be more common in summer months, as warmer water holds less oxygen. Stonybrook University professor Christopher Gobler noted that the waters of Shinnecock Bay have been unseasonably warm lately, which may have contributed to this latest mass death.

Long Island is no stranger to low-oxygen fish kills. In May and June of 2015, three major fish kill events occurred in the Peconic River, killing hundreds of thousands of bunker fish. Scientific investigation found that the waters were not toxic, but that the river had experienced extended periods of low oxygen. The same explanation was reached for similar fish kills in the river in 2008 and 2009.

The low-oxygen levels in the Peconic were found to be related to high water temperatures, high concentrations of fish, and perhaps most importantly, large amounts of early-blooming algae, whose growth was boosted by nutrients (nitrogen and phosphorus) added to the water from human sewage and other runoff.

Algae survive through photosynthesis, which means they release oxygen during the day. But at night, they "breathe" the same way we do: taking in oxygen and releasing carbon dioxide. And when algae die, the chemistry of their decomposition removes yet more oxygen from the water. All in all, big algal blooms can quickly create a fatally low-oxygen environment, particularly when fuelled by human pollutants in warm waters populated by lots of fish.

Algae can also form infamous "red tides", large toxin-releasing blooms that can accumulate in fish and spread through the food web. These happen nearly every summer in the Gulf of Mexico, and a particularly dramatic case occurred last year on the west coast of North America, stretching from California to Alaska. Aside from forcing many fisheries to close for fear of toxic harvests, that incident was also linked to widespread illness in birds, sea lion strandings and whale deaths.

The list of factors that can cause mass fish mortality is actually pretty extensive. A fish kill in Arkansas in 2011 was suspected to be caused by disease since it only affected drum fish, while a case in Chesapeake Bay that same year was thought to have been the result of cold stress from an unusually frigid winter.

As for the Shinnecock Canal, officials have stressed that this new fish kill was unrelated to toxins or disease, or an unhealthy ecosystem. In fact, this aquatic accident wouldn't have happened without a thriving fish population, though the Suffolk County Health Department has assured residents it will test the water for pathogens and pesticides, just in case.