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Why is our Tappan Zee Project so cool?

By Susannah Black
April 10, 2015

I’ll tell you.  If you’re interested.  This may take a little while, about 5 minutes…

Here goes.

We started restoring oysters up there in 2010 as partners in NY/NJ Baykeeper’s Oyster Restoration and Research Project.  Together with our partners, we built a small reef (50 square meters) and seeded it with 50,000 oysters grown in our facilities on Governors Island.  The oysters were the first that had gone through the stressful transition from free swimming larvae to sessile, shell bearing juveniles in captivity in New York Harbor.  This was historic for us because before that time we did not know if oysters could metamorphose in New York Harbor water.

The oysters survived that critical time and then spent a season growing like crazy in the ecodock.  It was at this time that the ecodock became the largest concentration of oysters to exist in NYH in over 100 years.

On the Ecodock: Hauled Out

On the Ecodock: Hauled Out

We advocated to have the oysters planted on the six reefs before the end of August while they still had a few months of warm water.  Oysters only pump water during warm weather (when the water is over 50 degrees, generally from May through November).  During the “growing season” oysters can pump up to 50 gallons of water per day.  This pumping helps to keep them free of sediment and allows them to grow as sediment accumulates which helps to keep their bills above the level of the sediment.  As it happens the reefs were not ready for planting until the beginning of November.  By that time, based on water temperature, we knew that the oysters were no longer pumping.  They were dormant for the winter and at the mercy of water currents and sediment for the remainder of the fall, winter and spring.

The spring of 2011 was a wet one.  Up by the Tappan Zee we had what is known as a prolonged freshet.  There was so much melt water and runoff coming down the Hudson that the water was totally fresh for a period of two weeks.  Oysters need some salt in the water.  Anywhere from 2-5 parts per thousand is considered the lethal limit for oysters (full ocean is 35, here on Governors Island it’s generally in the 19-25 range). The freshet happened to coincide with the water temp passing that critical 50 degree point.  So our oysters woke up after a winter spent essentially hibernating and had no salt in the water for a period of two weeks.  Not surprisingly they all died, all 50,000 of them.

Hudson River Surface Salinity, 3/21/11.   Credit: Stevens Institute of Technology

Hudson River Surface Salinity, 3/21/11.
Credit: Stevens Institute of Technology


The young oysters on the reef had died but there were old oysters, wild oysters, on the rocks surrounding the reef that had survived.  This was the first of many instances over the last several years where oysters in New York Harbor have survived when, according to the literature, they shouldn’t have.

We still don’t know what made the wild oysters better able to handle the lack of salt in the water.  Was it that they were used to it? Oysters are basically starving in the spring and vulnerable to stressors.  It’s possible that our oysters were shocked to death when they emerged from their winter nap.  It’s also possible that the wild oysters are genetically better able to cope with the fresh water because they have survived for many generations in that environment.

Wild oysters from just south of

Wild oysters from NY Harbor south of Tappan Zee

This creates so many interesting research opportunities.  Could we take oysters from up there, spawn them in our moderately saline environment and then return them to the site? What if we took our own oysters and acclimated them more gradually to those conditions?  Are the oysters up north a genetically distinct group? Or are all the oysters in New York Harbor part of one large population whose members are more similar than they are different?

Rather than put our energies into answering these questions we decided to continue to develop the two reefs that showed the most promise at Governors Island and in Soundview Park at the mouth of the Bronx River. We haven’t been able to explore these questions until now.

In the fall of 2011, environmental surveys were conducted as part of the Environmental Impact Statement (EIS) for the construction of the new Tappan Zee bridge.  The project was moving forward quickly with the support of the Governor and the President.  The EIS was not expected to reveal any important habitat in this area of the Hudson.

Photo Credit: New York State Thruway Authority

Photo Credit: New York State Thruway Authority

Amazingly, surveyors found oysters.  Lots of oysters. They found big oysters and small oysters, in densities up to 300 per square meter.  These oysters were surviving in deep water (over 30 feet)– water that was fresh each spring for some days or weeks.  All in all it was determined that the new bridge would displace over 13 acres of oyster reef habitat.

The Department of Environmental Conservation awarded the permit to the Thruway Authority on the condition that they restore this habitat.

Photo Credit: New York State Thruway Authority

Photo Credit: New York State Thruway Authority

So now the Thruway Authority is on the hook for 13 acres of reef (65 million oysters) that no one knows how to build.  No one knows what oysters they should use, how they should build the reefs, where to build them, how big the oysters should be when they get put down, what predators (if any) will eat the oysters, how well oysters will reproduce up there or whether or not diseases will be a problem.

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In early tests to get a better picture of water quality in the area, scientists installed four SONDEs (electronic water quality monitors) on the bottom of the river.  These devices were equipped with floats on acoustic releases.  The idea was that you could get the boat close and then emit a special sound into the water, this would trigger the release and the float would come to the surface to allow for retrieval after a few months of data collection. Each of these units cost about $25K so you can imagine the dismay of the scientists when they reached their sites, emitted their sounds and no floats came to the surface.  The scientists called in others, who arrived with multi-beam sonar imaging technology and professional divers.  They quickly determined that the visibility was too bad and the current too strong for diving.  Eventually, they were able to recover one unit.  The other three were lost….

Naturally, it is at this stage in the game that they call in Harbor School/BOP.  They need people who can dive in these challenging conditions, design and build unique reef infrastructure, install and relocate underwater objects, design and carry out research projects to start answering some of these questions– and, of course, grow the oysters.

Harbor diving students get their pre-dive brief from Joe Gessert.

Harbor diving students get their pre-dive brief from Joe Gessert.

I just think it is so cool that the Thruway Authority is looking to teenagers to figure out how to build their 13 acres of reef.  Of course we will have tons of help from all of our partners, but we are becoming the experts in many of these areas.  And that’s being recognized.

–Pete Malinowski