St George School Investigates the Mystery of the Alewives

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Guest blog post by Alison England, middle school science teacher from St. George School in Tenants Harbor.
With help from the Vital Signs Freshwater Mini-grant, St George School students are following leads in our community mystery of why alewives are not returning to our marsh following several years of restocking and the brief "appearance" of a small group of alewives coming into our marsh two years ago.

We are investigating the effects of salt water intrusion into our marsh on the larger tides. We'd like to find out how the salinity of our freshwater marsh changes during the tide cycle of a flooding tide, how far into the marsh salinity changes are measurable, and how persistent the saltwater intrusion is. Here is a description of some field work as written by one of our students for our blog.

Where are the alewives?

Contributed by Maggie G.

The watershed of our marsh is in trouble, especially for the alewives.

As you might already know, the alewives that used to thrive in our town disappeared for over 30 years, leaving everyone wondering ‘What happened?’ Two years ago we got another shock; they came back! But there might be another problem on our hands now…

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Photo by A. England

In science class, we have been studying the salinity in the marsh. Using the refractometers that measure the salinity in water, we took two samples from the marsh and three different points near the stream and the culvert on May 16th. We took the samples back to the classroom and measured how much salt was in the water in each sample. Fortunately, water coming directly from the marsh was found to have zero parts of salt per thousand parts of water (0 ppt). In the stream itself we found anywhere from 4-27 ppt because of the tidal influence. We do think that some tides bring salt waters right over the outlet and directly into the marsh.

Now, you may be thinking “Okay, so there’s a little bit of salt in the waters of the marsh. What’s the big deal? And how does this relate to the alewives?” Here’s the problem about the salt in our watershed; research suggests that alewife eggs develop best in less than 5 ppt of salinity. Now is where the other part of our studies comes in.

Along with studying salinity, we have been studying tides that can flood into the marsh. According to the ‘rule of twelfths” of a tide cycle, we can go down to the marsh at any time and find out if the coming (or leaving) tide will (or has) flood (or flooded) the marsh. Here’s an example of how to calculate the final height of the tide; Say it was 1 hour before the coming 8.7 foot high tide. First, we would measure the water in the culvert, and let’s say that that we measured 10 inches in the culvert. Then we go back to the classroom and start calculating, knowing the time we made the observation, the time of the high tide, and the predicted depth of the tide. First, we find 1/12 of the 8.7 foot tide since there is another hour of water to come in on the tide. One-twelfth of 8.7 feet is 0.73 feet. Then we find out how many inches 0.73 feet is. That answer is 8 and 1/2 inches. The next step is to add this to our original measurement of 10 inches for a total of 18 and 1/2 inches. Since the difference in elevation between the bottom of the culvert and the outlet of the marsh is approximately three feet, this high tide of 8.7 feet, with 18 – 1/2 inches of water in the culvert would not flood the culvert.

In conclusion, the marsh’s alewives run may be in even more trouble, even if you can’t see the damage as its happening. To keep you updated, you could check out our website.

The Vital Signs Freshwater Mini-grant is generously supported by a grant from the Maine Outdoor Heritage Fund (MOHF). MOHF is a program through which proceeds from the sale of a dedicated instant lottery ticket (currently Lucky Catch) are used to support outdoor recreation and natural resource conservation. For more information visit MOHF.