Town opening Hummock, Sesachacha ponds this week
BY PETER B. BRACE
This morning at around 8:15 a.m. at low tide, the town is opening Hummock Pond to the ocean.

Seawater will rush into Sesachacha Pond excavator is digging a closed trench between Hummock Pond and the ocean, leaving a dam of sand at the pond end that its operator will pierce when the trench is completed. The ponds must be opened during certain narrow windows of time under just the right weather conditions for the openings to really help the pond.

To start with, the ponds, swollen with autumn rain, are best drained at low tide, the ponds acting like locks spilling out their water to reach equilibrium with the ocean so saltwater can flow in. In the case of Hummock, a strong north or northeast wind helps push the water out and west wind does the job for Sesachacha. Hydrodynamics and weather conditions aside, Town Biologist Keith Conant said the main reason for opening these ponds around this time of year is ecological.

"We can't do it during the summer and you don't want to do when the fish are in hibernation [later in the fall or winter] and you want to connect with seasonal migration," he said. "You want to let the juvenile herring in and the eels out, so it's connected to the time of year."

American eels and blue back herring are two species that need these salt ponds to survive. The eels are known as catadromous because they do their spawning out in the Sargasso Sea in the Central Atlantic Ocean, but live most of their lives in river estuaries and salt ponds. Herring are labeled anadromous because the adolescent herring come into the ponds in the fall to spawn and swim out in spring to live in the ocean.

After years of court battles with the Massachusetts Department of Environmental Protection (DEP), Nantucket secured local control of its great salt ponds in October 1993, scoring a standing permit from DEP to open its ponds on a biannual basis. This permit allows Conant to keep the channel open between ponds and ocean should it close prematurely, to allow a good swap of environments and a recharge of oxygen and food.

After 13 years of opening both ponds twice a year, Conant, who is encouraged by the health of both bodies of water, said that they are both better off because of the openings.

Though Sesachacha Pond has been in trouble for a while with fertilizers from the cranberry bogs and effluent from nearby septic systems leaching into the pond and causing algae blooms, which deplete oxygen, he believes the pond openings are helping.

"There seems to be a pretty good fish population in there now, but it's a phytoplankton populated pond," he said.

Conant has been stocking Sesachacha with oysters for several years leading to a natural population continuing to grow each year. This is good for the pond because as filter feeders, oysters live off the bad nutrients that can kill the pond. Though he would like the salinity level of this pond to reach 20 parts per thousand), currently it is at 17 these days, a good sign of the pond's improving health.

Conant said that Hummock Pond is not looking too bad either. This pond usually has salinity at 12pps after an opening and drops down to six after a good rain. The salinity is good enough, he said, to allow barnacles to grow well up toward the head of the pond.

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