Wednesday, April 29, 2009

You Can Go Home Again

The Bronx has a few new residents. Though, to be honest they are actually native Bronxites—more returnees than new arrivals. They're also silver.

In March, a trap net set in the Bronx River by the Natural Resources Group of NYC Parks and Recreation captured half a dozen alewife (Alosa pseudoharengus). Like salmon, alewife are an anadromous species, meaning they are born in fresh water, spend much of their life in the ocean and return to fresh water to spawn. But they don’t return to just any patch of fresh water—they return to the same river or stream where they were born. By all indications, the fish that appeared in NRG's nets were likely born somewhere in the quiet water above the dam at 182nd St. three years ago.

Their reappearance is not particularly miraculous, but it does speak to the surprising ability of a heavily urbanized waterway to support a diversity of life. Together with the appearance of Jose the beaver in 2006, it also speaks to a general resurgence in the overall health of the river that has occurred as a result of hard work on the part of community members, local non-profits, and government agencies. The return of the alewife is a good example of what can happen when this partnership mobilizes.

In 2006, a load of 200 adult alewife from Bridle Creek in Connecticut were introduced to the Bronx River (the actual release happens from 4:24 to 5:35 in this video) in the hopes that they would mate and their offspring, or fry, would return when they were ready to spawn three or four years later. Analysis of the scales on several of the fish caught in NRG nets showed that the fish were three years old.

"Unless alewife suddenly came into the Bronx River or unless they had always been coming and we hadn't seen them, it's reasonably likely that this represents a return of some of the animals that were born in the river in 2006," said Joe Rachlin, an aquatic biologist at Lehman College who helped plan the release.

There is no historical evidence that alewife, in fact, ever called the Bronx River home, but they are common in nearby streams. Studies by Rachlin and others have also shown that the river offers the right habitat for spawning and for hatchlings and that, despite appearances, the river contains almost all of the fish species known to have inhabited the waterway. The fact that alewife have never been found in the Bronx River until now may, therefore, be less an issue of water quality and more a result of the fact that the river has been dammed continuously since the 1600's, effectively preventing the adults from reaching their freshwater spawning grounds.

Both the young and adults are an important part of a healthy river’s ecosystem. They feed on microscopic zooplankton in the water column and, in turn, serve as prey for waterbirds and mammals. In 2006, however, the U.S. National Marine Fisheries Service declared (pdf) alewife and blueback herring (collectively known as river herring) "species of concern." The steep declines in stocks of both fish since 1990 are largely due to the human tendency to dam the coastal rivers and streams that alewife need to complete their lifecycle.

That raises a difficult question on the Bronx River (and on rivers up and down the East and West Coasts): What to do with the dams that currently stand between the ocean and the rest of the river and that serve no real purpose. But that's a story for another post. For now, welcome back A. pseudoharengus. We missed you.

Photo courtesy of Rocking the Boat

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