What's killing the most important fish in the sea?

on August 14, 2007 at 2:15 PM, updated March 19, 2009 at 10:11 AM

A natural cycle?

Dead menhaden - known locally as bunker - float in the thick guck of a Great Kills Harbor backwater.

Death overtakes a hardy species
The National Parks Service says the thousands of dead fish washing up on Island beaches is "routine," a natural occurrence. Island fishermen scoff at the notion, saying it's clear that water pollution is killing the fish.

Which side is correct? In a way, they both are.

The fish in question is the menhaden. Down-easters in Maine call them pogys. Around New York they're known as bunker. In some fishing grounds they're called bugmouths; in others, alewives or fat-backs. Scientists refer to them as brevortia tyrannus. Many names, same fish. And an interesting critter it is.

Menhaden are very oily and they taste awful. But they are still commercially useful, though, as a source of oil. They are also ground up for animal food and fertilizer. So, as with any easy-to-catch-and-sell commodity, the fishermen tend to overdo it, and in some areas have brought the menhaden population dangerously near collapse.

Fact is, the fish may be worth more to us in the sea. The menhaden has been described as "the most important fish in the sea" because of its ability to clean up the water. A "filter feeder," the menhaden cannot choose what it eats. Instead it swims with its mouth open, scooping up all that flows in and filtering up to four gallons of water per hour. The more menhaden we have swimming around our shores, the cleaner the water will be.

John Ruskin, a poet, artist, social critic and all-around deep thinker once opined that "No human being, however great, or powerful, was ever so free as a fish." He had it wrong. Fish have no freedom to stray from their habitats. The finny denizens of the warm, shallow littoral zone could no sooner exchange places with the cold, deepwater domain of the Atlantic mackerel than they could with a freshwater rainbow trout. So the menhaden are stuck in their coastal neighborhood, for better or worse.

That's where we come in. The water can be full of pesticides and fertilizers and mercury and other things that are unhealthy, even to such un-picky eaters. What's more, when heat and mud and sewage overflows (the result of storms) and other factors combine to lower the oxygen content of the water, the hapless menhaden may quite simply suffocate and drown. A bad situation, oxygen-wise, can be made worse if the menhaden are herded together and terrorized by a voracious school of predatory bluefish.

Oxygen depletion happens. So, in that sense, the die-offs are normal. Pollution is a leading cause of oxygen depletion. So the fishermen who don't buy the dismissal of the die-off as unavoidably "routine" are correct, too.