Nature Notes: The Drive to Spawn

A reproduction strategy called “swarming,”
After hatching in the Sargasso Sea, young eels make their way into coastal freshwater streams and ponds, where they grow into adults. Decreased access to suitable places to mature may be accelerating the species’ decline. U.S. Fish and Wildlife Service

    Alewives have entered Big Fresh Pond in North Sea in waves beginning two Mondays ago. Most of the ospreys are back, their returns scheduled, it would seem, to coincide with the movement of river herrings — alewives, shads, blueback herrings — from marine waters into fresh to spawn. The double-crested cormorants’ return seems to be tied to the same rhythmic phenomenon.

    There are no seabirds more adept at catching fish than cormorants. They show up at the mouth of Mill Pond in Water Mill every spring precisely at the same time that the alewives begin to enter it from Mecox Bay. Cormorants are the marine counterparts of wolves: They work together to round up the alewives into tight groups to make capturing them more efficient. A cormorant has little trouble swallowing a 10-inch-long alewife in one bite. Bald eagles have also been lurking over the fishways — they probably deign to take an alewife or two, but they specialize in scavenging dead deer and toy dogs and cats where available.

    The alewives come in such large numbers, hundreds at a time into Big Fresh Pond, that the predators can’t take them all. It’s a kind of reproduction strategy called “swarming,” the same as oysters laying thousands of eggs at a time. A tiny fraction, but just enough, will get through and survive the gauntlet of growing to adulthood to keep the species going on an even keel.

    While the adult alewives are running upstream, a tiny baby fish is liable to be going the same way. Alewives, smelt, shads, sturgeons, and many others run upstream to spawn and are said to be anadromous. The tiny baby fish going up with them are one of the very few fish species that go downstream into the seas to spawn when mature — the American eel, Anguilla rostrata. It and the European Anguilla species are said to be catadromous.

    Because the baby eels, or elvers, are transparent to the degree they are — they are called glass eels when they leave marine waters and enter fresh ones — we rarely see them. No doubt would-be predators don’t see them all that well, either. They have another quality going for them: They can surmount almost any kind of barrier on their way to the lakes and ponds where they will spend their adolescent years — they can scale small walls, get through almost any kind crevice or hole, even shinny over wet land using their characteristic sinusoidal gate, much like snakes.

    When one takes into consideration that these little guys come all the way up from the Sargasso Sea, a vast gyre bounded by four separate currents on the west, north, east, and south, where the adult eels spawn, it wouldn’t be fair to travel a thousand miles northwest and northeast, to the American and European coasts, only to be kept from making it to their final goal, a freshwater pond or lake where life is easy and food is plentiful.

    According to Howard Reisman, our bona fide local ichthyologist, there are 15 species of Anguilla in the world, but only three go to such great lengths to spawn. The Japanese eel is similarly catadromous. Elvers reach maturity in fresh waters; the mature eels swim a thousand miles into the south central Pacific where they spawn and then die, just as the American and European ones do. Pacific salmon belonging to several species swim upstream to spawn like their Atlantic counterparts, but as with the three species of eels, die after shedding their eggs and sperm.

    One can see alewives, salmon, and sea-run trout swimming upstream to spawn, but it is rare to see the elvers doing the same. That is why I was so excited upon hearing the news that Matt Stedman of Montauk and a member of the Third House Nature Center witnessed several glass eels emerging from a culvert pipe and entering Big Reed Pond in Montauk on Sunday. I’ve witnessed such an event only twice in a lifetime of 78 long years: The first time when I saw a single glass eel slowly making its way up Brush’s Creek in Laurel from Peconic Bay on the North Fork at about the age of 8, the second when I witnessed for 30 minutes or more a steady stream of glassy elvers moving up Scoy Run through a culvert under Alewife Brook Road in Northwest, East Hampton, in April of about 2009.

    The American eel is a species of special concern, as it is becoming more and more of a rarity. Part of its demise is tied to its popularity as bait for striped bass and several other large fishes that ply the Atlantic. Second, many of their maturation ponds or the streams to them no longer exist, have dried up, or have been closed to elvers in some other way. The oiling of freshwater ponds and treating them with DDT to control mosquitoes in the second half of the 20th century didn’t help them either.

    Male eels are more estuarine than the females, which stay in fresh water until they are five or six years old, three feet long, and as big around as a tennis ball. When the appropriate signal is given they go downstream. The males, which have become silvery and are called silver eels, follow or join in somewhere along the way, and they all proceed to the sea and head to the spawning grounds, a one-way trip, as it were, to collect and shed their eggs and spawn in a swirling mass made up of both sexes deep down in the sea.

    Larry Penny can be reached via email at