This article is made possible, in part, by funds from Maine Sea Grant and the Oak Foundation.

Rumors of their arrival ripple along the docks and across the surface of bays and estuaries. Fishing boats head out to the offshore ledges and banks searching for herring. Enthusiasts of all ages head down to the wharf or out in skiffs to try for pogies. Lobstermen call around to see who has fresh bait and to check prices.

Herring and menhaden may be small-only 10 or so inches long-but they support valuable commercial fisheries, fill the bait-bags for Maine’s lobster fishery and are eaten by just about every predator in the Gulf of Maine.

Herring are the staple bait for Maine’s $245 million lobster fishery. As herring stocks recovered from extreme depletion in the mid-1970s, bait was plentiful and cheap, with the ex-vessel price for herring nearly constant at about six cents per pound from 1977-2002, according to the Maine Department of Marine Resources. But since 2002 the price has increased every year, reaching 13 cents in 2008. This year, fisheries managers have set a lower catch limit on the herring fishery in the inshore area of the Gulf of Maine, based on scientific assessment of the size of the herring population, leaving lobster fishermen worried about a dramatic price increase for herring bait. They are thinking about alternatives, like menhaden.

Some lobster fishermen already use menhaden, also known as pogies, for bait. But menhaden have not been plentiful off the coast of Maine since the late 19th century, when abundant schools fueled construction of more than 20 oil and fertilizer factories in the state. Today, 90 percent of menhaden are caught off Virginia and North Carolina and go to one factory: Omega Protein in Reedville, Va., the world’s largest producer of omega-3 fish oil and North America’s largest manufacturer of fishmeal.

Menhaden, like herring, are small, silvery schooling fish that spend their winters along the mid-Atlantic coast south of Cape Cod, and migrate north into the Gulf each spring when conditions are ideal for feeding and spawning. Herring typically migrate north during April, and disperse onto Georges Bank and throughout the Gulf of Maine; menhaden usually arrive a few weeks later, in late May or early June. Both tend to aggregate where their food concentrates.

So what determines whether-and when-herring and menhaden show up along the coast of Maine? Many factors likely contribute including temperature, salinity, localized food availability and population size.

Herring seek out productive offshore areas like Platt’s Bank, Fippinnes Ledge and Cashes Ledge, 20 miles or more from shore, although they sometimes come closer to shore along frontal regions and local upwelling zones. According to Bill Overholtz, fisheries biologist with the National Marine Fisheries Service in Woods Hole, Mass., herring filter-feed or “graze” on zooplankton and “particle feed” on larger animals, including larval fish and euphasiids (krill). Lew Inzce, a marine ecologist at the University of Southern Maine, recounts a scene on Platt’s Bank in 2005, when huge numbers of krill attracted predators from herring to humpback whales. The feeding frenzy continued for five weeks. Incze and his team returned to the same area for the next three years and never saw the phenomenon again. Incze notes that herring quickly “clean out” the available food in an area and move on, similar to tuna, whales and other pelagic species that aggregate in different areas each year depending on where the eating is good.

Menhaden prefer warmer water and feed on smaller plankton, and so they usually travel closer to shore than herring and often aggregate in shallow waters at the mouths of rivers and estuaries. Menhaden feed by filtering large volumes of seawater, retaining microscopic particles (between 5 and 15 microns, which in most places means phytoplankton but can include zooplankton and miscellaneous particles of organic matter). Other species of marine fish are not equipped to filter phytoplankton from the water, and this habit of feeding on a plentiful food supply allows menhaden to prosper in very large schools.

In addition to local patchiness in food availability, researchers have looked at whether water temperature, salinity or population size could affect the timing and location of migrations during particular years.

Janet Nye, a post-doctoral researcher at Northeast Fisheries Science Center in Woods Hole, has documented year-to-year differences corresponding to temperature. “Herring respond very quickly to changes in temperature, on a year to year basis they will be further north if it’s a warm year and further south if it’s a cold year.”

Matthew Cieri, a biologist with the Maine Department of Marine Resources, said that there is some suggestion, very preliminary, that freshwater may affect menhaden schools. “In years with a lot of runoff, we tend not to see menhaden as much due to the freshwater influence. There were lots of fish in 2008, but not in 2009 which was a wet year and the fish stayed south of Cape Cod,” he said.

Menhaden presence in the Gulf of Maine, the northern extent of their range, is linked most strongly to the survival of juveniles in the Chesapeake Bay region.

“For most species, more adults means more young. This relationship doesn’t hold for menhaden because environmental factors-which influence the larvae and young as they move to inshore nursery areas-are a stronger influence,” Cieri said. “Menhaden are one of those species, like Atlantic herring, that have really strong year classes. As fish get older, they tend to move farther north. The stronger the year class in Chesapeake Bay (the more fish), the more likely we are likely to see them as three to four-year-old adults in Maine.” Unfortunately, recruitment in the mid-Atlantic has been declining for the last three decades.

Herring and menhaden have gone through cycles of abundance and depletion in the Gulf of Maine-likely linked to both fishery and environmental drivers-and as a result it has been difficult for researchers to infer reasons for these historic changes and predict how the fish will behave any given year. Without being able to predict where and when the fish will show up, those betting on baitfish face uncertain odds.

Heather Deese, Ph.D in oceanography, is the Island Institute’s director of marine programs. Catherine Schmitt is communications coordinator for Maine Sea Grant.