Most days, the ocean can seem deceptively calm, with no signs of the breeding, feeding and migrating mayhem below the surface. The Gulf of Maine is home to an diversity of animals that live on and under the waves, that crawl along the seafloor, or burrow deep into the mud. Some live in the Gulf for their entire lives, some visit seasonally, accidentally, or on their journeys between the rivers and seas farther afield. While they are here, they eat.

On any given day in the Gulf, animals are moving around, searching out shelter and places to grow or breed, but mostly they are looking for food. And most of what they are eating is each other. The complicated feeding relationships between them comprise the ever-changing marine food web.

To start to grasp today’s Gulf of Maine food web, one might as well start with the American lobster. Lobsters in a natural environment—without lobster traps—eat urchins, clams, mussels, snails and polychaetes, said Jonathan Grabowski, a marine ecologist at Northeastern University, and formerly at Gulf of Maine Research Institute.

Of course, Maine’s waters are definitely not empty of lobster traps. Grabowski has worked with colleagues, including Graham Sherwood, a marine ecologist at Gulf of Maine Research Institute, to study how the presence of traps, and bait, influences lobster diets, by comparing lobsters in areas with and without lobster traps.

Grabowski, Sherwood and their team employed both gut content studies and stable isotope analysis to determine lobster diet. Gut content studies “provide a snapshot of what an animal ate last night,” according to Grabowski, “while stable isotope analysis provides an integrated measure from body tissue of what an animal has eaten in the last week, month or longer, depending on how quickly that tissue turns over.” Both methods have their pros and cons. Together, they showed the researchers that lobsters in areas with baited traps had a significantly higher percentage of herring and other pelagic forage fish (which are used as bait) in their diets, compared with the lobsters in the non-trapped areas.

So who eats lobster? According to Grabowski, small fish like sculpins, sea ravens and cunners eat lobsters that are smaller than three or four inches. Larger lobsters fall prey to striped bass, sandy dogfish and other larger fish like cod. In many areas of the Gulf of Maine, however, large fish are now scarce, and recent studies suggest that the most important natural predators of lobster along the coast of Maine today are actually other lobsters. Bigger lobsters eat smaller lobsters. And we humans eat a whole lot of lobsters—90 million pounds of Maine lobster last year.

In addition to Grabowski and Sherwood’s research, a number of other studies over the past few decades have shown that parts of the Gulf of Maine with more large fish, like Cashes Ledge, have fewer large lobsters, but large lobsters do reign in inshore coastal waters where larger fish are now rare.

What about other species in the Gulf of Maine? How much do their diets vary over time, and between areas, and what influence do we have on these other parts of the foodweb? Scientists have used gut-content studies to study feeding relationships for decades and NOAA fisheries service maintains archives of stomach-content data from multiple scientific trawl surveys it has conducted every year for the last 50 years. With the emergence of stable isotope analysis, newer lipid isotope analyses, and computer modeling capabilities, researchers can now present spaghetti-like diagrams showing who eats whom in the Gulf of Maine, and on nearby Georges Bank. But even those complex images are drastic simplifications, because relationships are constantly changing.

Michael Fogarty, a marine ecologist and Director of the Ecosystem Assessment program at NOAA’s Northeast Fisheries Science Center (NEFSC), points out that, while specific populations have varied dramatically over the last 50 years since NEFSC began regular surveys, the total biomass of marine life in the Gulf of Maine region has remained relatively constant. In other words, while the numbers of individual fish species have risen and fallen, the total weight of fish living on George Bank has remained between 700 million metric tons and 1100 million metric tons since 1963.

Fogarty said that the constant biomass indicates environmental conditions that generally lead to a somewhat stable total productivity of the ecosystem. The widely varying relative abundance of specific fish like cod and herring, on the other hand, points to the highly complicated predator-prey and competitor relationships that result in an ever-changing balance of which type of animals dominate the system. And on Georges Bank, as along the coast of Maine, humans have been a central player in the food web since well before any scientific studies began.

Despite the wealth of information available about feeding relationships of herring, cod, haddock, flounder, scallops, lobsters, urchins, dogfish, tuna and every other creature in the sea, the current fishery management system still essentially manages fishing pressure on a species-by-species basis, despite calls by scientists, fishermen, managers and environmentalists for “ecosystem-based management.” The New England Fishery Management Council is expected to vote this fall on a new management approach that would attempt to create more truly integrated, ecosystem-based management of fishing effort across the species groups, and at different levels of the food web. But it remains to be seen whether, or how, what we now know about who eats whom will be used to help manage how much fish we catch—and eat.

This article is made possible in part by funds from Maine Sea Grant.

Heather Deese holds a doctorate in oceanography and is the Island Institute’s vice-president of programs. Catherine Schmitt is communications coordinator for Maine Sea Grant.