Science Insights:
Do salt marshes serve as fish nurseries?
Delving beyond the sound bite

By Peter H. Taylor

THERE'S NO QUESTION that salt marshes are incredibly rich, productive and valuable parts of the Gulf of Maine ecosystem. They contribute copious amounts of vegetation to the food web. They help to filter pollutants from the water. They provide habitat for numerous species of fish, birds and invertebrates. They are beautiful places to paddle a canoe, watch birds or go fishing. For these reasons and more, salt marshes deserve protection and restoration.

But whenever ecologically minded people talk about salt marshes, it’s almost a sure bet that the word “nursery” will get tossed in. The dogma is that some species of marine fish call salt marshes home when they are young, before they move offshore and are caught as adults. Indeed, the notion of salt marshes as fish nurseries has become “common knowledge” among resource managers, conservationists, environmental educators, and many laypeople. Usually it’s treated as a basic tenet of coastal ecology in public outreach materials, educational programs, magazine articles, and non-scientific books and people routinely cite this nursery role to help justify protecting and restoring salt marshes. Based on common knowledge, one might assume that Gulf of Maine salt marshes virtually teem with juvenile haddock, cod and other commercially important fish and that destroying a salt marsh would be like destroying a hospital nursery.

Although salt marshes intuitively seem like good nursery habitats, I hadn't heard much direct scientific evidence for the idea, particularly in the Gulf of Maine. Is it true that marine fish use salt marshes as nurseries? Which species? Do young fish rely on the marshes, or just occasionally happen to be found there?

Recently I dug into the scientific literature to find out. Clearly, salt marshes support commercial and recreational fisheries, and overall ecosystem health, by contributing to the food web and improving water quality. But the question of salt marshes as fish nurseries is an unresolved, thriving area of basic and applied research. I learned that the evidence for the nursery role is patchier and the whole story more complex than one might expect. “Salt marshes are nurseries” really is just a sound bite that masks key facets of the subject. That's OK. Often the best way to communicate is to boil complexities into a form that is quickly digestible by the majority. However, an accurate understanding of habitat functions and linkages is essential for ecosystem-based management. Resource managers and other coastal decision-makers around the Gulf of Maine need more than the sound bite.

1. What does “nursery” really mean?

Often people declare a salt marsh or other habitat as a nursery simply because young fish are present. That's not sufficient. A working group at the National Center for Ecological Analysis and Synthesis recently focused on clarifying the definition: To qualify as a nursery, a habitat must contribute a disproportionate number of juveniles into the adult population. In other words, more young reach adulthood from that habitat, acre for acre, than from other habitats. The disproportionate contribution can arise due to any combination of these four factors in the nursery habitat: (1) more young per unit area, (2) faster growth, (3) higher survival, and/or (4) better success moving from nursery to adult habitat. Many studies purporting to examine the nursery role of salt marshes don't look at these factors, making it impossible to discern the nursery value. I'm not aware of any studies from the Gulf of Maine that provide this information.

2. Nursery value differs among sites

Tom Minello of the National Oceanic and Atmospheric Administration's Fisheries Service laboratory in Galveston, Texas, and colleagues analyzed more than two dozen studies on the abundance, growth, and survival of fish and free-swimming invertebrates in salt marshes around the world. They found that the nursery value of these habitats varies among and within geographic regions. For example, salt marshes seem to play a greater nursery role in the Gulf of Mexico than along the southeastern U.S. Atlantic coast. The study also suggested that, within geographic regions, the nursery value of a salt marsh is influenced by its orientation along the coast, current patterns, tidal flushing, and proximity to other habitats such as seagrass beds. Because rigorous studies of nursery functions are scarce, Minello's analysis did not include any data from the Gulf of Maine. However, the findings hint that (a) it's impossible to infer the nursery significance of salt marshes in the Gulf of Maine from studies in other regions, and (b) marshes around the Gulf of Maine probably vary in nursery value.

3. “Nurseries'” economic impact is unknown

Sometimes non-scientific authors attempt to place dollar values on the nursery role of salt marshes something like, “One third of fished species depend on salt marshes as nurseries, and commercial fisheries are worth $300 million per year. That means salt marshes are worth $100 million annually as fish nurseries.” This type of conjecture goes well beyond what's actually known. Although more than a dozen species of marine and estuarine fish are known to occur in the Gulf of Maine's salt marshes, scientific data do not yet answer the question of how many species caught offshore actually use and rely on salt marshes as nurseries. Even if a species is found in salt marshes, other young fish of the same species might live in non-salt marsh habitats, and the salt marshes may or may not truly function as nurseries, as discussed in section 1 above. It is premature to put a dollar figure on the economic value of Gulf of Maine salt marshes as nurseries for commercially important fish.

4. Salt marshes and the “food web relay”

Often the “nursery” idea gets top billing, but salt marshes perform many other valuable functions in the ecosystem. One way that they benefit fisheries is through the food web. Few animals eat salt marsh plants, but after the plants die they become colonized by bacteria, fungi and protozoans, making a rich food called detritus. Detritus is the initial step in a “food-web relay” that ultimately feeds commercially important species. First worms, crabs and other invertebrates eat the detritus on the marsh surface. At high tide, mummichogs, silversides and other small fish swim across the flooded marsh surface to feed on the detritus and invertebrates. At low tide, the small fish retreat into deeper creeks. Larger fish such as winter flounder and striped bass venture into the creeks and feed on the small fish. In turn, the larger fish swim out of the marsh, which connects the salt marsh's food web with that of the coastal waters beyond. Through the food-web relay and the export of nutrients, salt marshes help to sustain commercial and recreational fisheries.

5. Other habitats act as nurseries

Salt marshes are widely discussed as nursery habitats, but it is less recognized that other shallow-water habitats in the Gulf of Maine may also serve as nurseries. For example, eelgrass and kelp beds host young-of-the-year Atlantic cod, mussel beds and cobble bottoms offer young sea urchins a refuge from predators, and rockweed habitats along rocky shores are used by juveniles of many species, including herring, pollock and winter flounder. These other habitats might help to sustain fisheries, and they should be kept in mind for habitat conservation and restoration.

Let's step back and look at the big picture again. The lack of scientific evidence doesn't mean that salt marshes in the Gulf of Maine aren't fish nurseries. We simply can't say yet. Nor does it mean that salt marshes shouldn't be protected and restored. There are plenty of reasons to retain that priority, aside from the nursery function. Nor should incomplete scientific knowledge deter progress with ecosystem-based management in the Gulf of Maine. Knowledge is never complete, the precautionary principle prescribes protection even if cause-and-effect are not scientifically proven, and adaptive management can take into account the emerging understanding of salt marshes and other habitats.

What about all those outreach and education publications for students and non-scientists? My suggestion is to revise the sound bite: “Scientists postulate that salt marshes act as nurseries for some fish species.” I think it has the same impact and it's accurate.

Peter H. Taylor is a consultant for the Gulf of Maine Council's Science Translation Project. For information visit: www.gulfofmaine.org/science_translation.

Contact Peter at: peter@waterviewconsulting.com