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September 18, 2003
Little Fish may make Big Splash on the Cape
By Jonathan Talbot

With a little help from the Marine Biological Laboratory and a native fish, cranberry growers in Massachusetts may soon be heroes throughout the Northeast in the fight against West Nile virus, Eastern Equine Encephalitis, and other diseases transmitted by mosquitoes. Growers may also enjoy a higher standard of living for their efforts to reduce the number of mosquitoes that make some Cape Cod visitors wonder about returning for another season. "Basically, we have a lot of stagnant water around here, and a lot of tourist money and skin, and it’s a bad combination," says Bill Mebane, superintendent of the Aquaculture and Engineering Division at the MBL.

The Laboratory is collaborating with the state’s Cape Cod Mosquito Control Project to develop a manual for cranberry growers to raise large numbers of the banded sunfish Ennecantus obesus in often-fallow cranberry bogs. The round-tailed native of Cape Cod has a seemingly insatiable appetite for the comma-sized mosquito larvae, which typically live at the surface of stagnant water bodies. The long-term plan is for cranberry growers to sell the fish to hobbyists, gardeners with increasingly popular water features, and to convince the state to stock some larger bodies of water.

Cape Cod is a good place to be a mosquito, and increasingly a more difficult place to make a living growing cranberries. The glacial deposit that formed the Cape created more than 3,500 potential freshwater mosquito habitats on the Cape, according to Gabrielle Sakolsky, an entomologist with the state agency. Add to that ambitious green thumbs adding countless water features into gardens, and you have an area littered with prime breeding grounds for summer’s version of a mandatory blood drive. As for cranberry growers, prices continue to fall, threatening one of the area’s signature industries.

The mosquito agency does what it can to combat the little vampires, but faces an uphill battle, reapplying bacteria that kill mosquito larvae every 12-14 days to larval habitats during the summer. This method of controlling mosquitoes, though effective where practiced, requires considerable effort. "When you apply that bacteria to a swamp, you need a crew to apply it and walk through the whole site," Sakolsky says. Individual swamps range from less than an acre to about 20 acres. "A major advantage of using fish," she says, "is that you can release them at one site, and they go find the problem."

The Story

In 1995, the CCMCP received funding to explore using a native species of mosquito-eating fish on the Cape to control larval mosquitoes. After considering a number of fish, Sakolsky says the mosquito control agency chose the banded sunfish. Getting enough fish, however, proved a challenge.

"We could not find a lot of them," Sakolsky recalls. The agency seined ponds and looked in other water bodies but could not find enough to stock additional waters.

Even if there were enough fish ready to catch in bogs and ponds around the state, which Mebane says is unlikely, those fish are the property of the Commonwealth, he says, and cannot be arbitrarily removed from one body of water and introduced into another. "It would be like selling deer from Otis," he adds. The project could only go forward if the fish could be reared in captivity with appropriate permits from the state. Those reared fish, though the offspring of wild-caught fish, are not the property of the Commonwealth, though permits are still needed to introduce them into new bodies of water, Mebane says.

So, Sakolsky approached the MBL to see if it could spawn the needed numbers of fish in tanks, and then use those results to develop a manual for cranberry growers to spawn fish in their bogs. The agency then surveyed ponds around the Cape to find fish to begin the breeding. It collected several fish in a pond near Wareham and handed them over to the MBL.

Mebane’s team at the MBL team promptly figured out how to accelerate the sunfish’s spawning. MBL staff placed the fish in tanks at its Marine Resources Center, then manipulated the light and temperature in their new watery homes, mimicking an accelerated change of seasons. A final boost of a hormone stimulated the fish to produce eggs. "They spawned tons and tons of beautiful babies, " said Mebane, which the MBL then gave back to the mosquito specialists.

After the Lab produced the fingerlings, and after receiving permission from the state’s Department of Fish and Wildlife, the mosquito control agency placed them in a few small water bodies near Barnstable. "We had to stock them in places with no inlet or outlet, and with no other fish" to comply with state regulations, Sakolsky says. The agency introduced 50 fish in into a 2-acre cedar swamp, and 50 fish into a half-acre grassy pool. The next year, a total of 104 fish were added to the two sites.

These fish initially lived in containers which limited them to feeding on mosquitoes. Inspection of sampled fish from these populations showed that they were eating lots and lots of mosquito larvae. In the next phase of the experiment, the agency released the fish from their cages, Sakolsky says, and later examination of a few of these fish showed that even free-swimming, they tended to eat a lot of mosquito larvae, and not much else. They also managed to survive the winters.

Most significantly, these small-scale experiments showed that the number of adult mosquitoes in the area dropped dramatically after the introduction of the fish.

Precedents and Potential Problems
Although using fish on a large scale to control mosquitoes would be new to Massachusetts, says Mebane, many other states already consider it a trusted form of mosquito control. "Some states use the exotic Gambusia the way residents of Massachusetts use mosquito repellent," he says, referring to a genus of fish known as ‘mosquitofish.’

Mebane says those working on the sunfish plan have considered the potential undesirable impacts of introducing large numbers of captive-bred fish into the larger environment, even though they are native to the Cape and the other areas in the Northeast envisioned as likely places to stock them.

The most obvious concern, Mebane explains, is that the introduced sunfish might escape their intended waters, reproduce wildly, and wreak ecosystem havoc. Some past introductions of exotic species in the pursuit of pest control have had disastrous consequences. Mebane points to the explosion in Gambusia introduced into warm waters in the Southern United States. The warm-water fish usually die off during cold U.S. winters, but some can find warm patches of water to survive, and those stocked in waters too far south feel right at home. States which use Gambusia to control mosquitoes must restock the fish each year, but in some southern states, Mebane says, some fish survived the winters, and dramatically altered bass and frog populations.

Banded sunfish, says Mebane, will work better than Gambusia. Firstly, banded sunfish are native, presently found in cranberry bogs, and are known to survive Cape winters, as well as the agricultural chemicals, harvesting, and fluctuating water levels that are part of bog life. Cranberry growers, for example, typically flood bogs to control insects and to assist in harvesting the berries. Banded sunfish have also been reported to burrow into mud to wait out dry spells, only to reemerge when the waters return. As a result, the sunfish will not need to be restocked each year as Gambusia typically do, Mebane says. What gets Mebane most excited about the banded sunfish, however, are its peculiar needs to spawn. Those needs presented a challenge to spawning them in captivity, but make the fish unlikely to wreak ecological havoc, Mebane says.

The sunfish, he explains, require both low-mineral (or ‘soft’) and highly acidic water to reproduce-a rare kind of water found in only a few natural settings, including cedar swamps and peat or cranberry bogs. Mebane says that there is little chance of the sunfish reproducing outside the acidic bogs where they will be introduced, and where they are found in small numbers today. "The chances are slim to none," he says, that the sunfish will escape their intended habitat, swell in numbers, and compete with other species.

"What really changes a system is when a species is introduced that remains there and keeps reproducing," as the Gambusia do, he explains. Even if a few sunfish were to find their way out of the cranberry bogs into waters where the project managers did not want them to go, Mebane says, the scientific consensus is that they would survive the winter, but not reproduce. The adventurous fish would have no heirs to enjoy any newfound homes, and would simply die off with time.

Large numbers of banded sunfish might also alter the ecology of the cranberry bogs where they will be reared. The MBL’s research, however, suggests that the cranberry bogs do not have many other fish to be crowded out by even large numbers of banded sunfish. Mebane says that his team pulled a 100-foot seine through flooded bogs near Wareham, and found ‘essentially no other fish.’

Mebane says that the banded sunfish may take some food away from other species of fish, and possibly consume food other than mosquitoes. Mebane hopes that the state will soon put some of these fish in a pond with no inlet or outlet, and compare other fish populations before and after the introduction to see whether the sunfish displace, for example, local sunfish or bass, by competing for food.

Predators might also gobble up the introduced fish before they can munch on mosquito larvae floating at the water’s surface. The fish, however, are so well colored and are able to get into such dense weeds, says Mebane, that he thinks predation will be minor. "They get into weeds as thick as a hairbrush, and I don’t know how anything will get to them. They’ve developed some really good, elusive mechanisms." Mebane also says that those same traits mean they should not have much of a visual impact on water gardens where they are introduced. They’ll just eat a lot of mosquito larvae.

Raising the fish in tanks is difficult, Mebane says, precisely because of the need for highly acidic but soft water. "It’s difficult to get biofilters established in the soft water, primarily because the nitrifying bacteria need the minerals to grow," while the substrate chosen for the breeding tanks needs to be insoluble at low pHs to avoid increasing the hardness of the water. MBL designed a tank system that used peat moss to soften the water, removing dissolved minerals. In 1995, water quality was so critical to spawning the fish that the MBL had to make repeated trips to the fish’s initial home to draw water for the tanks back at the MBL. Mebane says that the current research focus is learning how to rear the fish starting from tap water, because "not everyone who wants to raise these fish is going to drive to that pond."

The Markets

Mebane has high hopes for what rearing banded sunfish in bogs could mean to local cranberry farmers. "The Gambusia market is a very lucrative market," he says. Although small numbers of the sunfish already live in some cranberry bogs, Mebane says, spawning in captivity is needed to produce enough fish to survive and grow to a size that they can then be sold and transported. Even if a grower actively uses a cranberry bog to raise cranberries, Mebane says, the sunfish would thrive in adjoining irrigation canals.

The MBL will do market research, Mebane says, once it has enough larvae on hand to respond to at least some consumer demand. The vision is to sell the fish through garden catalogs, websites, and email lists to pet shops and water gardeners. Funding for this research comes from the South Eastern Massachusetts Aquaculture Center (SEMAC)

The sunfish may also provide coveted eggs to biomedical researchers such as those at the MBL. "They have these beautiful, clear eggs," says Mebane. Researchers often value clear eggs for the ease of directly observing the processes at work inside them.

Mebane also thinks there is a market among hobbyists for the attractive fish. The banded sunfish have a distinctive, very rounded tail. Most sunfish, Mebane says, have a slight lobe or fork to their tale. When in their mating colors, they also have distinctive black bars, and iridescent purplish gills. Although banded sunfish have been raised and traded by hobbyists for years, Mebane says that no one has ever tried to produce so many fish, or to control the process so much.

While both Mebane sand Sakolsky focus on the potential the sunfish have to take up residence in water gardens, Sakolsky looks to their use in other locations. She says, for example, that cranberry bogs restored to wetlands may make a good place for the fish to work on mosquito larvae. Even stocking fallow cranberry bogs alone, without the fish being stocked elsewhere, should make some difference, Sakolsky adds.
Sakolsky says that for the foreseeable future, release of the fish will likely be limited to bodies of water with no entrance or outlet, because of permitting constraints imposed by the state fish and wildlife authorities.

"We’re never going to get rid of them all," Sakolsky says of the Cape’s mosquitoes, but she expresses confidence that her agency will find a way to reduce the pests to a level at which they no longer cause nuisance or economic problems.

Essentially dormant since 1995, the project recently kicked into high gear, Mebane says, due to increasing concerns about mosquito-borne diseases, a desire to control the parasites without dangerous chemicals, the growing popularity of water features, and the falling prices of cranberries.

"Up until the end of the 1990s, mosquitoes were just an annoyance factor. Then in 1999, people with water features were raising mosquitoes which could carry West Nile," says Sakolsky.

"A lot of people have fish ponds that are not well kept," says Mebane. "They do not keep fish in them, and they become breeding grounds for mosquitoes."

The banded sunfish are found in the wild in New Jersey, Connecticut, Rhode Island and Massachusetts, says Mebane.

"Now that we know a lot of the possible negatives about moving species around, we can address them. If it still looks viable, doggone it, let’s go for it," exclaims Mebane. "It could be an incredible thing for the cranberry growers and everyone frustrated by mosquitoes."

The Marine Biological Laboratory, located in Woods Hole, Massachusetts, is an independent scientific institution, founded in 1888, that undertakes the highest level of creative research and education in biology, including the biomedical and environmental sciences.