Introducing mosquitofish

Western mosquitofish, small freshwater fish typically about two and a half inches long, have been distributed far and wide from their native habitat in the southern U.S. and northern Mexico. Because they feed on insect larvae, including, depending on the circumstances, substantial numbers of immature mosquitoes (hence their name), humans have introduced them to waterways throughout the world.

In fact, these introductions are still occurring today – if you live in California’s Alameda County, you can request a delivery of mosquitofish right to your own pond.

Of course, biological controls rarely function exactly as intended. Mosquitofish are aggressive toward other fish, and have even been known to replace native fish species – which, in some cases, are more “efficient mosquito control agents” than the introduced mosquitofish that displaced them.

Mosquitofish also eat lots of other types of insects, as well as zooplankton and aquatic vegetation – meaning the effects of their eating habits can ripple outward from multiple parts of their food webs. Far from solely reducing mosquito populations, mosquitofish can cause big changes in their new homes (which is why some states regulate the expansion of their range).

Water bodies are not isolated from the environments that surround them – scientists call the flow of nutrients and resources between aquatic and terrestrial areas a ‘boomerang flux’ or ‘reciprocal subsidy.’ For example, many aquatic insects (including mosquitoes) spend their larval stages underwater, and later emerge as winged adults – the energy they incorporate into their bodies from the aquatic environment is transferred to terrestrial environments when those adults are eaten by predators like spiders or birds. New research recently published in the journal Freshwater Science suggests that introduced mosquitofish can alter that transfer of energy.

A team of scientists working at Utah’s Fish Springs National Wildlife Refuge, a series of spring-fed wetlands in the middle of the desert 125 miles southwest of Salt Lake City, measured how well aquatic insects survive to adulthood in the presence of western mosquitofish, as well as two native fish species.

The researchers set up experimental tubs of spring-water, complete with sediment and aquatic vegetation, to which they added either one species of fish or a combination (they also kept fish out of some of the tubs, so they could compare the results from the fish-filled tubs to the fish-less ones). Then they captured the adult aquatic insects that emerged from the tubs’ water surface – presumably the insects that did not survive their underwater larval stages became fish food. (None of the adult insects they collected were mosquitoes.)

The scientists found that the biomass of aquatic insects that emerged unscathed from the tubs that didn’t contain any mosquitofish was much larger than the biomass captured above the tubs where they were present: about 70 percent of the insect biomass made it out of the tubs containing native fish species, while only about 40 percent survived the mosquitofish tubs.

As the researchers note in their paper, “[w]estern mosquitofish have the potential to negatively affect the flow of energy from springs to the terrestrial environment,” in the form of reduced adult insect biomass. Though we humans may cheer when those adult insects are mosquitoes, we should remember that mosquitoes are not the only insects western mosquitofish consume, and the effects of introducing the fish to new locations are rarely as straightforward as we might like.

Western mosquitofish are often intentionally introduced to ponds and lakes in an effort to reduce mosquito populations; such introductions can have unanticipated negative consequences. 

(Image by NOZO via Wikimedia Commons/Creative Commons license)

Hypoxia and habitat loss

Habitat loss happens as a result of many different human activities: deforestation, for instance, or urban development. When I think about habitat loss, the habitat that I picture disappearing is usually terrestrial in nature; a forest, or a meadow or mountaintop. But aquatic habitats are easily degraded, too, with deleterious effects on their inhabitants.

Though most fish and other aquatic organisms don’t breathe air like humans do, they still depend on oxygen – in the case of fish, they absorb oxygen from water as it passes across their gills. Underwater areas low in dissolved oxygen, or ‘hypoxic zones,’ (as well as ‘anoxic zones,’ places completely free of measureable oxygen) represent habitat loss for fish.

Hypoxic zones are usually a seasonal phenomenon, caused by a chain of events that typically starts with excess nutrients entering a body of water, which leads to algal blooms that ultimately deplete underwater oxygen as they begin to die and decompose. Hypoxia can be extreme enough to cause fish-kills, massive die-offs of hundreds or even thousands of fish.

Hypoxic zones can affect fish in other ways, too, as new research recently published by a team of scientists working on Lake Erie highlights. The researchers working in Lake Erie found that the edges of hypoxic zones are more dynamic than previously thought, with some intermittent periods of normal oxygen levels; they also found “higher fish densities near the edges of hypoxia,” presumably because fish and other mobile aquatic organisms can be displaced by hypoxic zones as they seek areas where oxygen is still available.

When fish are concentrated into a small area, they may be easier to catch – and indeed, the scientists found that, depending on where trawls are taken or nets are set, “catches may actually increase in areas affected by hypoxia.” Many fish population estimates are based on how many fish commercial fishers land, so if hypoxic conditions are allowing fishers to catch more fish than they normally would, the very set of circumstances that fish are trying to escape could lead to an overestimation of their numbers. Such an overestimation could put the population at risk of overfishing if managers set higher quotas than the true number of fish can support.

Habitat loss and degradation threaten many species, both on land and in the water. By studying how fish respond to hypoxic events, hopefully we can reduce some of the risk that they face.

An algal bloom in Lake Erie, in early October 2011. Decomposing algae during and after a large bloom can result in hypoxic zones.

(Image by NASA via Flickr/Creative Commons license)

Food web reverberations

Great blue herons are excellent hunters and fishers; at four feet tall, with a wingspan of six feet, an adult heron can be a formidable sight as it wades through marshy shallows, waiting for the perfect moment to strike its prey. Great blue herons are also regulators of the complex interactions that connect them to their fellow eelgrass meadow-dwellers, according to a study recently published in the journal Oikos.

Events that impact one corner of a food web have a way of reverberating throughout the rest of its threads – the organisms that share an ecosystem are linked through many direct and indirect connections. When effects ripple from one end of a food web to the other, scientists call the ensuing changes a ‘trophic cascade.’ The new study suggests that changes to great blue heron populations might set off just such a cascade of changes.

The scientists monitored patches of eelgrass meadow (a major foraging area for great blue herons during the summer months) where herons could freely feed, as well as patches from which the herons were excluded. They monitored organisms at multiple levels of the food web: fish (which are eaten by herons), insects (which are eaten by fish), and the algae that cling to eelgrass blades (which is eaten by insects).

By the end of their nine-week experiment, they found that excluding herons from the meadow influenced all levels of the food web. Fish were more abundant where herons were absent (perhaps, the authors speculate, because fish were “seeking refuge” there), and in those areas where fish were more plentiful, one type of insect commonly eaten by fish was less abundant. The researchers were expecting algae to proliferate in those same areas (where, because herons were absent and fish more abundant, there were fewer algae-eating insects), but instead they found that algae were more abundant in the areas with herons, highlighting the complexity of food webs and the unanticipated effects that can occur in response to changes.

This study “highlights the ecological importance of predatory wading birds,” the authors note. A change in the number of great blue herons in a local ecosystem will affect the birds themselves, of course, but it will also affect all the other organisms connected to them through the food web.

Fish are a staple food for great blue herons.

(Image by Pen Waggener via Flickr/Creative Commons license)

Saltmarsh sparrows

The saltmarsh sparrow (“a secretive bird with skulking habits and a barely audible song,” according to the Cornell Lab of Ornithology) makes its home in salt marshes along North America’s Atlantic and Gulf coasts. It’s listed as “vulnerable” on the IUCN list of threatened species, which is not as bad as “endangered,” “critically endangered,” or “extinct in the wild,” but somewhat worse than “least concern,” and “near threatened.”

In other words, conservation-minded scientists and wildlife managers should be interested in mitigating the threats the bird faces in the wild, which revolve around habitat loss.

Salt marshes are complex ecosystems regularly inundated by tides in which tiny variations in elevation – sometimes just a few centimeters – can mean the difference between daily or twice-per-month flooding. As a result, completely different plant communities develop in low marsh and high marsh areas. Saltmarsh sparrows tend to build their nests in the high marsh areas, among plants that grow only at higher elevations. 

Humans have not always recognized the important services salt marshes provide (they help prevent coastal erosion, for example, and also provide food and shelter for fish and other aquatic species), and development, including water-control efforts and in-fill projects, have destroyed or degraded a lot of the salt marsh habitat that once lined much of the U.S. coast. Invasive species, particularly Phragmites australis, or common reed, and rising sea levels further threaten the marshes and saltmarsh sparrows that are left.

Salt marsh restoration projects typically involve elements designed to reestablish natural conditions, like removing or altering barriers to tidal flows or eliminating Phragmites stands. But, as a team of scientists from the University of Connecticut recently reported in the journal Restoration Ecology, even when managers are able to successfully alter a salt marsh plant community through restoration, that may not translate into increased habitat for saltmarsh sparrows.

The researchers, working in 18 different Connecticut marshes, compared remnant plots of healthy marsh to plots that had been restored, either through direct removal of Phragmites or renewed tidal flow. They found that tidal flow restoration appeared to control Phragmites growth and allow for the reestablishment of native low marsh plant communities, but not the plants typical of high marsh areas, where saltmarsh sparrows like to build their nests; consequently, there were fewer saltmarsh sparrows, and fewer nests, in the tidal flow restoration plots compared to the remnant marsh plots.

“[D]espite its successes,” the authors write, “saltmarsh tidal restoration is not providing conservation value for some of the most vulnerable species that use tidal marshes.” They go on to suggest that an emphasis on restoring high marsh areas, in addition to low marsh areas, is necessary to help species like the saltmarsh sparrow.

The saltmarsh sparrow may be a secretive, skulking bird, but it’s a fundamental part of salt marsh ecosystems. In order to fully rebuild salt marshes, future restoration efforts will have to target all elevations, even if they’re only a few centimeters apart.

Saltmarsh sparrows are four to five inches long, and weigh less than an ounce.

(Image by nebirdsplus via Flickr)