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![[Images] Five photos of different landscape](/images/imstr1.jpg "[Images] Five photos of different landscape")
Issue 6
Catfish, Crayfish, and Mussels
by Zoë Hoyle and Jim ClevelandIt’s May in Oxford, Mississippi, still cool, not muggy yet. It’s time to haul out the waders and get the electric current going.
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The fine-lined pocketbook mussel has developed special structures to attract the sunfish its specialized larvae, called glochidia, depend on for the intermediate stage of their development. (photo by Wendell Haag) |
The boat is just one of many tools the team uses to research warmwater fish, freshwater mussels, and crayfish in the forested wetlands of the Lower Mississippi Alluvial Valley (LMAV). Part of the SRS Center for Bottomland Hardwoods Research located at Stoneville, MS, the members of the Oxford team work together to understand the aquatic ecosystems of the Southeast, and have contributed to a new understanding of the diversity and imperiled status of aquatic species in the region.
The team conducts numerous studies in the LMAV, looking at the effects of deforestation, stream channelization, and other land practices. Fish, mussels, and crayfish can all be critical indicators of water quality and biodiversity, and the data the team develops on ecology biology, and community structure and function inform efforts to restore, conserve, and afforest in and around the streams and rivers of the area.
As part of a larger, comprehensive 5-year baseline study, the team developed sampling protocols and conducted the first standardized, quantitative inventory of fish and fish habitat in the National Forests of Mississippi. The field work, which also included an inventory of freshwater mussels and the distribution and habitat associations of crayfish, was completed in 2004. Though the findings are still being analyzed, results include a geo-referenced database that covers physical and biological conditions for 364 stream reaches in over 7 ranger districts, and 110 fish species, 16 crayfish species, and numerous mussel species.
“While the national forests represent only 5 percent of Mississippi’s land area, they are important habitat for 110 species of freshwater fish out of the 220 or so that we know occur in the State,” says Warren. “So these are critical places to preserve these stream fish, and mussels and crayfish as well. Understanding the populations in these areas can also provide us with information we need to help restoration efforts on other ownerships.”
Space, Time, and Fish
Today the team is headed east of Oxford to a put-in on the Little Tallahatchie River in the Holly Springs National Forest, where they will launch the boat for a trial run. This part of the Little Tallahatchie River is a good place to measure the effects of channelization on freshwater aquatic organisms; there’s access here to a relatively unaltered section of the river, as well as to a section that was diverted and channelized for flood control. Channelization and incision—a condition where the stream has entrenched into the land along its banks—are the rule rather than the exception in the LMAV, and present a major challenge for aquatic conservation and restoration efforts.
The team previously used three small tributaries of the Little Tallahatchie for a 17-month study on how fish assemblages—the community of fish defined by the species present, their life cycles, and how they interact with one another—vary in relation to the seasons. Two of the streams are channelized with incised banks, while the third stream remains relatively unaltered. The channelized streams are wide and shallow with shifting sand bottoms, almost no canopy cover, and very little in-stream wood or other cover for fish. The unaltered stream is extensively shaded, with in-stream wood forming debris piles, riffles, and pools.
“There hasn’t been much work done on how fish assemblage structure in channelized, incised, sand-bottom streams varies in relation to time and space,” says Warren. “Establishing this variability in relation to range and life cycle is extremely important to conservation efforts. Monitoring data, used to assess the influence of human actions or natural events on fish assemblages, cannot be interpreted reliably unless you know how the assemblage reacts over time prior to the event you’re looking at.”
The aquatic team uses a combination of techniques to sample and monitor fish, ranging from snorkeling (when the water is deep enough and clear enough to count fish) to electrofishing. Many warmwater fishes—shiners, darters, madtoms—are small and difficult to see even when the water is clear. For the fish assemblage study, the team used a stationary seine and electrofished downstream towards it so that the stunned fish were swept by the current into the seine. Because of the tight mesh of the seine, they were able to capture smaller sized and younger fish often missed with other techniques.
For this study, they captured almost 18,000 fish representing 13 families and 52 species. They found high variability in fish assemblages over time in the channelized streams, though each of the streams maintained its own unique characteristics. “High temporal variability in the more degraded streams suggests an environmental factor is coming into play, rather than lack of food or predation,” says Warren. “In other studies, we found that the presence of even minor amounts of in-stream wood is associated with fish faunas indicative of more stable and predictable conditions over time.”
Nature’s Water Filters
Back out on the water, research fisheries biologist Wendell Haag, now in waders, holds up a large, dark shell with a thin, sharp wing. “This mussel digs into the bottom of the stream, leaving the wing pointing up. You can guess how it got the name Alabama Heelsplitter.”
Haag devotes long hours to the study of freshwater mussels, wading through streams to locate and sometimes collect species with colorful names like pimpleback, monkey face, mucket, pistolgrip, and pocketbook. Extensive surveys of mussel populations in the LMAV were not done until the 1980s, and most of those were done on the larger rivers.
The aquatic team has focused on finding out more about the freshwater mussels in the small streams around the Little Tallahatchie (though this is just one aspect of their South-wide inventories, and they have also done studies in other parts of the LMAV). From 1993 to 2001, they surveyed for freshwater mussels at 135 sites in north-central Mississippi, with a large number of the sites lying within the Holly Springs National Forest. One result of their efforts was the discovery of eight previously unknown populations of the rayed creekshell, which is considered a species of special concern by the American Fisheries Society.
With their work extending across the southeastern region, Haag and Warren are emerging as leaders in the field of freshwater mussel research. The two have been in the forefront in identifying the unique relationships with fish that mussels depend on to carry out their reproductive strategies, uncovering previously unknown links between 20 different mussel species and their fish hosts.
“Reproduction is one of the most highly sensitive events in an organism’s life history, and reproductive traits are often useful indicators of how a species will respond to environmental changes,” says Warren. “Knowledge of an organism’s reproductive ecology is critical to its successful management and conservation.”
Understanding this delicate dance between species, which involves mussels attracting their fish hosts with a wide range of inventive lures, is as necessary to stream function and water quality as any other aspect. It’s a connection Haag makes readily.
“Freshwater mussels filter water, take the food resources they need, and deposit the rest in discrete packets called pseudofeces which other organisms then break down. Freshwater mussels in the Southeast as a whole have suffered greatly from the loss of habitat over time, and many species of both mussels and fish are now faced with extinction. We can only benefit from restoring the organisms and functions of these systems.”
Of Catfish and Crayfish
Researcher Susie Adams pilots the boat upstream into the area where the old stream intersects the newer, man-made channel. She asks McWhirter to take over the wheel and steer her closer to the bank, where she leans out of the boat to examine the yellow-fringed blossoms of swamp privet—a shrubby native that grows in wetlands and along streams throughout the Lower Mississippi Alluvial Valley. Adams studies ecological relationships that link land and water; in this case, whether channel catfish act as dispersal agents for swamp privet seeds. “Seeds of the swamp privet eaten and defecated by catfish are still capable of germination,” says Adams. “Although common in the Amazon River basin, this is the first example of fish dispersing seeds in North America.”
Along with research wildlife biologist Paul Hamel—the fourth member of the Oxford team—and other colleagues, Adams is looking at how the fruits are beneficial to catfish, how often the fruit is available to them, and whether birds such as cedar waxwings also play a part in seed dispersal for swamp privet. It may seem an odd focus, but it’s part of the SRS team’s growing interest in aquaticterrestrial linkages.
“Aquatic-terrestrial linkages occur at the interface between land and aquatic ecosystems and can assume many forms,” says Warren. “Research so far suggests that there are many more of these linkages than previously suspected, especially in forested wetlands and bottomland streams, and that they will have important implications for managing bottomland hardwood and riparian forests.”
But Adams logs a lot more time with crayfish than catfish. She has spent a good part of the last 2 years studying the distributions of crayfish inhabiting small streams throughout the National Forests of Mississippi, and studying the ecology and life histories of several crayfish species native to north Mississippi.
“There’s really very little information about crayfish in the Lower Mississippi Valley,” she says. “We don’t know where different species occur, their best habitat, or the ecological interactions among species.”
It appears that the aquatic team’s interest in crayfish may result in the description of at least one new species. “There is a surprising amount of undescribed variation in crayfish,” says Adams. “Whether this will lead to descriptions of species or subspecies, or just better accounting of the variation within species remains to be seen. We are collaborating on some studies of crayfish genetics to help clarify some of this confusion.”
From Drought, A Lesson in Resiliency
The aquatics team has also done extensive research on the effects on fish and crayfish of the record-breaking drought in Mississippi that ended in the fall of 2000. Because of the drought, numerous small streams or stream segments in northern Mississippi that normally flowed year-round dried up or were reduced to small, stagnant pools—presenting a good opportunity to study how fish and crayfish recolonize after water flow returns. The team sampled fish and crayfish in 7 stream reaches that dried and 5 that remained flowing throughout the drought, with the sampling starting before the drought and lasting through recovery.
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Members of the SRS aquatics team on a preliminary survey of the Little Tallahatchie River in northern Mississippi. From left to right: Technician Amy Commens, research fish biologist Susie Adams, and technician Gordon McWhirter. (photo by Jim Cleveland) |
Recolonization was slow in the winter, but picked up in the spring months. By June 2001, the fish populations in the dried streams were much the same as they were before the drought. “In previous studies, rapid recolonization was observed immediately after the disturbance,” says Adams. “In this case, the rapid response came months later, partially due to the timing of the disturbance in relation to the life cycles of the fish. This illustrates how important it is to understand the temporal cycles of fish assemblages in managing for disturbance.”
Far less is known about crayfish response to drought, and, before this study, there was no published work on the species common to the area. “Numerically, the patterns of recovery in crayfish were very similar to fish, except that crayfish populations increased slightly more rapidly in the winter,” says Adams. “Crayfish sizes drifted toward smaller individuals, indicating that reproduction may be more important as a mechanism of recovery than immigration from other stream segments.”
“A critical part of the recovery of fish communities is the ability of fish to move among the networks made up of these small tributary streams,” says Warren. “Effective management must ensure the freedom to move, especially during times of stress such as drought. To really understand these processes, we need to extend our population studies over larger spatial scales and multiple seasons.”
“All of our studies are designed to understand the aquatic system so that we can provide information for the scientifically based management and recovery of aquatic animals in the stream and riparian ecosystems of the Lower Mississippi Valley—and in the Southeastern United States as a whole.” It’s a daunting task, but Warren’s team is up to it.
For more information:
Mel Warren at 662-234-2744 x246 or mwarren01@fs.fed.us
Susan Adams at 662-234-2744 x267 or sadams01@fs.fed.us
Wendell Haag at 662-234-2744 x245 or whaag@fs.fed.us
Jim Cleveland is a freelance writer based in Calhoun City, MS.