Science sometimes makes big leaps in response to small questions. Our question was: How can we capture more fish species, and smaller members of those species, in large rivers? The answer was a simple merger of a standard large-meshed trawl with small mesh. This gear, and the technique for its use, have since become popular around the world.
Trawls (a net pulled by boat, similar to a shrimper’s trawl used in the oceans) were being used by some large-river scientists to sample fish populations, but they were not catching many rare or small fish.
To estimate what kinds of fish, how many, and of what sizes were passing through a standard trawl, the small mesh material was placed outside the trawl body. As fish passed through the larger inner-panel mesh, they were captured in the smaller-meshed, outer panel (and funneled to the end of the net, called the cod-end). This experimental dual-mesh trawl was implemented as part of a research effort and was shown to greatly increase the number of species and individual fish caught compared to the original trawl.
The unique pass-through design of the experimental trawl allowed us to demonstrate that we were catching fish with the old trawl, but most of the fish were simply passing through the large mesh and never made it to the cod-end. The new design became known as the Missouri Trawl by the late 1990s.
Initially the technique of trawling was considered too similar to a deep-water seine to justify consistent use. However, that thought changed based on the many new fish records that were being captured when using the trawl.
For example, in 1998 the first known “young-of-the-year” (less than 1-year-old fish), federally endangered pallid sturgeon was captured in a Missouri Trawl. Then, from 2000–2001, the Missouri Trawl was used to obtain unprecedented information on the abundance and distribution of sicklefin chub and sturgeon chub, which at the time were candidate species for federal protection. The data was critical to the U.S. Fish and Wildlife Service in determining the status of the populations.
The effectiveness and ease of use of the Missouri Trawl continued to gain it popularity as Big Rivers and Wetlands Field Station staff completed several how-to-trawl workshops. During this time, we began developing variations of the original 16-foot-wide Missouri The Missouri Trawl is an effective sampling gear in deep, swift rivers. It was designed to skim the bottom of streams and rivers where no other gear can be effectively deployed. The trawl has helped scientists better understand what kinds of organisms live in such places and where they are distributed. Below and on the following page are some examples of rare and unusual organisms captured by the trawl in Missouri. The Missouri Trawl is an effective sampling gear in deep, swift rivers. It was designed to skim the bottom of streams and rivers where no other gear can be effectively deployed. The trawl has helped scientists better understand what kinds of organisms live in such places and where they are distributed. Invitations for workshops soon followed from neighboring state researchers.
During the early 2000s, trawling workshops were conducted in Tennessee, Nebraska, Illinois, Kentucky, Arkansas, Iowa, and Mississippi in conjunction with ongoing large river monitoring and research efforts in those states. Each workshop yielded new fish records and further increased biologists’ interest in using the technique. However, it wasn’t until invitations began coming from Minnesota, New York, Pennsylvania, Vermont, and Canada that field station staff began to understand the relevance of the research to a much larger resource contingency—perhaps worldwide in scope. Researchers from the Netherlands, Poland, Australia, and South America have all expressed an interest in using or have already used Missouri Trawls in their programs. A demonstration in China was completed in 2009 as part of large-river scientific exchange program and the Mini-Missouri Trawl was used on the Yangtze River.
The Missouri Trawls are used to capture young individuals of many fish species because understanding early life information (reproduction) is important to the management of fish. If the species doesn’t reproduce there will soon be no fish for anglers to catch. The trawls are being used to study early life information for paddlefish, channel catfish, blue catfish, shovelnose sturgeon and many other species. They are also used to investigate young crappie abundance in some lakes because other methods do not capture them.
Trawling is useful for capturing species that are rarely sampled by other gear. Missouri Trawls have captured more Pseudiron mayflies than any other sampling gear used. The Pseudiron mayfly is known to be eaten by the federally endangered pallid sturgeon. They have also been used to capture Ohio shrimp (a Missouri species of conservation concern), and new research is uncovering their usefulness for sampling young mussel populations in large rivers.
Trawling is a useful method for sampling in small to large waters. While the Missouri Trawls are particularly useful to sample small, rare fishes in deep, swift-water habitats, they are now used throughout the United States and internationally in smaller rivers and large creeks. They can also be used by detaching them from a boat and pulling by hand in water that is too shallow to motor through.
Trawling has become widely used since the creation of the Missouri Trawl in 1997 and has been incorporated into many sampling protocols. As a large-river scientist from Illiniois told us, “This technique will forever change how we sample large rivers.” A Canadian researcher who works on smaller rivers and lakes reported that the trawl also helped enhance the knowledge of small fishes in the lower Great Lakes and tributaries.
Independent researchers are now creating new variations of older trawl designs that sample throughout the water column and some are attempting to chase down invasive species like the Asian carps. Sampling technique books now suggest trawling as part of a thorough fish-sampling protocol, further identifying the importance of the technique.
The Missouri Trawl is an effective sampling gear in deep, swift rivers. It was designed to skim the bottom of streams and rivers where no other gear can be effectively deployed. The trawl has helped scientists better understand what kinds of organisms live in such places and where they are distributed. Following are some examples of rare and unusual organisms captured by the trawl in Missouri.
Pseudiron Mayfly (Pseudiron centralis)
Localized common names for this rarely seen mayfly are white sand-river mayfly and flat-headed mayfly. The species is widespread across North America, but is not known from the St. Lawrence River and some drainages west of the Rocky Mountains. It is generally considered to be rare and is often listed as a state endangered or threatened species. However, because this species lives in large, sandy rivers, which are difficult to sample, it is probably more common than records indicate. In the Mississippi River bordering Missouri, it is frequently encountered when using the trawl and can be abundant in the river above St. Louis. Unlike most mayflies, this species lives primarily on the surface of sandy-bottomed rivers.
The nymphs are mostly yellow with black eyes but males are darker, having browns and reds over the body and bluish eyes. It is a ferocious predator eating mostly fly (chironomid) larvae (also called red worms), but also eats other mayflies. They crawl over the sand like crabs as they hunt prey. Eggs are deposited in the sand and lay dormant over winter. They hatch in May to June. The larvae grow quickly, completing their life cycle in one summer and will emerge in August as adults. Unlike some mayflies for which they are famous, this species does not emerge in masses covering bridges and roadways.
Ohio Shrimp (Macrobrachium ohione)
The genus Macrobrachium is known as the “large river shrimps.” Most are marine, but four species enter freshwater. Only one species, the Ohio shrimp, is found in Missouri. This species enters freshwater rivers of the Eastern Seaboard from Virginia to Florida and of the Gulf Coast from Alabama to Texas. In the interior United States it has been found in the Mississippi and Ohio river systems from Oklahoma to Ohio. It has also been found in the lower Missouri River where it is extremely rare.
Prior to 1940, the Ohio shrimp population was so abundant it supported a commercial fishery in the Mississippi River below St. Louis. Towns along the river, such as Cape Girardeau, often had “shrimp fries.” By the late 1940s the species became rare in the Mississippi River above Cairo. The last known collection of Ohio shrimp from this reach was at Cairo in 1962.
However, they were rediscovered by MDC scientists in 1991 from the Mississippi River near Cape Girardeau. Apparently, the species persisted in the river undetected in low numbers until a Mississippi River monitoring program was established at Cape Girardeau.
Ohio shrimp can be 4 inches long as adults. They are translucent when alive and have numerous brownish-orange spots or speckles covering the body. The head has a rather distinctly serrated rostrum (a spear-like protrusion projecting forward from the head). Ohio shrimp eat a variety of foods, including vegetation, invertebrates, and fish. They can quickly swim forward or backward to capture prey or avoid being eaten.
Ohio shrimp are amphidromous meaning individuals of various life stages regularly migrate between fresh and salt water. Reproduction occurs in freshwater but the young develop in saltwater. Juveniles migrate up freshwater rivers to mature, especially females. Ovigerous females (those carrying eggs) have been found in the Mississippi River near Cape Girardeau. This poses an interesting and yet unsolved mystery: If ovigerous females reproduce in Missouri, and young need saltwater to develop, can some fraction of the young survive in freshwater? Or do the ovigerous females swim all the way to the Gulf of Mexico to reproduce?
Blue Sucker (Cycleptus elongatus)
This species is rarely seen by anglers but when caught it garners attention. It has been given several colorful names such as blackhorse and schooner. This is a big-river fish distributed widely in the Missouri, Mississippi, and Ohio river systems, but it occasionally moves into larger tributaries. Blue Sucker reproductive migrations were
well known before dams were placed on many rivers, but are much reduced today. They were considered by several states to be an endangered or threatened species.
The blue sucker is a slender fish with a small head and mouth, and a long sickle-shaped dorsal fin. The back and sides are dark blue in some individuals but others may appear olive to pinkish. The belly is white. Like most suckers, it feeds on the bottom of rivers consuming a variety of immature insects (of which the Pseudiron mayfly nymph is a likely food item). A moderately large fish, it can weigh more than 20 pounds, but individuals larger than 5 pounds are rare. They spawn in Missouri in April over rock, gravel, and firm sand.
Larval blue suckers were rarely reported prior to the 1990s, probably because of the difficulty in being able to sample the relatively deep and swift water occupied by the young. Trawling is an effective method to capture larval and juvenile blue suckers. Given the efficacy of trawling and improved understanding of where these fish live, they are no longer thought to be rare over some of their range.
Trout-perch (Percopsis omiscomaycus)
This species is an evolutionary intermediate between the Salmoniformes (trouts) and the derived Perciformes (perches). On its back, posterior to the dorsal fin, is a fleshy, rayless structure called an adipose fin. Adipose fins are found on more primitive fishes such as catfishes and trouts. This is a thick-bodied, straw-colored fish with several rows of dusky, brownish spots traversing its sides. There are only two species of trout-perches (family Percopsidae), the other is found in the northwestern United States and is called the sand roller (P. transmontana). Trout-perch is a wide-ranging species found from Alaska to Maine and from the Hudson Bay to Virginia. It is common over most of its range but is uncommon in Missouri. It is best known in Missouri from Locust Creek.
This is a small fish usually less than 5 inches long. Little is known about its habits and life history in Missouri. It probably spawns in March along the edges of streams in shallow water. It eats aquatic insects and other small invertebrates. It is known to move from deep water during the day to shallow water to feed at night. Trawling has been very effective in sampling this species. We have documented trout-perch at depths of 30-60 feet during daylight hours in various parts of its range.
Cystal Darter (Crystallaria asprella)
This species gets its name because it is found in clear streams with clean sand and gravel and because it is well camouflaged by its color and markings; it appears as crystalline as the quartz sand deposits in which it is found.
The crystal darter is found in moderately large rivers from Minnesota to Louisiana and from Alabama to Indiana. It is considered to be uncommon to rare over most of its range. An eastern form found in the Upper Ohio and Cumberland rivers of Tennessee and Kentucky has been described as a new species, diamond darter (C. cincotta),
and it has been presumably extirpated from most of its range except in the Elk
River, West Virginia. In Missouri, crystal darters are found in the Gasconade and Meramec rivers in the northern part of its range and in the Black, and St. Francis rivers to the south. It is no longer thought to exist in the Floodway ditch system in the south.
The crystal darter is slender with a pale-yellow body and four or five brownish crossbars. The sides have several oblong dull blotches. The belly is white. This is a large darter that can be up to 7 inches long. Little is known about its habits and life history in Missouri. It probably breeds in late winter or early spring. It grows quickly; up to 6 inches long at the end of the second year of life. Food items include immature insects, especially midges, blackflies, and caddisflies.
Prior to the development of the Missouri Trawl, data suggested that this species was rapidly disappearing from Missouri. It was last collected from the St. Francis River and Floodway ditches in 1964. By the late 1970s, it was no longer seen from the Meramec River. Extant populations were known only from the Gasconade and Black rivers. However, trawling since 2000 has yielded many specimens in the Meramec, Gasconade, and Black rivers. Crystal darters observed in aquaria will bury into sand with only their eyes exposed. This was presumed by scientists but had not previously been documented.