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Table 2: Fish biomass estimates in Mississippi River habitats. Values in parentheses are standard error, sample size.

Reach

Habitat

Year

Method

Biomass
(kg $ ha-1)

Reference

UMR

Backwater

1947-1952

Rotenone

361 (90, 8)

Pitlo 1987

UMR

Channel border (slough)

1946

Rotenone

333 (141, 2)

Pitlo 1987

UMR

Backwater

1977-1984

Rotenone

596 (171, 4)

Pitlo 1987

UMR

Channel border (slough)

1979-1984

Rotenone

327 (92, 8)

Pitlo 1987

UMR

Backwater (side channels)

1976-1981

Rotenone

558 (478, 2)

Pitlo 1987

UMR

Channel border

1983-1984

Primacord

748 (413, 6)

Pitlo 1987

UMR

Main channel

1996-1997

Benthic trawl

21 (3, 114)

Dettmers et al. 2001

LMR

Backwater (oxbow lakes)

1984

Rotenone

741 (263, 5)

Lowery et al. 1987

LMR

Backwater (abandoned channal connected to river)

1984

Rotenone

34 (B, 1)

Lowery et al. 1987

LMR

Backwater (abandoned channel not connected to river)

1984

Rotenone

911 (559, 2)

Lowery et al. 1987

LMR

Borrow pits

1981

Rotenone

687.9 (132.6, 25)

Cobb et al. 1984

LMR

Dyke pools, <0.5 ha


Rotenone

153

Baker et al. 1991

LMR

Dyke pools, 0.5-4.0 ha


Rotenone

2,065

Baker et al. 1991

COMMERCIAL FISHERIES

There is no commercial fishing in the Headwaters. The National Marine Fisheries Service (NMFS) maintained commercial harvest statistics for the Mississippi River until 1977. Based on these records, annual catch in the UMR ranged from 12-16.5 million kg and followed a general downward trend (NMFS data presented in Risotto and Turner 1985). The Upper Mississippi River Conservation Committee (UMRCC) has compiled commercial harvest statistics for the UMR plus the reach of open river upstream of confluence with the Ohio River (i.e. the MMR) since 1945. In contrast to the NMFS landing statistics, the UMRCC data show wide fluctuations over time and a slight positive linear trend, despite the dominating influence of high common carp landings in 1958-1975 (Figure 5, Table 3). Highest harvest reported by the UMRCC is less than half the landings reported by the NMFS. Furthermore, the decline in catch during 1965-1973 evident in the NMFS statistics coincides with a period of peak catch in the UMRCC data. Although the statistics from both the NMFS and the UMRCC are based on self-reported data and may be biased, I consider the UMRCC data more reliable. The NMFS data are collected from diffuse sources. The UMRCC data are collected by each member-state fisheries agency. The combined catch of common carp, buffalo fishes, catfishes and freshwater drum is more than 90 percent of the total fish catch in the UMR. Catches of all these species or species groups except common carp have trended upward (Table 3). Catch of common carp was generally high during 1958-1975 and has decreased since; harvests have approximately doubled for buffalo fishes, catfishes and freshwater drum during 1945-1999 (Figure 5). Commercial harvest in the UMR likely is more driven by selling price and market demand than catch rate (J. Rasmussen and R. Maher pers. comm.).

In the LMR, NMFS statistics for 1954-1977 show catches of 6-12 million kg and increasing over time (Risotto and Turner 1985). No catch statistics comparable to those of the UMRCC exist. Self-reported commercial harvest statistics have been collected by the Tennessee Wildlife Resources Agency since 1990 and by the Kentucky Department of Fish and Wildlife Resources since 1999. Annual catch from the Mississippi River bordering Tennessee (river km [Rkm] 1 151-1 458, approximately 259 km2) during 1991-2000 varied from 36-125 tonnes (Figure 6) but trended upward (Table 3). Landings of blue catfish and flathead catfish have increased substantially, while harvests of common carp, buffalo fishes, channel catfish and freshwater drum have been highly variable. In Kentucky waters of the Mississippi River (Rkm 1 458-1 534), catch ranged from 18-56 tonnes during 1999-2001. As in Tennessee, buffalo and catfishes predominated the catch. Commercial landings are measured in Louisiana but are not assigned to specific waters. The other states with jurisdiction over the LMR either do not measure commercial catch or do so sporadically. Fluctuations in catch probably do not reflect variation in catch rate but, as in the UMR, are driven by price and market demand. However, LMR catches also vary as commercial fishers direct their fishing effort to other waters. For example, some fishers in the upper portion of the LMR will fish both the Mississippi and the Tennessee rivers and choice of fishing site is dictated by fishing conditions in both rivers.

Figure 5. Commercial fish harvest in the Upper Mississippi River. A. Total fish harvest and value. B. Harvests of common carp, buffalo fishes, catfishes, and freshwater drum.

Table 3: Trends in commercial harvest (metric tons) of fish in the upper Mississippi River and Tennessee waters of the lower Mississippi River. N is sample size (number of years).

Location
Species or species group

Linear trend

N

R2

P

Upper Mississippi River

Total fish

Harvest = -34927 + 20.066(year)

52

0.13

<0.01

Common carp

Harvest = 25227 - 11.801(year)

52

0.10

0.02

Buffalo fishes

Harvest = -25951 + 13.724(year)

52

0.63

<0.01

Catfishes

Harvest = -12111 + 6.503(year)

52

0.29

<0.01

Freshwater drum

Harvest = -14373 + 7.605(year)

52

0.46

<0.01

Paddlefish

Harvest = 78 - 0.021(year)

54

0.00

0.91

Shovelnose sturgeon

Harvest = -479 + 0.254(year)

55

0.11

0.01

Tennessee

Total fish

Harvest = -6368 + 3.233(year)

10

0.11

0.35

Common carp

Harvest = 563 - 0.279(year)

10

0.02

0.67

Buffalo

Harvest = 992 - 0.484(year)

10

0.02

0.66

Channel catfish

Harvest = 2307 - 1.147(year)

10

0.09

0.41

Blue catfish

Harvest = -8492 + 4.263(year)

10

0.63

<0.01

Flathead catfish

Harvest = -3387 + 1.701(year)

10

0.68

<0.01

Freshwater drum

Harvest = -172 + 0.089(year)

10

0.01

0.85

Although relatively minor components of the UMR commercial fishery, shovelnose sturgeon and paddlefish fisheries are significant management concerns. Substantial efforts are underway by State and Federal fisheries agencies to conserve or increase the stocks of paddlefish; yet, commercial fishing is still allowed in two of the five states. Paddlefish harvests have fluctuated widely, but without any long-term linear trend (Figure 7, Table 3). As observed by Rasmussen (in press), shovelnose sturgeon harvests have fluctuated but trended upward. Most noticeable is the sharp increase in 2000-2001. The upsurge is attributed to rapidly increasing value of the roe resulting from international declines in sturgeon roe harvest. Concern for these fish is warranted, because population information is limited and the long life and late maturity make these fish susceptible to recruitment overfishing. High mobility of the fish and the roe fishery mandates multi-jurisdictional management.

As in any fishery, appropriate harvest is an important fisheries management issue. Using NMFS commercial fishery statistics, Risotto and Turner (1985) found estimated fish harvests from the Mississippi River fell within the realm of expected harvests based on global harvest-drainage area and harvest-river length relationships developed for large rivers by Welcomme (1979). Further, small and trend-less variations in catch over 25 years and stable catch at varying effort levels led Risotto and Turner to conclude the Mississippi River was harvested at near optimal levels. The average harvest for the LMR was 11 000 tonnes and average effort was 7 000-8 000 fishers per year during the 25 year period (Risotto and Turner 1985). However, the substantial differences in catch magnitude and trend between the UMR and NMFS data detract from the well-intentioned analyses by Risotto and Turner (1985). Pitlo (1997) demonstrated over harvest of high market-value channel catfish in Iowa waters of the UMR. Implementation of a 15-inch (38 cm) minimum size limit in 1985 increased both yields and recruitment index of the channel catfish; as of 1997 the population was continuing to expand. The general trend of increasing harvest from the UMR suggests the stocks presently are not over harvested in the upper portion of the Mississippi River. The trend of increasing harvest of total fish and high-value catfishes in the Tennessee reach also suggests stocks are not over harvested in at least a portion of the LMR. Indeed, the commercial harvest from Tennessee waters of the Mississippi River is low compared to other Tennessee waters. During 1996-2001, the annual harvest of blue catfish from Tennessee waters of the Mississippi River averaged 1.1 kg ha-1 and harvest of flathead catfish averaged 0.4 kg ha-1. In nearby Barkley Lake and Kentucky Lake, impoundments of the Cumberland and Tennessee rivers, respectively, annual harvest of blue catfish was 5.5-8.6 kg ha-1 and annual harvest of flat-head catfish was 1.0-1.3 kg ha-1. The catfish fisheries of Barkley and Kentucky lakes are not considered over harvested (R. Todd pers. comm.); thus the low harvest from the Mississippi River suggests its catfish stocks may support considerably greater harvest. The size and age structures of the commercial fisheries are not routinely monitored. However, beginning in 1988 a standardized fishery assessment program was implemented in the UMR. Evaluation of length frequency distributions of important sport and commercial fishes (Gutreuter et al. 1997; Gutreuter et al. 1998) indicates adequate recruitment and length distributions show no evidence of overfishing. UMR catch rate trends from the standardized assessment indicated one commercial species declined and two commercial species increased in abundance; overall, there was no evidence for a general decline in abundance (Gutreuter 1997). At this time, the commercial fish stocks in the Mississippi River appear stable and, at least in portions of the LMR, may support additional harvest. Shovelnose sturgeon stocks should be closely monitored.

Figure 6. Commercial harvest in Tennessee waters of the Mississippi River.

Figure 7. Commercial harvest of shovelnose sturgeon and paddlefish, Upper Mississippi River.

RECREATIONAL FISHERIES

The Mississippi River is a bountiful recreational fishing resource. The Headwaters is entirely within the state of Minnesota and the Minnesota Department of Natural Resources frequently conducts creel surveys on portions of the river and the river lakes. Fishing effort ranges from 13-47 hours ha-1 and harvest ranges from 5-16 kg ha-1 (Albert 1995; Bublitz 1996; Sledge 1998, 2000; Ekstrom 1999). Fishing effort, catch rate, harvest rate and mean size of fish caught have remained steady or trended upward over the past 20 or more years. Prevalent recreational species include northern pike, channel catfish, smallmouth bass, white crappie, black crappie and walleye.

In the UMR, recreational fishing effort and catch have been measured sporadically on several pools. Averaged for 7 of the 26 pools, annual harvest ranged from 8.1-9.4 kg ha-1 from 1962-1973. Throughout the UMR, annual recreational fishing effort was 4.6-5.2 million hours and harvest was 1.2-1.4 million kg during the 1962-1973 time frame. Catch and effort were stable over this brief time frame. More recent creel surveys estimated fishing effort of 18-64 hours ha-1 and catches of 13-100 fish ha-1 in four different UMR pools (Fleener 1975; Ackelson 1979; Watson and Hawkinson 1979; Farabee 1993). Recreational fishing effort and harvest are relatively low in the headwaters and UMR; for comparison, angler effort averages 88 h and harvest 13.3 kg ha-1 year-1 in U.S. reservoirs (Miranda 1999), many of which are serial reservoirs on rivers (like the UMR) but have longer retention times (c.f. Jenkins 1967). Analysis of abundance trends in the UMR during 1990-1994 indicated that three sport species declined and two sport species increased; declining species were fishes associated with backwaters while species that increased were riverine (Gutreuter 1997).

The recreational fishery has not been measured in the MMR or LMR reaches of the open river. Personal observations on the LMR suggest that freshwater fishing catch rates are relatively high; but effort and thus catch and harvest, are extremely low. Because of the large size, swift and dangerous currents, the presence of large commercial craft and lack of public access, recreational fishing on these reaches has been largely discouraged. Providing access is difficult because of the large annual fluctuations in river level and separation of many of the remaining floodplain lakes from the river during low water stages (see below). Management agencies are only beginning to recognize the potential fisheries that the Mississippi River offers and measures are being initiated to improve access and public education regarding the fishing opportunities. Although catfishes are important to both recreational and commercial fisheries and channel catfish suffered overfishing before increasing the minimum length limit, recreational fish stocks do not presently appear overfished and, especially in the LMR, can withstand increased harvest.

MANAGEMENT OF THE MISSISSIPPI RIVER

Ten state agencies manage the Mississippi fisheries resources by establishing and enforcing harvest methods and limits and by providing boating and fishing access. The U.S. Fish and Wildlife Service and the five bordering states manage about 184 000 ha of lands and wetlands adjacent to portions of the UMR and MMR (Theiling et al. 2000). However, the U.S. Army Corps of Engineers (COE) is mandated by the Federal government to control flooding throughout the Mississippi basin and maintain commercial navigation on the Mississippi River from Minneapolis, Minnesota to the mouth. Up to the present, flood control and navigation have dominated management of the Mississippi River-floodplain ecosystem. Thus, the COE manages the habitat and, therefore, is the principal manager of the Mississippi River downstream of St. Anthony Falls. Most of the major fisheries management issues in the Mississippi River are related to flood control and navigation.

SEDIMENTATION

The Mississippi River has always carried sand and sediment to the Gulf of Mexico. Agricultural development of the Mississippi River basin has increased sediment inputs; however, for the LMR, some increases have been offset by impoundment of the UMR, the Ohio River and, principally, the middle Missouri River. Although most of the sediment originates in the watershed (a relatively small amount results from bank erosion and re-suspension related to navigation traffic [Bade 1980]), it is the management of the river for navigation (i.e. impoundment, channelization and dredged material disposal) and flood control that has resulted in rapid rates of sediment accumulation and habitat loss in off-channel and floodplain habitats. In the UMR, impoundment has slowed the water flow, causing navigation pools to become sediment traps. Directing flows through the navigation channel has reduced flows through side channels, increasing sedimentation during low flows and decreasing scouring during high flows. Thus side channels and backwaters are most impacted by sedimentation; the expected life of backwater habitats may be as short as 50 years (Simons, Schumm and Stevens 1974; Bade 1980; Breitenbach and Peterson 1980). These are productive habitats and essential for one or more life stages of many species (e.g. Christenson and Smith 1965; Schramm and Lewis 1974; Holland 1986; Shaeffer and Nickum 1986; Rasmussen 1979; Grubaugh and Anderson 1988). By providing warmer water and refuges from the current, they are especially important overwintering habitats (Pitlo 1992; Bodensteiner and Lewis 1992; Sheehan, Lewis and Bodensteiner 1990); but their value rapidly diminishes as sediment reduces water depth (McHenry et al. 1984; Bhowmik and Adams 1989; Gent, Pitlo and Boland 1995; Knights, Johnson and Sandheinrich 1995). Although the time to fill various habitats varies within and among pools, without continued regulation the river will restore its historic channel dimensions. For the pools of the UMR, this means continual sedimentation until the channel cross-sectional area equals the collective cross-sectional area of the pre-impoundment channel. Site-specific dredging is required annually to maintain the navigation channel. While the amount of sediment excavated may be small relative to the cumulative sediment load, this material may substantially affect biota depending on the disposal method. Disposal lateral to the navigation channel can alter channel border and backwater habitats. Some fishery benefits have been gained by using dredged materials to build islands in the channel border zone (Johnson and Jennings 1998).

In the open river reach flows remain essentially unchanged and the loss of habitat from sediment deposition probably resembles natural processes that occurred 10 000 years B.P. However, channelization, which not only maintains channel depth and width but also trains the channel course, prevents the river from carving new channels. Sediment deposition occurs in backwaters, both those on the floodplain and in abandoned channels confluent with the river. These backwaters appear to support the greatest biomass of fishes and provide important, if not essential, habitat for one or more life stages of most native Mississippi River species (Beckett and Pennington 1986; Baker et al. 1991; Table 1). In the LMR, Schramm et al. (1999a) estimated 8 400 ha of backwater habitat within the riverbanks and 53 300 ha on the floodplain. However, existing lakes are rapidly filling (Gagliano and Howard 1984; Cooper and McHenry 1989). Borrow pits created when soil was excavated from the batture lands (the floodplain from the levee to the river) to build the levees in the 1930s can provide important fish nursery areas (Sabo and Kelso 1991; Sabo et al. 1991). However, many of these borrow pits have filled during the 60-70 years since their construction. In the upper portion of the open river, abundance of several riverine species appears to be increasing while backwater species are decreasing (Bertrand 1997) although based on only 5 years of data, a similar trend was noted in the UMR (Gutreuter 1997).

Sediment deposition in the open river reaches also occurs in secondary channels, former main channels or new channels in the making that are separated from the present main channel by large sandbars or islands. The secondary channels are usually 5 km or longer. Dyke fields that divert water to align and hydraulically dredge the navigation channel reduce flows through many of these secondary channels. Dyke placement initially increases habitat diversity by the addition of hard bottom substrate (rock riprap), by creating deep scours at the tip and immediately downstream of the dykes and by providing reduced or zero current pools further downstream of the dykes. These areas temporarily harbour high diversity, density and biomass of fish (e.g. Pennington, Baker and Bond 1983a; Nailon and Pennington 1984; Beckett and Pennington 1986; Baker et al. 1987, 1988a) and are inhabited by different fish assemblages than open side channels (Barko and Herzog in press). However, the dykes slow the flow through the secondary channel; the water-borne sediment is deposited downstream of the dykes, filling the scours and pools, eventually covering much of the rock riprap dyke and resulting in net loss of both aquatic area and habitat diversity. In an effort to conserve aquatic habitat and diversity, the COE is evaluating the benefits of large (15-30 m wide) notches in dykes to allow more water to flow through the secondary channel, thereby reducing sedimentation. This technique has been used successfully on the UMR and Missouri rivers. Elevation of the bottom of the notch is usually at or below the Low Water Reference Plane to ensure some flow through the secondary channel during low water stages. During low flow, catch rates of lentic fishes (e.g. shads, white bass) are higher downstream of un-notched dykes, whereas catch rates of rheophilic catfishes are greater downstream of notched dykes (Schramm et al. 1998, 1999b). Steep natural banks support relatively high densities of fish (Pennington, Baker and Potter 1983b; Baker et al. 1988b; Driscoll 1997; Driscoll, Schramm and Davis 1999). These productive banks with irregular current-washed walls of clay and clay-sand substrate, deep holes, variable currents and concentrations of large woody debris are essentially unique to secondary channels; such a bank in the main channel would be armoured with articulated concrete mattress and rock riprap to prevent erosion. Filling of LMR secondary channels will eliminate steep natural banks.

THE FLOODPLAIN, CONNECTIVITY AND THE FLOOD-PULSE CONCEPT

Current thinking about floodplain-river ecosystems predicts fish production is a function of floodplain inundation (Junk, Bayley and Sparks 1989; Bayley 1995). The floodplain provides energy that is consumed directly (e.g. plant or animal material produced on the floodplain between floods) or indirectly (e.g. through trophic webs) by fishes colonizing the floodplain during the flood pulse. The floodplain also provides essential or desirable spawning and nursery habitat for many native fishes. Further, the floodplain contributes to riverine fish production when terrestrial plant and animal material and fish produced on the floodplain are flushed into the river with flood flows or drain into the river with receding water levels. In keeping with the flood-pulse concept, fisheries production should show some relation to the amount of food, spawning, or rearing resources available; thus, fish production would be related to the area of floodplain inundated. Levees isolate the river from much of the historic floodplain beginning at Pool 15 in the UMR and throughout the open river reach. Flood proofing at the Mississippi River valley, particularly the open river where vast areas of the floodplain have been reclaimed for agricultural and related developments, is a polarized issue. One alternative favours protecting personal property and agricultural production and the economy (local, regional and national) associated with it. Another option proposes minimizing economic loss resulting from repeated disaster recovery payments and crop and flood insurance payouts by the Federal government and achieving the fish, wildlife and economic (e.g. hardwood timber) benefits expected from a larger and presumably more functional floodplain. Proponents of this non-structural alternative advocate levee removal, notching, or relocation further from the river to reconnect at least portions of the floodplain to the river. Such an action would necessitate Federal government purchase of private lands or flood easements from willing sellers and even relocating some towns that would be impacted by floodwaters. Advocates argue that costs would be lower than repetitive emergency relief and flood insurance payments.

Economic value of personal property and commodities lost and gained on the restored floodplain can be estimated, but the effect on fish and wildlife and the subsequent value gained or lost is unknown. Despite this, evaluations of the relationship between fish production and floodplain inundation in the Mississippi River are few. Results in the UMR lend support to the applicability of the flood-pulse concept to the Mississippi River. Growth of littoral zone (floodplain-dependent) fishes was higher during a year of protracted flooding than in other years, but growth of a riverine species did not differ over the same time frame (Gutreuter et al. 1999). Studies in the LMR failed to find expected relationships between growth and abundance of age-0 and 1 Mississippi River fishes and measures of floodplain inundation (Rutherford et al. 1994; Rutherford et al. 1995). Risotto and Turner (1984) found no relationship between commercial harvests and area of floodplain inundated. These LMR evaluations suggested that failure to find expected positive relations between fish growth or abundance and floodplain inundation may be related to the reduction of active floodplain area. Employing a bioenergetic approach, Eggleton (2001) failed to find a clear linkage between catfish growth and floodplain inundation. The evidence in support of the floodplain as a primary determinant of fish production in the Mississippi River is far from compelling; but, as discussed below, floodplain function may have been compromised by the interaction of several alterations to the river and its floodplain.

In the LMR, catfish growth was not significantly related to area or duration of floodplain inundation; however, a strong positive relationship emerged between catfish growth and extent of inundation when water temperature exceeded 15C, a threshold temperature for active feeding and growth by catfishes (Mayo 1999; Schramm, Eggleton and Mayo 2000). These results need validation with a longer time series. However, in support of the importance of temperature, the increased growth of littoral zone fishes observed by Gutreuter et al. (1999) occurred during an unusually late summer flood when water temperatures were conducive to active feeding and rapid growth.

Considering the flood-pulse concept is largely based on and supported by studies of rivers in tropical and subtropical climates (Junk et al. 1989), consideration of a thermo-temporal component (Schramm et al. 2000) or the coupling of temperature and flooding (Junk et al. 1989) may be appropriate for the temperate Mississippi River with warm water fish assemblages. Although water temperature data are not available, the thermal conditions of the current flood pulse probably are quite different from historical conditions. During 1928 to 1942, two coincidental changes dramatically affected hydrology in the LMR. The first is the aforementioned levees. Although numerous small levees have existed since the mid 1800s (Baker et al. 1991) the river still remained connected to much of the floodplain until the continuous mainline levees were built during 1928-1937. The other change was cut-offs. During 1929-1942, 16 meander loops were bypassed by constructing cut-off channels (Baker et al. 1991). These cut-off channels shortened the river by 245 km and subsequently increased the slope. The hydrologic consequences of the cut-offs were less frequent, lower and shorter duration flood pulses (Figure 4). Presently, the floodwaters inundate the floodplain relatively briefly and subside earlier in the year; consequently, the water is colder. Schramm et al. (2000) found thermal conditions on the floodplain suitable for spawning and growth of warm water fishes occurred only twice in six years during 1993-1998. Before levees were constructed, floodwaters spread over a broad, flat floodplain; the waters likely warmed quickly and receded slowly as flood discharges subsided. Conversely, the same discharges confined by levees into a narrower floodplain produced deeper inundation, shorter retention time and substantial current in many areas (Welcomme 1985; Satterlund and Adams 1992). These waters warm slowly (Schramm et al. 1999; Schramm et al. 2000; Eggleton 2001) and probably recede quickly. From this reasoning, the net result of the cut-offs and levees is a greatly reduced floodplain of only infrequent value for feeding or reproduction of warm water fishes. The inconsistent recruitment of several floodplain-dependent fishes sampled in LMR floodplain ponds (levee borrow pits Cobb et al. 1984) lends support to this contention. The role of the floodplain and the effect of the current hydrologic regime on Mississippi River fisheries require further evaluation.

The LMR continues to adjust to the change in slope that resulted from the cut-offs. To regain its original slope the channel is degrading (incising) upstream of Rkm 700 and aggrading downstream. Channel degradation results in a lower elevation for a given discharge. This in turn results in lower summer and fall water levels in some floodplain lakes and severs their connection with the river.

In the impounded UMR, a flood pulse still occurs (Figure 5) and both the timing and the duration of the flood pulse are similar to pre-impoundment conditions. Although the area seasonally inundated by the flood pulse may be less than before impoundment, except during years of exceptionally high precipitation, there is no evidence to suspect that thermal conditions of the flood pulse differ from historic conditions. However, dams and regulation of minimum water levels necessary for navigation have eliminated the flood-drought pulse, a summer dewatering of the historic floodplain and the fall rise. The summer draw-down would benefit consolidation and aeration of the sediments and growth of terrestrial vegetation. Fisheries managers have recommended summer pool drawdowns to improve habitat and benefit UMR fish populations (J. Rasmussen pers. comm.). Obviously, a major summer drawdown to mimic natural conditions is incompatible with navigation. Although brief, the fall-rise may have substantially contributed to fisheries production. Peak plant senescence occurs in autumn and coincides with substantial elaboration of benthic macroinvertebrate biomass (Anderson and Day 1986). The fall-rise probably flushed the plant material into the river where it could be used by the benthos (Grubaugh and Anderson 1988).

NAVIGATION

In addition to the habitat alteration from creating and maintaining a navigable channel, navigation and directly related activities affect fish populations. Commercial navigation upstream of Baton Rouge consists of barges pushed by towboats. The greatest economy is achieved by a single, powerful towboat pushing the largest number of barges. Although fish mortality is difficult to quantify, entrainment of larval and juvenile fishes by towboat propellers is significant (Bartell and Campbell 2000). Additional mortality results from stranding associated with wakes from the tows or when approaching barges cause temporary drawdown (Adams et al. 1999). In the UMR, Gutreuter, Dettmers and Wahl (in press) estimated adult fish losses from entrainment at 2.65 clupeids, 0.53 shovelnose sturgeon and 0.53 smallmouth buffalo per km for each towboat. Use of the UMR by recreational powerboats is relatively high (e.g. Watson and Hawkinson 1979; Farabee 1993; Carlson, Propst, Stynes et al. 1995; Gutreuter et al. 1999) and likely also contributes to fish mortality from wave action and stranding. However, because recreational vessels are shallower draft and have smaller propellers, losses from entrainment would be expected to be lower.

Driven by local economies and competition from agricultural production in other countries, commercial river traffic is forecast to increase. Bartell and Campbell (2000) estimated recruitment losses in four pools of the UMR from a 25 percent increase in traffic ranged from 1 420 fish for walleye to 88 million fish for emerald shiner. Linear increases in losses were predicted for further increases in traffic. However, the predictions do not include the effects of cumulative stress to the fish from increased frequency of entrainment, increased habitat disturbance (e.g. increased sediment suspension, more bank erosion), loss of habitat from activities and development associated with increased commerce (e.g. barge staging areas, docking areas) and elevated probabilities of toxic spills (Breitenbach and Peterson 1980), all of which may adversely affect fish survival and may operate in a synergistic, multiplicative fashion. Furthermore, entrainment losses probably are site specific; entrainment is expected to be higher in a narrow, shallow channel where a larger portion of the water column passes through the propellers than in a wider and deeper channel. In the latter case fish can more easily escape entrainment. Present traffic levels kill fish, but catch and harvest data indicate the various populations are able to support existing fisheries. Whether navigation-related mortality acts in a compensatory or additive fashion with natural mortality (i.e. whether the estimated foregone production actually reduces population size and, in turn, catch and harvest) awaits resolution.

WATER QUALITY

The Mississippi River flows through a sea of intensive agriculture dotted with islands of urban development. As such, the river is the inland sink for fertilizers, pesticides and domestic and industrial wastes. During the 1940s-1960s the river and its aquatic life were severely impacted by pollution. Segments of the river downstream of Minneapolis and St. Paul, Minnesota suffered severe oxygen depletion (Fremling 1964, 1989). Improved wastewater treatment and agricultural practices have reduced nutrient and toxic chemical loads. Although the river still shows the effects of agriculture, industry and urban development and persistent toxicants remain in the sediments, water quality is improved (Meade 1995; Fremling and Drazkowski 2000; Sullivan et al. 2002) and the river supports fish throughout its length that generally are safe for human consumption. Yet, fish health remains impacted by various contaminants, in particular bioac-cumulative organic compounds, throughout the river (Schmitt 2002). Nutrient dynamics have undoubtedly been changed by habitat alterations. The UMR pools are sediment traps and, thus, remove nutrients and toxins associated with sediment. Impoundment also likely contributes to biological processing of nitrogen and phosphorus. Conversely, biological filtration may be reduced in the MMR and LMR, where the spatially reduced floodplain and the briefer, colder and faster-flowing flood pulse likely results in less assimilation of nutrients. These conditions may, in turn, contribute to downstream problems, such as nutrient accumulations and hypoxic conditions in the Gulf of Mexico (Rabelais et al. 1996; Rabelais, Turner and Wiseman 2002). Comprehensive assessments of contemporary water quality are given by Meade (1995), Schmitt (2002) and Sullivan et al. (2002).

INTRODUCED SPECIES

Confluence with waters draining 3.25 million km2 and connection to international commerce via the Gulf of Mexico and the Great Lakes makes the Mississippi River highly vulnerable to invasion by non-native aquatic species. Non-native animal species presently established in the Mississippi River include the common carp, grass carp, silver carp, bighead carp and zebra mussel (Dreissena polymorpha Pallas). Except for the common carp, the impacts of these species in the Mississippi River are not known. However, populations are expanding and competition with native fauna is likely. A comprehensive assessment of potential Mississippi River invaders and their impacts is available in Rasmussen, Pitlo and Steingraeber (in press).

SUSTAINING FUTURE FISHERIES

At present, the River’s native fish assemblage appears intact (Fremling et al. 1989; Gutreuter 1997; Weiner et al. 1998), but a substantial number of species are considered rare and I found no information to elevate their abundance status. With the exception of sturgeons, sport and commercial fisheries show no signs of overfishing and may even support increased effort and harvest. However this level of apparent abundance may be short lived as additional backwater habitat disappears, the remnant active floodplain only intermittently contributes to fish production and nonnative species invasions take their toll. Water quality is improved but fish are still stressed by contaminant burdens. While the river has been managed to achieve maximum economic benefit for man, fisheries resources have been largely ignored.

The UMRCC has identified strategies to sustain UMR fisheries and other natural resources (Duyvejonck 2002). These include: improve water quality, reduce erosion and sedimentation, reclaim the floodplain to allow channel meanders and to increase habitat diversity, provide for an effective flood pulse and periodic low flows, connect backwaters to the main channel, manage the channel and dredge material to improve habitat, prevent the spread of non-native species and provide native fish passage at dams. With minor exceptions, these strategies are also applicable to the open river. Engineering technologies are available to accomplish these strategies and some restoration has been done (e.g. Bade 1980; Knights et al. 1995; Johnson and Jennings 1998). However, fundamental to these strategies and the general conservation and management of the fishery resources of the Mississippi River are (1) implementation of a system-wide assessment program and (2) societal recognition of the multiple values of the Mississippi River.

A system-wide assessment program is essential to comprehensively assess the status of the fish assemblage and individual populations, identify management needs, provide essential biological and ecological information to guide management and restoration efforts and evaluate the progress of management and restoration activities. A resource assessment program (Long-Term Resource Monitoring Program), initiated on the UMR in 1988, has implemented a systematic assessment of UMR fisheries and aquatic resources that provides information useful for management decisions. This program should be expanded to the entire navigation reach of the river. Advances in sampling methodology, such as benthic trawling (Gutreuter, Dettmars and Wahl 1999; Dettmers et al. 2001) and electrofishing (Burkardt and Gutreuter 1995; Pugh and Schramm 1998; Schramm and Pugh 2000), increase sampling efficiency, expand the range of habitats that can be sampled, allow better comparability among habitats and likely will contribute to a better understanding of the system as a whole. A comprehensive, long-term assessment program is expected to stimulate further development and refinement of sampling methodologies. Application of geospatial technologies will help monitor system changes and contribute to more effective assessment. Advances in remote sensing technologies may be especially applicable to monitoring and managing sedimentation, measuring fish abundance and measuring physical conditions where the fish live (in contrast to surface measurements). Advances in analytical (statistical) procedures may help assess trends and evaluate habitat suitability and requirements. Fisheries assessment has been confounded by the dynamic nature of the river and interpretation of fisheries data has been hampered by high variability. Habitats and the physical and biotic conditions that create them are not discrete; they are continua in space and in time. Statistical procedures that evaluate multiple variables (e.g. Johnson and Jennings 1998) or gradients (e.g. Brown and Coon 1994; Eggleton 2001; Barko and Herzog in press) are less encumbered by variation (which may be inherent to the system) and may prove efficient tools for evaluating habitat change and management efforts.

The second fundamental need is to change social perception of the river, especially the MMR and LMR and to establish value for the natural resources of the Mississippi River. There is much to be done to restore the Mississippi River and the technologies are available. Projected changes in the river-floodplain ecosystem foretell increasing management and restoration needs. The Federal government spends several hundred million U.S. dollars annually to maintain navigation and flood control in the LMR. Less than 1 percent of this amount is spent for assessment and management of fisheries resources. Recreational use of the Headwaters is substantial. Recreational use of the UMR is valued at US$1.2 billion per year (Carlson et al. 1995); of the estimated 12 million annual visits to the river, 49 percent were for fishing. Our society, including lawmakers, needs to be aware of the changes in this vast system that are adversely affecting its ecological function and value to man. Without societal support, management and restoration of the Mississippi River to achieve fishery and other natural resource benefits will not be a priority. Achieving societal support is difficult in the UMR, but will be even more difficult in the LMR where the river is largely inaccessible and where recreational use is probably three orders of magnitude lower.

ACKNOWLEDGMENTS

Hydrographic data used in this report were provided by Richard Engstrom, Allen Phillips, Tim Rodgers, U.S. Army Corps of Engineers. James Rogala, U.S. Geological Survey, provided spatial data for the Upper Mississippi River floodplain-river ecosystem. Commercial fisheries data were provided by Jon Duyvejonck, Upper Mississippi River Conservation Committee, Karen Hukill, Kentucky Department of Fish and Wildlife Resources, Michelle Marron, Wisconsin Department of Natural Resources and Robb Todd, Tennessee Wildlife Resources Agency. Steve Ellis and Wayland Hill, U.S. Army Corps of Engineers provided valuable information about levee construction and river hydrology. Jon Duyvejonck, Steve Gutreuter, U.S. Geological Survey, Bob Hrabik, Missouri Department of Conservation and Ron Nassar and Jerry Rasmussen, U.S. Fish and Wildlife Service, provided useful reviews of earlier versions of this manuscript. Andy Bartels, Wisconsin Department of Natural Resources, Melvin Bowler, Iowa Department of Natural Resources, Daniel Dieterman and Konrad Schmidt, Minnesota Department of Natural Resources, Robert Hrabik, Jerry Rasmussen and Stephen Ross, University of Southern Mississippi provided input on the status and distribution of Mississippi River fishes (Table 1).

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