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Seasonal diet composition of adult shovelnose sturgeon in the Middle Mississippi River.

INTRODUCTION

Due to commercial harvest and habitat degradation worldwide, sturgeon numbers have been declining for nearly 100 y (Keenlyne, 1997; Shuman, 2003). Mature female sturgeon are selectively harvested for caviar, rendering populations especially vulnerable. Because sturgeon delay sexual maturity, reproductive potential is further reduced by selective harvest (Colombo et al., 2007; Tripp et al., 2009a). This reproductive loss coupled with habitat degradation has led to the collapse of multiple sturgeon populations and may be threatening one of the last commercially viable populations in the world (Billard and Lecointre, 2001; Ludwig et al., 2002; Secor et al., 2002; Tripp et al., 2009a, b). Shovelnose sturgeon Scaphirhynchus platorynchus are the most abundant member of the family Acipenseridae within the Mississippi River and support commercial fisheries in portions of the Mississippi and Missouri rivers (Kline and Golden, 1979; Carlson et al., 1985; Hurley et al., 1987). In the free-flowing Middle Mississippi River extending from Cairo, Illinois to St. Louis, Missouri (MMR; RKM 0-200) shovelnose sturgeon populations have declined perhaps due to habitat degradation and commercial harvest (Keenlyne, 1997; Morrow et al., 1998;Jackson, 2004; Colombo et al., 2007; Tripp et al., 2009a, b). Market price for shovelnose sturgeon eggs is high compared with other commercial fish species in the Mississippi River basin (Becker, 1983). In order to maintain a harvestable shovelnose sturgeon population, we need to eventually understand their life history, which guides management and ultimately drives regulations.

As with all organisms, energy intake affects shovelnose sturgeon growth and survival. Diet composition and quantity can dramatically influence condition and thus influence reproduction. Shovelnose sturgeon diets have been quantified in several large rivers: Platte River (Shuman, 2003), upper Mississippi River (Hoopes, 1960; Helms, 1974; Carlson et al., 1985), and Missouri River (Held, 1969; Modde and Schmulbach, 1977; Carlson et al., 1985; Megargle, 1997; Berry, 2002; Braaten et al., 2006; Wanner et al., 2007). Although many shovelnose sturgeon diet studies have been completed, we are unaware of any studies that have been conducted in the MMR, which is free-flowing and geologically unique to the remainder of the Mississippi River drainage. Composition of diets by season is also unknown in this system. Thus we sought to determine diet composition of shovelnose sturgeon in the MMR across seasons. Since multiple shovelnose sturgeon diet studies have been completed we decided to look at the relationship seasonal diets have with temperature and discharge. This information will allow us to further understand their ecology and help with management decisions.

METHODS

To quantify adult shovelnose sturgeon (fork length 319-786) diet composition in the MMR, fish were sampled monthly during Jan. 2005 through Nov. 2005 using stationary bottom-set gill nets [5.08 cm bar mesh, 45.70 m long, 3.05 m deep]. During each month, six nets were set for 24 h off the tips of wing dikes, parallel to the flow in an area of converging water velocities at Modoc, IL (RKM 201-198), Chester, IL (RKM 191-188) and Grand Tower, IL (RKM 127-124). These locations were chosen based on prior knowledge of high shovelnose sturgeon densities (Tripp, 2007). Fork length (FL) was measured to the nearest 1 mm for each fish. Relative condition (Wr) was calculated for shovelnose sturgeon using a standard Ws equation (Quist et al., 1998). Water temperature (C) was collected at the surface during each sampling trip and from this we modeled a continuous graph. River stage height (m) was determined from the U.S. Geological Services' stream gage at Chester, IL.

Each month, a subsample of the first 20 adult shovelnose sturgeon collected at each site was preserved on ice and taken back to the lab. A mid-ventral incision was made from the anus through the pelvic girdle, exposing the stomach. A section of the alimentary tract was removed between the esophagus and the junction of small intestine and the spiral valve, hereafter termed the stomach (Modde and Schmulbach, 1977). The contents of the stomach were flushed and stored in 10% buffered formalin.

Stomach contents were identified to family for orders Ephemeroptera, Diptera, Odonata, Coleoptera, Plecoptera, Decapoda, and Trichoptera (Merrit and Cummins, 1996). All other less common food items present were identified to order. Infrequent food items in the sturgeon stomachs were grouped and labeled as "other." Shovelnose sturgeon diet composition was examined by seasons; winter (Jan.-Feb.), spring (Mar.-May), summer (Jun.-Aug.) and fall (Sept.-Nov.). Stomach contents were quantified for each prey type as frequency of occurrence (i.e., percent of stomachs that contained prey), percent by number (i.e., number of prey items divided by total number of stomach contents) and percent of total mass (mass of prey over total mass of stomach contents; Bowen, 1996; Chipps and Garvey, 2007; Wanner et al., 2007). After the prey items were identified and counted, they were then weighed for wet mass to the nearest 0.0001 gram. Chi-square analysis was performed by means of SAS for percent composition by mass among seasons for the five major prey items (Family: Chironomidae, Hydropsychidae, Ephemeridae, Corophiidae and other).

RESULTS

Stomachs were removed from 562 (winter n = 71; spring n = 163; summer n = 149; fall n = 179; fork length mean = 617 mm; standard deviation = 64.57; fork length 319-786 mm) adult shovelnose sturgeon. Only 170 stomachs (winter n = 65; spring n = 95; summer n = 6; fall n = 4) contained prey items. Of those stomachs, we identified prey items (winter n = 36; spring n = 77; summer n = 6; fall n = 4; total n = 123) based on size structure per 2.5 cm length classes for 10 random stomachs within a length class to keep from being bias; however, all of the summer and fall stomachs were identified due to their small sample size. We calculated Wr for sturgeon among these seasons: 99.7, 99.6, 93.1 and 96.1 respectively.

Shovelnose sturgeon consumed a variety of benthic organisms (Table 1) and diet composition depended on season (Fig. 1). Comparing among seasons, percent composition by mass of prey items differed between winter and spring ([chi square] = 39.37, df = 4, P < 0.0001), where diets were dominated by Ephemeridae in winter and shifted to Hydropsychidae by spring. Diet also differed between winter (i.e., dominated by Ephemeridae) and summer (i.e., dominated by Hydropsychidae; [chi square] = 136.59, df = 3, P < 0.0001). Spring and summer diet composition by percent mass were dominated by Hydropsychidae (Fig. 1); however, during summer percent mass of Hydropsychidae increased significantly ([chi square] = 51.85, df = 4, P < 0.0001). Diet composition in fall differed from winter (i.e., [chi square] = 130.30, df = 4, P < 0.0001), summer ([chi square] = 140.24, df = 4, P < 0.0001) and spring ([chi square] = 123.48, df = 4, P < 0.0001) and was dominated by Corophiidae (Fig. 1).

The relative number of prey in diets differed within seasons. In winter, benthic organisms in diets varied widely (Fig. 1). Chironomidae was highest in percent number and Ephemeridae was highest in percent mass (Fig. 1). In spring organisms varied widely, but Hydropsychidae dominated in percent number and percent mass (Fig. 1). In summer, richness of benthic organisms in diets was low. Hydropsychidae comprised 90 percent of percent number and also percent mass (Fig. 1). In fall an exotic amphipod (Apocorophium lacustre; Corophiidae) dominated the percent number and also percent mass of diets (Fig. 1). This family is normally a saltwater, coastal inhabitant which smothers native invertebrates and then remains in the vicinity to compete for fine particulate organic matter (Ysebaert et al., 2000; Evans et al., 2004). Although documented upstream in the Missouri river, Ohio River, and in the upper Mississippi River (Grigorovich et al., 2008), this is the first documented finding of this species in the MMR.

River stage was highest during winter and spring, thus resulting in highest period of discharge (Fig. 2). This coincided with the high variety of benthic organisms identified (Fig. 1) and also high percent of shovelnose sturgeon stomachs containing prey (Fig. 2). River stage was at its lowest during the summer and fall months, which resulted in low discharge (Fig. 2). The low river stage coincided with the low variability of benthic organisms (Fig. 1) and low percent of shovelnose sturgeon stomachs containing prey items (Fig. 2). High temperatures occurred during summer, fall temperatures were similar to those in spring and lowest temperatures occurred during the winter (Fig. 2).

DISCUSSION

The shovelnose sturgeon has been described as an opportunistic feeder (Modde and Schmulbach, 1977; Berry, 2002; Wanner et al., 2007). In support of this, we found that a variety of larval benthic aquatic insects comprised shovelnose sturgeon diets in the free-flowing stretch of the MMR, although this depended on season. This dominance of aquatic insects in the diet of the shovelnose sturgeon has been previously reported for the Platte River (Shuman, 2003), upper Mississippi River (Hoopes, 1960; Helms, 1974; Carlson et al., 1985), and the Missouri River (Held, 1969; Modde and Schmulbach, 1977; Carlson et al., 1985; Megargle, 1997; Berry, 2002; Braaten et al., 2006; Wanner et al., 2007). Variability among seasons in diet composition likely depended on seasonal environmental conditions and patterns of invertebrate recruitment.

Seasonal invertebrate recruitment, invertebrate drift, and changes in invertebrate density are driven by the timing and discharge rates of the fiver and their interaction with spring warming (Modde and Schmulbach, 1977). Beginning in winter and also in the spring there was high discharge; this may have resulted in a higher percent of shovelnose sturgeon stomachs containing prey items. This high percentage of stomachs containing prey recorded shows that the shovelnose sturgeon are actively foraging. Foraging success may be high during this time because of catastrophic drift associated with high discharge (Beauchamp, 1932; Waters, 1961). Catastrophic drift occurs when organisms that normally do not disperse in flow are dislodged from the benthos and become available to predators. The high diversity of prey items found within the shovelnose sturgeon diets during the winter and spring suggests that they are opportunistic, consuming organisms that likely reflect their availability in the environment, particularly in the drift. Also contributing to taxon-rich diets and more stomachs containing prey in winter may have been because of reduced evacuation rates due to low temperature. Even though it is believed that lower temperatures may reduce consumption of mid-temperate fishes (i.e., Seaburg and Moyle, 1964; Molnar et al., 1967). Water temperatures during the spring months being around shovelnose sturgeon's maximum feeding efficiency at 21.78 C (Kappenman et al., 2009) could have attributed to the high percentage of stomachs containing prey items.

Continuing to support the concept that drifting organisms are important energetically to shovelnose sturgeon was the dietary shift to Hydropsychidae as flow declined and temperature increased during the summer months. At this time, obligatory drifting organisms, primarily Hydropsychidae larvae (Modde and Schmulbach, 1977), were likely most abundant in the flowing river as drift. However, the dietary shift to Hydropsychidae should be used in caution because of our small sample size (n = 6). The decline in the number of apparently foraging shovelnose sturgeon during summer may have been a combination of low prey availability coupled with reduced consumption rates, given that shovelnose sturgeon, like the Gulf of Mexico sturgeon, likely reduce feeding at high temperatures (Gu et al., 2001). Shovelnose sturgeon may also decrease consumption due to water temperatures exceeding their optimal temperature of 22.48 C (Kappenman et al., 2009). High temperatures during summer also may have decreased the percentage of stomachs containing prey items by increasing evacuation rates. Summer seems to be the most stressful period for the shovelnose sturgeon because of the low discharge rates causing low abundance of drift organisms and, therefore, causing them to exert more energy into obtaining prey. Most shovelnose sturgeon's annuli are laid down during the summer (Whiteman et al., 2004), illustrating that summer is a period of low to no growth and therefore a stressful period. This coincides with the drop in relative condition of our shovelnose sturgeon from spring to summer months and further represents summer as a stressful period.

Although fall temperatures were comparable to those in spring; the richness of prey items plus the number of apparently foraging individuals were low in the fall. However these patterns were more similar to that of the summer season and were likely due to low availability of drifting organisms due to low discharge. Also, reproduction and emergence of drifting insect taxa may have been low during this season. As such, the invasive amphipod was likely the most abundant organism in fall and likely reflects a switch from foraging on drift to the benthos. However there was a low sample size of shovelnose sturgeon containing prey items (n = 4), so use caution when using this for prey composition during this season.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

This study of seasonal diet composition has shown that discharge is a very important component of the shovelnose sturgeon's feeding habits. Apparently the shovelnose sturgeon consumes mainly drift organisms and has trouble obtaining prey when there is low discharge. High temperatures make summer a stressful season for shovelnose sturgeon, along with this, spring leading into and fall following summer are very important seasons for shovelnose sturgeon's survival. Conservation strategies for the shovelnose sturgeon especially, during the spring months in periods of low discharge, would be to create river habitat structures, to help dislodge drift organisms from the benthos making it easier to obtain prey organisms. Also future studies conducted on the main prey organism's habitat by season could provide a better understanding of the foraging ecology of shovelnose sturgeon. To this end, this study has provided a detailed portrayal of shovelnose sturgeon seasonal diet composition which provides another aspect into their complex life history. Using information gained in this study will provide insight into conservation efforts that need to be undertaken to ensure sustainability of the shovelnose sturgeon.

Acknowledgments.--Special thanks to Mike Hill for assistance in field collection of the shovelnose sturgeon. We would like to thank Nick Wahl, Dawn Sechler, Darcy Ernat and Heather Calkins for helping with identification of aquatic insects. Also we would like to thank Dr. Matt Whiles for identifying the exotic amphipod to species. Funding for this research was provided by the St. Louis District U.S. Army Corps of Engineers.

SUBMITTED 9 JUNE 2009

ACCEPTED 19 NOVEMBER 2010

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JUSTIN R. SEIBERT, QUINTON E. PHELPS, (1) SARA J. TRIPP AND JAMES E. GARVEY

Fisheries and Illinois Aquaculture Center, Department of Zoology, Southern Illinois University, Carbondale 62901

(1) Corresponding author: e-mail: qphelps@siu.edu
TABLE 1.--Total number of prey items in shovelnose sturgeon diets
(n = 123), % occurrence (%O), % number (%N) and % mass (%M)
throughout all seasons collected in the Middle Mississippi River
(MMR), Jan. 2005 to Nov. 2005

Prey item                     n         %O         %N         %M

Diptera
  Chironomidae                8867     90.70     31.01       4.21
  Ceratopogonidae              406     44.96      1.42       0.05
  Tipulidae                      8      3.10      0.03       0.04
  Dipteran pupae               236     35.66      0.83       0.12

Trichoptera
  Hydropsychidae            10,781     92.25     37.49      17.40

Ephemeroptera
  Ephemeridae                 5899     72.87     20.70      59.65
  Leptophlebiidae              103     25.58      0.36       0.27
  Heptageniidae                103     26.36      0.36       0.18
  Baetidae                      19      4.65      0.07       0.08

Coleoptera
  Amphizoidae                    5      3.10      0.02       0.01
  Elimidae                       6      3.88      0.02       0.08
  Dryopidae                      4      2.33      0.01       0.09
  Gyrinidae                      3      0.78      0.01       0.03
  Carabidae                      5      3.10      0.02       0.02
  Chrysomelidae                 19     10.85      0.07       0.26
  Halipidae                      3      1.55      0.01       0.001
  Dytiscidae                     2      1.55      0.01       0.02

Plecoptera
  Capniidae                     13      3.88      0.05       0.03
  Perlodidae                    92     31.01      0.32       0.56
  Perlidae                       7      4.65      0.02       0.07
  Chloroperlidae                 2      1.55      0.01       0.01
  Nemouridae                    10      1.55      0.04       0.03
  Peltoperlidae                  1      0.78      0.004      0.002
  Taeniopterygidae              52     18.60      0.18       0.17

Anostraca                      497      0.78      1.75       0.34

Odonata
  Gomphidae                     35     17.05      0.12       1.09
  Coenagrionidae                51     17.83      0.18       0.30

Amphipoda                       44     16.28      0.15       0.18
  Corophiidae
  Apocorophium lacustre        985      3.10      3.46       1.20

Mysida                           1      0.78      0.004      0.03

Megaloptera                      7      3.88      0.02       0.09

Decapoda
  Palamonidae                   45     10.08      0.16       0.31

Hymenoptera                      2      1.55      0.007      0.15

Hyplotaxida                      1      5.43      1.06      12.41

Hiodontiforms
  Hiodontidae                  302      0.78      0.004      0.41

Other                           10      5.43      0.04       0.12
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Author:Seibert, Justin R.; Phelps, Quinton E.; Tripp, Sara J.; Garvey, James E.
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1U1MA
Date:Apr 1, 2011
Words:3893
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