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Effects of Static versus Flowing Water on Aquatic Plant Preferences of Triploid Grass Carp a

a

Robert T. Pine , Lars W. J. Anderson & Silas S. O. Hung

b

a

U.S. Department of Agriculture , Agricultural Research Service, Aquatic Weed Control Research Laboratory, University of California , Davis, California, 95616, USA b

Department of Animal Science , University of California , Davis, USA Published online: 09 Jan 2011.

To cite this article: Robert T. Pine , Lars W. J. Anderson & Silas S. O. Hung (1989) Effects of Static versus Flowing Water on Aquatic Plant Preferences of Triploid Grass Carp, Transactions of the American Fisheries Society, 118:3, 336-344, DOI: 10.1577/1548-8659(1989)1182.3.CO;2 To link to this article: http:// dx.doi.org/10.1577/1548-8659(1989)1182.3.CO;2

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Transactions of the American Fisheries Society 118:336-344. 1989

Effects of Static versus Flowing Water on Aquatic Plant Preferences of Triploid Grass Carp ROBERT T. PINE AND LARS W. J. ANDERSON U.S. Department of Agriculture, Agricultural Research Service Aquatic Weed Control Research Laboratory. University of California Davis. California 95616. USA

SILAS S. O. HUNG Downloaded by [University of California Davis] at 10:39 19 February 2015

Department of Animal Science. University of California. Davis Abstract.— Triploid grass carp Ctenopharyngodon idella were presented with three aquatic plant species (sago pondweed Potamogeton pectinatus. Eurasian watermilfoil Myriophyllum spicatum. and American pondweed P. nodosus) in outdoor canals with static and flowing water in winter, spring, and summer. Plant consumption by triploid grass carp in winter was low but increased dramatically in spring and summer. Based on plant shoot lengths, triploid grass carp preferences in spring for static water were sago pondweed = American pondweed, American pondweed = Eurasian watermilfoil, sago pondweed > Eurasian watermilfoil; for flowing water, sago pondweed = Eurasian watermilfoil > American pondweed. Summer preferences for static water were sago pondweed = Eurasian watermilfoil = American pondweed; for flowing water, sago pondweed = Eurasian watermilfoil = American pondweed. Plants of all three species produced longer shoots in canals with flowing water than with static water. The differences in shoot length may have altered the triploid grass carp's consumption rate and preference. Flowing conditions also had varying effects on nutritional content of plants, as shown in proximate analyses of dry matter and percent of fat, ash, protein, crude fiber, nitrogen free extract, and acid detergent fiber. Ash content was consistently higher in plants of all three species from canals with flowing water. This may reflect a morphological response to flow by the plants. None of the variables of the proximate analysis of plants correlated statistically with preference. This suggests that accessibility and ease of mastication were more important in determining preference than nutritional quality of the plants. Algae consumption by triploid grass carp, however, made it difficult to discern correlations between nutritional factors and vascular plant consumption. There is considerable interest in the ability of triploid grass carp Ctenopharyngodon idella to remove undesirable aquatic macrophytes from farm pond reservoirs and irrigation ditches in northern California. Since introduction of diploid grass carp into the USA in 1963, there have been studies to determine their efficacy in eliminating certain aquatic plants and to assess their preferences for these plants in canals with static water (Avault 1965;Michewiczetal. 1972; Edwards 1974; Fowler and Robson 1978; Haller and Sutton 1978; Wiley and Gorden 1984; Leslie et al. 1987) and flowing water (Sutton and Vandiver 1986; Sutton et al. 1986). Manipulation of flow rates (and volume) in canal systems is a common management practice used by irrigation districts in the western USA. Furthermore, plant emergence is influenced by flow, which can change plant accessibility to grass carp. Therefore, one of the objectives of our study was to determine whether triploid grass carp change their preferences for and consumption rates of submersed aquatic plants in canals with static

versus flowing water. Sago pondweed Potamogeton pectinatus. Eurasian watermilfoil Myriophyllum spicatum, and American pondweed P. nodosus were selected because they are common in irrigation ditches and farm pond reservoirs in northern California. Several studies have been conducted to relate important nutritional variables of aquatic plants to the most effective stocking rates and age-classes of diploid grass carp for solving aquatic plant problems (Fischer 1970; Stott and Orr 1970; Tan 1971; Fischer 1972a, 1972b; Stanley 1974a, 1974b; Wiley and Wike 1986). Temperature is the most important factor that affects this species' preference and rate of consumption of aquatic plants, Stroganov (1963) found that, in winter, diploid grass carp engage in limited, irregular plant consumption and prefer aquatic plants over terrestrial plants. He noted that above 16°C, feeding becomes more intense and less selective, The second objective of our study was to examine the potential relationship of the proximate composition and acid detergent fiber content of

336

FEEDING BY TRIPLOID GRASS CARP


sago pond weed, Eurasian watermilfoil, and American pondweed to the preference and consumption rate of triploid grass carp in canals with static versus flowing water at different water temperatures. We determined the proximate composition of the fish to gain some information on their nutrient utilization. Here we present evidence that plant proximate composition was altered by static and flowing conditions but that triploid grass carp preference had no correlation with the proximate analysis variables of the aquatic plants we studied. Proximate composition of triploid grass carp carcasses and viscera was the same for canals with static and flowing water. Methods Triploid grass carp were brought to the Aquatic Weed Control Research Laboratory in Davis, California, in November 1985. The fish had been spawned at the Leon Hill Fish Hatchery in Lonoke, Arkansas, where heat shock was applied to newly fertilized eggs to produce triploid fish (Thompson et al. 1987). A Coulter counter was used by the hatchery to certify that all fish that were shipped were triploid (Wattendorf 1986). At Davis, triploidy of fish was reverified by flow cytometry with a modification of a technique developed by Alien and Stanley (1983). We used one 10,000-L fiberglass pool to hold the fish before starting experimentation. The pool was supplied with water at a flow rate to provide a 24-h turnover. It was also aerated and covered with shade cloth. Water temperatures in the pool varied from 15 to 23°C. Fish were fed ad libitum with an equal (by weight) mix of the three plants under study plus lettuce Lactuca salva for at least 1 month before the start of each trial. We used a new group of 12 fish in each of three studies (winter, spring, summer). The triploid grass carp were 3 years old and weighed 350-550 g. We conducted the study in four parallel canals that were 122 m long x 3 m wide (at the top), concrete lined, trapezoidal in cross section, and filled with well water. Two canals had static water and two had flowing water, and each canal was divided across the middle to make two replicate sections, one upstream and one downstream. A trammel net was suspended over each canal to protect fish from piscivorous birds. We controlled flow velocity in the canals by a gate valve at the upstream end; velocity was 0.67 m-s' 1 upstream and 0.25 nvs~' downstream. We repeated studies in winter, spring, and summer so that the effect of temperature on consumption rate could be stud-

337

ied. Although water temperature steadily increased from winter to summer in both systems, canals with static water were about 5°C colder than canals with flowing water in winter and 3° colder in spring and summer (Figure 1). Preference study.—We planted eight containers each of sago pondweed, Eurasian watermilfoil, and American pondweed in 44 cm x 34 cm x 5 cmdeep plastic containers, placed them in random species order in a canal section, and allowed the plants to acclimate to their respective experimental conditions (static or flowing water) for 1 month. Total number of containers used was 4 canals x 2 sections x 3 plant species x 8 containers = 192, and the amount of soil (Yolo clayey loam) in each container was equalized as much as possible. Containers were placed about 1 m apart to reduce light and nutrient competition. Numbers of plant species that could be studied concurrently were limited due to the high number of replicated experimental units needed for statistical comparisons. After 1 month, immediately before being stocked with fish, canals were partially drained, and a random sample of 25 plants in each container was measured in situ for shoot length and total plant count. These two characteristics had been determined to be the best nondestructive indices of biomass (Pine et al. 1989). Also, at the beginning of each study, three containers of each plant species were removed from each canal for measurements of shoot length, total plant count, and fresh and dry weight. Data then were used to estimate biomass of each plant species in each canal section. Biomass was equalized among sections by removing containers of plants before the introduction of the triploid grass carp into the treatment sections. At the start of each trial, 12 triploid grass carp were anesthetized with quinaldine sulfate at 10 mg/L for 15-30 min (lower water temperature and larger fish size required more time for immobilization) and measured for wet weight. Three fish were stocked into either upstream or downstream canal sections in each of the four canals; the other sections served as unstocked controls. A 180-d, 5-35°C recording thermograph (Peabody-Ryan Instruments) was placed on the bottom of a canal with static water; another thermograph was placed in a canal with flowing water. In the winter study, a 5-35°C minimum-maximum thermometer (Taylor Instruments) was used in the static water but was replaced with a recording thermograph in the spring and summer study. The following water quality data were determined at 1200 hours, be-

338

PINE ET AL. 30

STATIC o

15

WINTER

SPRING

SUMMER

SPRING

SUMMER

30

FLOVWG


015

WINTER

FIGURE 1.—Mean daily temperature of static and flowing water in winter 1986 and spring and summer 1987. Error bars show the range of six values taken at 0200, 0600, 1000, 1400, 1800, and 2200 hours; winter values for static water represent the means of minimum and maximum temperatures.

fore and during the experiments: total alkalinity, total hardness, conductivity, pH, dissolved oxygen, and turbidity (Table 1). A Hach kit was used to measure total alkalinity and total hardness, a Hydrolab System 8000 was used to measure conductivity, pH, and dissolved oxygen, and an HF Instruments Model DRT 15 was used for turbidity measurements. We ended experiments when visual inspection indicated the near elimination of one of the plant species. The winter trial lasted 2 months, whereas the spring and summer trials lasted 1 week each. At the end of each trial, canals were partially drained. Fish then were recaptured, anesthetized as before, and remeasured for wet weight. All plants in each container from both treatment and control sections were cut at the soil surface and measured for mean shoot length, plant number, and total fresh and dry weight. These values were used to estimate total amount of plant material consumed as a percent of plants in the unstocked control sections. Initial control values were subtracted from final values to determine plant growth and mortality during the experiment. Treatment values were then corrected by use of this information. Treatment values were compared to control values that had been averaged separately over canals with static and flowing water. Algae were separated from plants, and fresh weight and dry weight were determined to estimate algal biomass. We selected the spring experiment for analysis of nutritional variables. We determined the triploid grass carp's preference among the three different plant species by comparisons of shoot length

relative to control shoot length, gross energy consumed, and protein consumed. Proximate composition and acid detergent fiber of plants.—Algae Cladophora sp. made such a large contribution to the biomass within each canal that it was treated as a fourth plant species presented to triploid grass carp in the trials. The trace amounts of algae introduced with the plants during the initial experiment increased substantially in the canal environment. All analyses done for comparison of the original three plant species were also perTABLE 1.—Water quality during winter 1986 and spring and summer 1987 experiments in canals with static versus flowing water conditions. Samples were taken from canal 3 (static water) and canal 1 (flowing water). NTU = nephelometric turbidity units. Winter Variable

Nov25 Dec 17

Total alkalinity (mg/L) Total hardness (mg/L)

Static water 230.0 232.0 216.0 214.0 500.0 496.0

Conductivity (S/cm) Dissolved oxygen (mg/L) PH Turbidity (NTU)

Total alkalinity (mg/L) Total hardness (mg/L) Conductivity (S/cm) Dissolved oxygen (mg/L) pH Turbidity (NTU)

Spring Summer

May 31 Jul23 220.0 198.0 479.0

228.0 194.0 485.0

13.6 7.0 0.6

12.3 9.0 1.5

13.6 10.2 1.2

Flowing water 226.0 234.0 208.0 222.0 481.0 490.0

222.0 204.0 498.0

222.0 206.0 483.0

9.2 8.8 1.5

10.2 9.3 1.5

13.0 7.0 0.4

12.6 7.4 2.6

15.3 7.1 1.3


FEEDING BY TRIPLO1D GRASS CARP

formed on algae for both static and flowing conditions. Dried plants from control sections in the previous study were ground in a Wiley mill and placed in plastic bags. At least 500 g of dried, ground material of each plant species for each treatment were composited into one sample. Gross plant energy was measured with a Gallenkampf bomb calorimeter, proximate analysis by the AOAC (1984) method, and acid detergent fiber by the method of Van Soest and Wine (1967). Acid detergent fiber is undigestible lignocellulose plus ash and is measured because crude fiber analysis includes cellulose and hemicellulose, which are partially digestible and make the analysis less reproducible. Proximate analysis variables determined were: percent dry matter, protein, fat, crude fiber, ash, and nitrogen-free extract. Nitrogen-free extract represents the digestible carbohydrate in a sample and is determined by subtracting values for percent protein, fat, crude fiber, and ash from 100. Duplicate analyses were run on each sample. If the difference between the two results was greater than 1%, a third analysis was performed, and the closest two results were used to calculate the sample mean. Fish proximate composition. —Standard AOAC (1984) methods were used to determine the proximate composition of the fish. After the spring trials, we killed two fish from each treatment section with an overdose of quinaldine sulfate. The viscera and carcasses were separated and stored frozen in plastic bags for 2 months until analyses could be performed. Just before analysis, these samples were cut into small sections, freeze-dried, and then ground to a fine powder in a Wiley mill. Two subsamples were taken from both the ground visceral and carcass material of each fish and analyzed for percent ash, protein, and fat. Statistical tests used were analysis of variance and Duncan's multiple-range test. The significance level was set at P = 0.05. Results

Preference Study Due to the low water temperature, we observed low consumption of all three plant species in winter, and there were no significant differences in the fishes' preference for plants between canals with static water and those with flowing water (P > 0.05). As water temperature increased in spring, fish consumed a significantly higher amount of sago and American pondweeds in static water than

339

in flowing water. However, consumption of Eurasian watermilfoil and American pondweed was significantly higher in canals with static rather than flowing water in summer (Figure 2). Shoot lengths of plants tended to be shorter in canals with static than in flowing-water canals during all three experiments (Table 2). Mean shoot lengths were used to calculate the percentages shown in Figure 2 (100 x treatment shoot length/ control shoot length). Lower percentages indicated greater preferences by the grass carp. In the spring experiment, preferences in canals with static water were sago pondweed > American pondweed, American pondweed > Eurasian watermilfoil, sago pondweed > Eurasian watermilfoil; in canals with flowing water, they were sago pondweed = Eurasian watermilfoil > American pondweed (Figure 2). Fish gained significantly less weight in winter and spring than in summer in flowing water canals (Figure 3). No significant difference in weight gain was observed between canals with static water and canals with flowing water in winter, but a significant difference in weight gain was observed in the spring and summer experiments between canals with static and flowing water. Weight gain in static water was greater in winter than in spring and in flowing water was greater in spring than in winter. Water quality in canals was similar for both static and flowing water in all three experiments, although turbidity varied somewhat (Table 1). Changes in water quality occurred in both systems simultaneously with change of season. Algae grew significantly faster in flowing water than in static water during winter and summer trials (Figure 4). Algae may have interfered with vascular plant growth in static canals more in summer than in spring because control plants of all species were consistently shorter in summer (Table 2).

Proximate Composition and Acid Detergent Fiber of Plants Consumption of plant material by triploid grass carp, in terms of fresh weight and gross energy, varied for each of the four plant species under consideration in static and flowing water conditions (Table 3). However, the total amount of plant fresh weight and gross energy consumed in both types of canals was very similar. Preference based on gross energy of individual plants in the spring trial was Eurasian watermilfoil = American pondweed = sago pondweed in static water and American pondweed = sago pondweed > Eurasian watermilfoil in flowing water (P < 0.05).

PINE ET AL.

340

^Eurasian watermilfoil


-American pondweed

STATIC

FLOWING

STATIC

FLOWING

STATIC

FLOWING

WINTER

SPRING

SUMMER

EXPERIMENT

EXPERIMENT

EXPERIMENT

FIGURE 2.—Comparison of triploid grass carp preference and consumption rate of three plant species. The variable of comparison is the mean plant shoot length from canal sections containing fish as a percent of mean plant shoot lengths from control canal sections without fish. This was calculated for each plant species within each type of system (static and flowing water). If a bar is replaced by a zero, all plants were consumed. Each bar represents the mean of 10 values. Letters above bars represent within-experiment comparisons based on Duncan's multiple-range test of arcsine-transformed data. Bars with similar letters within a season are not significantly different (P > 0.05) from each other.

Proximate composition and acid detergent fiber of the four plant species were different between canals with static water and canals with flowing water, and the greatest differences between static and flowing systems were among percent dry matter, nitrogen-free extract, ash, and acid detergent fiber (Table 4). Preference for individual plants based on protein levels, obtained by multiplying percent protein by dry matter consumed for each species and dividing by 100, in the spring trial was Eurasian watermilfoil > sago pondweed = American pondweed in static water; sago pondweed = American pondweed > Eurasian watermilfoil in flowing water (P < 0.05).

Fish Proximate Composition Proximate composition of dry carcasses and viscera of triploid grass carp in the spring experiment was similar between static and flowing water canals. The percentages (mean ± SD) for carcasses and viscera, respectively, were: protein, 68.7 ± 1.4 and 55.8 ± 1.2; lipid, 11.9 ± 0.2 and 30.1 ± 0.6; and ash, 16.4 ± 0.4 and 5.4 ± 0.0. Discussion

Preference Study In our canals with static water, a thermocline may have existed with colder water toward the

TABLE 2. —Mean (±SD) shoot length measurements (cm) of sago pondweed, Eurasian watermilfoil, and American pondweed in winter 1986 and spring and summer 1987 in static and flowing water conditions. Measurements were taken from plants in canal control sections having no fish and treatment sections having fish at the end of the study; N= 10. Winter

Plant

Control

Treatment

Summer

Spring Control

Treatment

Control

Treatment

0.0 38.7±18.3 I3.5±19.2

12.9±2.3 46.9±0.4 20.4±13.7

0.0 4.4±l.l 0.0

15.9±10.4 41.7±22.1 79.8±26.7

22.8±4.5 84.8±5.5 75.8±13.5

10.1±I0.1 71.4±I2.3 66.7±7.57

Sago pondweed Eurasian watermilfoil American pondweed

7.2±2.9 I4.1±2.7 9.4 ±2.8

Static water 12.8±8.2 24.3±0.1 16.1±4.0 53.4±8.8 11.1 ±3.0 55.9±3.0

Sago pondweed Eurasian watermilfoil American pondweed

lt.7±2.5 20.5±4.5 18.9±3.2

16.3±5.0 23.7±5.5 12.3±3.8

Flowing water 24.8±14.0 51.0±0.8 59.9±3.1

341


WINTER


WINTER

SPRING

SUMMER

FIGURE 3.—Percent body weight gain of triple id grass carp in static versus flowing water in winter, spring, and summer. Values were calculated by dividing the final mean fish body weight minus the initial mean fish body weight by the initial mean fish body weight for canals with static and flowing water, and multiplying by 100. Each bar represents the mean of six values. Thin vertical values represent SDs. Letters above bars represent within-experiment comparisons based on Duncan's multiple-range test of arcsine-transformed data. Bars with similar letters within a season are not significantly different (P > 0.05) from each other. Initial body weights (±SD) of fish were: 348 ± 56, 453 ± 38, and 392 ± 94 in static water conditions; 407 ± 47, 446 ± 28, and 475 ± 133 in flowing water conditions in winter, spring, and summer experiments, respectively.

SPRING

SUMMER

FIGURE 4.—Average fresh weights (thin lines are SDs) of algae in control sections (without fish) of canals with either static or flowing water in winter, spring, and summer. Letters above bars represent within-experiment comparisons based on Duncan's multiple-range test. Bars with similar letters within a season are not significantly different (P > 0.05) from each other. Algae were removed from plants and soil in each container after each experiment ended and weighed after blotting. N = 30.

scribed in relation to the way herbivory can benefit plants (Belsky 1986). Each plant species may have a different rate of overcompensation, and further research needs to be done to clarify effects when plant preference is determined. When consumption rate is high, as in the spring and summer trials, overcompensation does not seem to be as bottom; canals with flowing water may have had important in affecting preference. more homogeneous temperatures because of mixIn spring and summer trials, when water teming within. Diploid grass carp cease to eat at water peratures were higher, more plants were contemperatures below 13°C (Stroganov 1963). sumed by triploid grass carp in canals with static Therefore, it is not surprising that triploid grass water than in flowing-water canals, and the fish's carp also consumed little food in either system in plant preferences appeared to change. Under static winter. Except for American pondweed, plants ac- conditions in spring and summer trials, sago tually seem to have grown faster than their rate of pondweed and American pondweed were preconsumption by triploid grass carp during the ferred over Eurasian watermilfoil. Although winter trials (Table 2), possibly because grazing by American pondweed seemed to be the most pretriploid grass carp may stimulate faster plant ferred species in flowing water in winter, it was growth. This effect caused by herbivore grazing is the least preferred in spring and summer trials. known as "overcompensation" and has been deControl of temperature could be used to manage TABLE 3.—Total fresh weight and gross energy of four species of plants consumed by triploid grass carp during spring 1987 in static and flowing water conditions. Values were corrected for plant growth during the trial by subtracting the difference between the initial and final control values from the treatment value. Values represent the mean of two replicates (±SD). Fresh weight consumed (g)

Energy consumed (MJ)

Plant

Static

Flowing

Static

Flowing

Sago pondweed Eurasian watermilfoil American pondweed Algae Total

440.5 ±128.1 588.3± 195.2 576.6±354.5 835.7 ±208.3 2,441.1

919.0±9.5 300.5 ±326.0 993.1±217.3 76.3±35.2 2,288.9

0.6±0.2 1.2±0.3 0.7±0.4 1.2±0.2 3.7

1.0±0.2 0.7±0.5 1.1±0.2 0.4 ±0.0 3.2

342

PINE ET AL.

TABLE 4.—Proximate analysis and acid detergent fiber analysis (Van Soest and Wine 1967) of control plants from canals with static and flowing water in spring. Each value represents the mean of two replicates (±SD). NFE = nitrogen-free extract; ADF = acid detergent fiber. Water condition

Percentage of dry matter as

Dry matter (%)

Fat

Ash

Static Rowing

8.7±0.1 9.7±0.2

1.4±0.6 2.2±0.1

36.2±3.2 45.3±28

Static Flowing

20.1 ±13.4 7.6±0.1

Static Flowing Static Rowing

Protein

Crude fiber

NFE

ADF

16.8±0.4 12.3±0.5

8.7±0.6 6.6±0.6

36.9±1.3 33.6±0.8

41.5±3.4 56.3±3.7

1.6±0.1 1.7±0.1

Eurasian watermilfoil 20.9±1.6 20.9±0.3 29.1±2.3 22.7±1.3

7.3±0.7 7.6±0.3

49.3±2.5 38.9±2.3

40.2 ±3.8 50.3±1.5

6.8±0.0 7.8±0.2

1.6±0.1 1.4±0.2

American pondweed 39.8±2.2 15.4±0.2 50.6±2.7 13.7±0.7

7.6±0.2 5.3±0.4

35.6±1.7 29.0±0.7

41.9±4.0 48.7 ±4.1

11.2±0.1 7.8±0.4

1.3±0.1 2.8±0.1

44.2±4.7 44.3±2.0

12.7±0.2 13.8±0.1

28.1 ±1.2 21.3±0.6

40.9 ±2.6 50.0±1.6


Sago pondweed

Algae

triploid grass carp plant consumption in irrigation canals. Release of water from the bottom of a storage reservoir would bring cold water into a canal to lower rates of consumption when desired. Slower feeding offish in a canal full of sago pondweed may maintain plant populations in the presence of both triploid grass carp and waterfowl. On the other hand, American pondweed is less important as food for waterfowl, and higher temperature water might be brought into a canal infested with American pondweed to eliminate the plants. None of the preference studies done by Avault (1965) with static systems included American pondweed, although both sago pondweed and Eurasian watermilfoil were used. These studies showed that sago pondweed always was preferred over Eurasian watermilfoil by diploid grass carp, and our spring study confirms these findings but with triploid grass carp (Figure 2). Plants in canals with flowing water were more vigorous than plants in canals with static water. Accessibility of an aquatic plant is usually determined by whether the plant reaches the surface. For instance, the lower stem of Eurasian watermilfoil is tough, fibrous, and normally rejected by the fish, whereas the tender, upper, new growth is readily eaten. Presentation of these plants under flowing conditions keeps new growth accessible to fish by submerging the plant. Sago pondweed in canals with static water, however, did not reach the surface, thus making it more accessible to fish and it was completely eliminated in spring and summer trials. One explanation for increased consumption of plants in canals with static water might be that the nutritional value of these plants was lower so

13.7±1.7 17.8±0.5

that triploid grass carp had to consume more food. The most preferred plant, sago pondweed, had a higher moisture level and lower lipid content so that it was less nutritious to triploid grass carp. Because fish in canals with flowing water had to expend more energy to maintain themselves against the current, one might have expected more plant biomass to have been consumed under these conditions. However, the more vigorous sago pondweed plants in flowing water contained more energy in the form of lipid and lower water content than those in canals with static water, thus enabling fish to consume less plant material while increasing net energy intake. Proximate Composition and Acid Detergent Fiber of Plants Proximate composition of the three aquatic plants used in our experiment was similar to data of Venkatesh and Shetty (1978). There were some major differences in the proximate composition of plants between canals with static water and canals with flowing water, and these differences possibly were correlated with morphological forms that plants developed as a response to either static or flowing water conditions. Acid detergent fiber is the undigestible lignocellulose of the plant and along with ash, normally is associated with cell wall material. In spring, ash content and acid detergent fiber were higher in plants from canals with flowing water than in plants from canals with static water. Acid detergent fiber was unusually high relative to ash content in Eurasian watermilfoil in canals with either static or flowing water. Because plants in a flowing system need to withstand the



fragmenting effects of a current, it is reasonable to conclude that they would produce more lignocellulose and incorporate more minerals to support their stems against shear stresses in flowing water. However, plants might be expected to have a higher moisture and protein content if they were in static conditions where stem strength was not as important. We found no correlation between energy or protein content of plants and plant preference based on shoot length. Cladophora algae were an important source of energy in static water, having more energy per gram than the other plants. This may help to explain the lack of correlation between plant preference and energy or protein. However, protein levels of algae were lower than those of vascular species in both systems. Eurasian watermilfoil had the highest protein and gross energy levels of the vascular plants studied, possibly because of its thick and fibrous stem, which would not be easy for fish to chew. The feathery leaves, which had a high surface area, also tended to collect more algae compared to sago or American pondweeds. Because of the small size of the triploid grass carp used in these trials, the large surface of the Eurasian watermilfoil and the associated algal mat may have been less accessible to grazing offish, thus explaining the lack of preference for this plant. Because sago pondweed was the most preferred of the three vascular species in both systems, handling time (i.e., accessibility of the plant to the fish and ease with which fish chew the plant material) may be the most important criterion for determining preference. Algae are both soft and accessible to fish and would have been consumed more than vascular plants if the high ash content did not interfere with the overall nutrition of the fish. Fish Proximate Composition Results of proximate analyses of eight sampled triploid grass carp were similar to a previous study (Tan 1971) in which fat was found to accumulate in viscera rather than muscles. However, Tan (1971) found that there was an inverse relationship between protein and fat content in diploid grass carp, whereas, in our study with triploid grass carp, an inverse relationship between protein and fat did not occur. Acknowledgments We thank Serge Doroshov and Joseph Cech for their critical reviews of this paper, the students and staff of the U.S. Department of Agriculture

343

Aquatic Weed Control Research Laboratory who helped when experiments were so large that extra hands were essential, and Paul Lutes for assisting with preparation of grass carp for proximate analysis. We also thank the Marin Rod and Gun Club for a stipend that it awarded in support of this research.

References Alien, S. K., Jr., and J. G. Stanley. 1983. Ploidy of hybrid grass carp x bighead carp determined by flow cytometry. Transactions of the American Fisheries Society 112:431-435. AOAC (Association of Official Analytical Chemists). 1984. Official methods of analysis, 14th edition. AOAC, Arlington, Virginia. Avault, J. W. 1965. Preliminary studies with grass carp for aquatic weed control. Progressive Fish-Culturist 27:207-208. Belsky, A. J. 1986. Does herbivory benefit plants? A review of the evidence. American Naturalist 127: 870-892.

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