Sep 26, 1973 - of Biology, Ithaca College, Ithaca, New York. 14850 ... use or increase it by eating small prey gen- ... To do this, it was necessary first to de-.
Omnivorous
zooplankton
and planktivorous
fish
John L. Confer and Pamela I. Blades1 Department
of Biology,
Ithaca
College,
Ithaca,
New
York
14850
The distance at which Lepomis gibbosus reacts to zooplankton is shown to be a linear fuilction of prey size. If fishes sweep out a cylindrical path as they forage the frequency of prey encountered is then proportional to the square of this reaction distance. With the inclusion of the probability of capture success after a fish pursues a prey, a model of prey selection can estimate the effect of predacious zooplankton on the energy ingestion of planktivorous fish. Since predacious zooplankton are much larger and more visible to the fish than their own prey, this more than compensates for the energy lost by the additional trophic level and the modcratc ability of predacious zooplankton to elude capture.
Carnivorous and omnivorous zooplankton are abundant. By their predation they alter the abundance of other zooplankton in a lake (Confer 1971; Dodson 1974), thus ahering the food supply available to planktivorous fish. Predacious zooplankton niay decrease the food supply of fish by eating prey that the fish might otherwise use or increase it by eating small prey gencrally ignored by the fish and, in turn, being eaten by the fish. We here estimate the effect of predacious zooplankton on the energy gain of planktivorous fish. To do this, it was necessary first to develop a model of the rekitive capture frequency of prey by planktivorous fish. Basic elements of this model are the distances at which different species and sizes of zooplankton elicit fish pursuit and the success of capture after pursuit is started. Lepomis gibbosus, the pumpkinseed, was used to determine the reaction distances and capture success with various zooplankton. Capture frequency may also be influenced if fish show selectivity by ignoring one type of prey or choosing one type when two are in view simultaneously. Ivlev (1961) and Werner and Hall (1974) have shown that selectivity decreases with deercasing prey densities. Our model predicts the relative capture frequency of prey by fish at low plankton densities when selectivity is minimal. Capture frcquencics predicted by the model are compared to 1 Present address : Department of Zoology, versity of New Hampshire, Durham. LIMNOLOGY
AND
OCEANOGRAPHY
Uni-
actual field analysts to determine the degree of concordance. We are indebted to F. H. Rigler and both referees for their patience in reviewing this manuscript a number of times. C. S. Holling and F. E-1.Rigler have contributed to the design of our study in ways that are significant but not easily included as a citation. Methodi Reaction distances were measured at room temperature with several fish starved for 24 h. A long, narrow aquarium, 200 X 10 x 6 cm, was used for all observations. A pair of fluorescent Gro-Lux lights, 100 cm long, was suspended 10 cm above the aquarium which was filled with dechlorinated tapwater. Pumpkinseeds display a reasonably distinct behavior when prey is sighted. A cruising fish will normally stop, aim directly at the prey, erect the dorsal fin more than normally, and then rapidly swim directly up to the prey. This swimming distancc was judged against a grid and called the reaction distance (RD). About a third of our observations were discarded because the point at which the fish reacted could not be clearly distinguished. Some observations were also discarded when the prey sank to the bottom and remained there motionlcss, or when the prey became trapped on the surface film. Different sizes or species of prey or both were alternated during any measurement period. Several fish, 2 to 5 cm long, were used during the study.
571
JULY
1975,
V.
20 (4)
572
Confer and Blades
Daphnia were measured from the top of the helmet to the base of the shell spine, copepods from the top of the head to the distal tip of the rami, all to 0.05 mm under 50X magnification. Capture success (CS ) is the ratio of the number of prey ingested to the number of prey pursued. When Lepomis was fed Daphnia for several days and then copcpods, the initial CS for copepods was low. AI1 reported values were compiled after Lepomis had been allowed to practice for at least a day. Diaptomus siciZis kept in the laboratory for more than a week appeared more sluggish than freshly caught spccimens. Capture success, determined with the same fish on the same day by alternating the prey, was significantly less for the freshly caught prey (x2 2 x 2). Consequently, all the values used for the CS of copepods were determined within 36 h of collection of the plankton. Our model includes the size of predacious zooplankton and the size of their prey. To add to published values of these size ratios, we measured predation by Episc7Wra lacustris on various instars of D. sic&, both collected from Fayetteville Green Lake, New York. Epischura could often bc seen eating prey in the concentrated samples. To duplicate the presumed in situ hunger levels Episclzura was isolated and starved for 24 h before being tested. One Epischura and four Diaptomus were added to each of 20 aquaria for each experiment. Surviving prey were counted at 24, 48, and 72 h and dead and eaten prey replaced at each counting. The predation rate from 24 to 48 h was about twice as high as during the other intervals, suggesting that experimental conditions influenced the absolute rates. Consequently, the data are used only to determine relative predation rates on different sizes of Prey * Results Capture success-The CS for L. gihbosus preying on several species of Daphnia w:\s nearly 100% and will be assumed to be 100%. The CS of copepods varied daily,
55 Correlation
50
coefficient
= 0.50
45
40 E “, 2 c f .L3
l
Dioptomus
leptopus, ad,
35
30
Mesocyclops
edax,
l
/
Diaptomus
sicilis,
ad.
/ Diaptomus
l
I
I
.5
IO
I
I.5
Zooplankton
sicilis,
C It
1
I
I
I
2.0
2.5
3.0
3.5
Size, mm
Fig. 1. The reaction distance of Lepomis sibboszls for different sizes and species of copepods.
apparently influenced by rapid fish learning and forgetting. Some species of copepod are highly successful at eluding capture. A CS of 79% for D. sic& compared to a CS of 39% for Diaptomus ashlandi, determined by alternating prey on the same date with the same fish, is significantly diffcrent (x2 2 x 2). No single value for the CS of copepods is entirely satisfactory. As an approximation of the in situ CS of copepods the average for six dates, 80%, will be used. 1 shows L. Reaction distance -Figure gibbosus RD for copepods and Fig. 2 shows the RD for Daphnia magna and Daphnia pulex. For copepods the fitted line is the least squares linear regression of prey size vs. the mean RD for a species-size category. Mean values were chosen rather than individual values because the large number of observations for Mesocyclops edax would bias the regression line so that it misreprescnts the general trend for copepods. For Daphnia each plotted point is the mean of
Plankton
573
predation
Table 1. Linear regression statistics for fish reaction distance on prey size. Daphnia magna data used for regression Eq. 1; Daphnia pulex, Eq. 2; four species of copepods using each observation individually, Eq. 3; four species of copepods using mean values for each species-size category, Eq. 4. N is the number of observations; a is the intercept in centimeters; b is the slope; S,, is the standard deviation of slope; cc. is the correlation coefficient.
0 .z $ ITz
15-
I
0.4 I
IO 5 1 I
.5
I
I
1.0
1.5
Zooplankton
I
I
I
1
2.0
2.5
3.0
3.5
Size, mm
Fig. 2. The reaction distance of Lepomis gihbosus for different sizes of D. magna and D. pulex.
at least ten observations, and the line is the least squares regression for individual observations. Regression statistics are in Table 1. When each observation is used individually the correlation coefficient for RD vs. copepod size is 0.50 (131 observations), 0.60 for D. magnu (168 observations), and 0.41 for D. pulex (79 observations ) . Individual values are variable, but the mean RD for a species-size category fits the respective regression lines fairly well. Supporting this is a linear correlation coefficient for the plotted D. magna mean values of 0.98. The RD for M. edax is considerably above the regression line for all copepod species. Since this value was based on 35 observations, we believe the deviation is real. Mesocyclops edax was certainly one of the most, if not the most, active prey we used. The reaction of L. gihbosus frcquently appeared to be triggered by prey movement and M. edax could readily be
Equation
N
a
b
1
168
6.4
152
15.9
0.60
c. c.
2
79
2.6
100
27.4
0.41
3
131
0.8
162
24.8
0.50
4
5
-6.6
197
57.1
0.87
seen because of its vigorous and frequent swimming movements. Because D. pulex is more translucent than D. magna, we expected it to be less visible and consequently to elicit shorter RD than D. magna of the same size. We tested this hypothesis for three size categories, selected on the basis of largest sample size: 0.9 mm long, 9 D, magna and 11 D. pulex; 1.7 mm, 6 and 9; 1.9 mm, 12 and 7. The diffcrcnce between the mean RD (“Student’s” t-test) was significant at the 95% confidence level for 0.9-mm Daphnia and at the 99% level for both 1.7- and 1.9mm Duphnia. The large difference in RD (Fig. 2) suggests that transparency gives a strong advantage to zooplankton exposed to fish predation. The regression line for the mean copepod values is steeper, with a lower intercept, than that for either Daphnia. Small copepods elicit significantly shorter RD (“Student’s” t-test) than either Daphnia of the same length: 6.0 cm for 0.9-mm D. sicilis; 10.2 cm for D. pulex; 14.7 CI~ for D. magna. The RD for the largest copepod tested, Diaptomus leptopus females 2.3 mm long, is not significantly different from RD for 2.3~mm D. magna-36.3 cm and 38.2 cm. Capture frequency by fish-Figure 3 suggests one way of visualizing relative encounter frequencies for prey which elicit differ-cm RD. In swimming a certain distance a fish will have scanned a larger volume of water for prey with larger RD and
574
Confer and Blades Table 2. Predicted electivity indices bmed on visibility and ease of capture. RD is the reaction distance; N is the relative density; VE is the relative probability of visual encounter; CF is the relative capture frequency; EI is Ivlev’s electivity index. I >\ ‘.__ //’ -_---_--------
.-
1 distance
h
-
Fig. 3,. Schematic representation tion of different sizes of prey.
\\ // -- 1-y’
---J of Fish percep-
a smaller volume for prey with a smaller RD. The ratio of these volumes, or the ratio of the probability of visual encounter, is proportional to the square of the ratio of the RD. The relative capture frequency for a species-size category (CF) is assumed to equal the product of RD2 times CS times the density of the species-size category (N): CF = RD2CSN.
(1)
This assulnes that the fish are hungry enough to pursue all sighted prey and that prey densities are sufficiently low that the probability of simultaneously encountering two prey is small. Werner and IIall (1974) have proposed that CF should be proportional to RD3. It seems true that at any instant the probability that a zooplankton will be within the fish’s vision is proportional to RD3. However, if a prey is not within sight the fish must move. When moving from prey to prey the fish will search out cylindrical volumes. Our model seems more appropriate for low prey densities when the swimming distance, h in Fig. 3, is larger than RD. Werner and IIall’s model may be more correct for high prey densities when the swimming distance is not much larger than RD, or when two or more prey are frequently within sight simultaneously. With different species of zooplankton, CF can be estimated with the appropriate regression equation, Table 1, and appropriate CS as follows: CF = (a + TIzZ)~CSN, where a is the intercept
(2)
of the regression
D. pulex 2.7 mm D. pulex 2.0 mm D. putm 1.3 mm D. putex 0.9 mm E. lamstris27 1.7 mm D. sicilis 0.9 mm nsuplii 0.2 mm
RD 31 cm
0.143
VE 0.357
CF 0.379
EI 0.45
24 cm
0.143
0.214
0.227
0.23
17 cm
0.143
0.107
0.114
-0.11
10 cm
0.143
0.037
0.039
-0.57
cm
0.143
0.271
0.229
6 cm
0.143
0.013
0.011
-0.86
1 cm
0.143
0.003
0.003
-1.00
0.23
(since a is not statistically significantly different from zero for any regression, it will be assumed to be zero and omitted from our calculations), m is the slope of the regression, and I is the length of the prey. Prey selection from a mixed assemblage of zooplankton can be predicted by this model. Field studies provide data by which the accuracy of the theoretical predictions can be judged. To derive the theoretical values for prey selection, we have arbitrarily assumed a zooplankton mixture of seven species-size categories, each of equal abundance (Table 2). Relative capture frequencies were calculated by means of Eq. 2. Theoretical electivity indices were calculated by means of the equation, (r - p)/( r + p), where r is the proportion of a species-size category in the fish gut and p its proportion in the environment ( Ivlev 1961). The theoretical predictions could be more rigorously tested if the field data provided the abundance of every species-size category of zooplankton at that depth where the fish were feeding. These data would be necessary because the electivity indices vary with the abundance of each size-category of zooplankton including those that are not eaten. Consequently, the following comparisons demonstrate only whether the theoretical values are in the same range as those observed in field studies.
Plankton Table 3. Predation by Epischura lacustris on Diaptomus sicilis in 65ml chambers at 9°C. One E. lacustris and four prey of specified instars were in each chamber except on 25 May. Number in parentheses is total number eaten after 3 days, with replacement of eaten and dead prey once a day; number next to parentheses is rate of predation in prey eaten per predator per day. Lxaptonlus sici lis adults small coDe. nauolius (0.3Lo.5Lnm) (0.81-0.9&nm) (0.16:0.36mm) (2 Episehura per 4 nauplii) iday (42) 0.72 Jun (81) 2.2 Jun (15) 1.1 (39) 1.9 1% 3 (0) 0 Jun (k?) 1.1 (1) 0.13 Jun Jun (25) 0175 (25) 0.69 Jul
Date 25 3 7 15 21 29 1
The theoretical electivity index for D. phx changes from positive to negative with Daphnia about 1.5 mm long. Galbraith (1967) obtained similar results by comparing the relative sizes of Daphnia in a tow net sample with those in the gut of rainbow trout and yellow perch. In 13 analyses the clcctivity index changed from positive to negative with Daphnia between 1.1 and 1.7 mm. The theoretical electivity index for small copepodites is negative and very low, that for large copcpoditcs slightly positive. The magnitude of both of these theoretical values agrees with the actual values reported for alewives by Hutchinson ( 1971). The theoretical electivity index for nauplii (and rotifers had they been included) is -1.00, in agreement with many studies of prey selection by the larger planktivorous fish. Fish fry, however, may prefer these smaller prey. It appears that many of the observed differences in capture frequencies can be explained by the visibility of the prey and their ease of capture. We will now use prey selection as predicted by this model to estimate fish energy gain with and without predacious zooplankton, Prey selection by predacious zooplankton -There is competition between predacious zooplankton and planktivorous fish, since prey eaten by one is no longer available to the other. The degree of competition depends largely on the overlap of the size of nrevd eaten bv, the two kinds of J. nredators. 4.
575
predation
Above, we developed a quantitative estimate of the size of prey eaten by the fish; here we present data on the size of prey chosen by predacious zooplankton. Prey selection by E. Zacustris is reported in Table 3. On both 7 and 15 June, when predation on nauplii and on early copepodites was determined concurrently, predation on the small copepodites was more intense. We conclude that small copepodites are the preferred prey of E. Zacustris: nauplii wcrc readily eaten, adults rarely. Table 4 presents further data on prey choice by predacious zooplankton. Dodson (1974) found that Diaptomus shoshone does eat Cladocera (Table 4). However, Anderson (1970) found that D. shoshone, Diaptornus arcticus, and Diaptomus neuadensis would not eat Cladocera when they were simultaneously offered copepods as prey. He found the mean ratio of predator to prey length for these three to be 2.5, 3.0, and 2.7 and for Cyclops bicuspid&us and Cyclops vernalis to be 0.85 and 2.2. In general predacious zoopIankton prefer prey smaller than themselves such as and early copepodites. rotifers, nauplii, Adult planktivorous fish generally ignore these small prey and ingest prey of a size that includes most carnivorous zooplankton. Chaoborus and Leptodora, which are highly transparent, seem to prefer Cladocera that are fairly visible. Cyclops bicuspiclatus may prefer small prey (McQueen 1969) or prey its own size or larger (Anderson 1970). Energy ingestion by fish-Wc can now estimate the ratio of energy intake by a fish with and without predacious zooplankton. We assume that energy content is proportional to weight for rotifers, cladocerans, and copepods. Size can be converted to weight by the equations provided by Edmondson and Winberg ( 1971). For fish feeding on Daphnia the relative rate of energy ingestion (EI ) is EL = CF K( 0.052Z3,2),
(3)
and for copepods EI = CFK( 0.055Z2,7),
(4)
.
576
Confer and Blades
Tabte 4. Lepomis gibbosus by the predacious zooplankton. ==
-i--_-e.
reaction
A-------zE=ii=;=i
-Y
distance
to predacious
< -- ----:.=--EL-
=z=2
c
-_-_ _ --.__ --_-
zzz
zooplankton zzz
--=-:E=J
and to the prey ~i~-_F=c---i--
(RD t0 p redacious zooplankton)* (RD to prey of predacious zooplankton)2 Prey frequently eaten by Prey seldom eaten,.by predacious zooplankton predacious zooplankton
selected ---
---
Fish's
Predator Prey Mesocyclops Diaptcww
edax, ad. fZoridanus, CIII DicIptOmUS floridanus, ad. ad. Cyclops bicuspidatus, Keratella sp. Ceriodaphnia sp, Diaptanus oregonensis, m. Diaptomus oregonensis, ad. Epischura ZacuStris, ad. Artemia nauplii Diaptomus minutus, ad. Diaptomus copepodites Dicptamcs nauplii Epischura lacustris, ad. Epischura lacustris, CII Epischura lacustris, m. Mesocyclops edczz, ad. Cyclopoid
copepodites
Size (mm) 1.26 0.56 0.97 1.1 "0.15
=~.a
-0.4 =0.9 1.7 =0.5 1.0 ~0.65 =0.15 0.5 0.2 1.35
1
200 6.5
2
>1a2 10.5
>1a2 "2.1
3
2521 5.0
-37. 2750
1.0
521. ,750
2.1 -14.1
-0.0
Cyclopoid nauplii 0.2 Daphnia "1.0 Daphnia 20.35 Diaphanosoma =1.0 Bosmina =0.5 h?ischurir Zacustris, ad. 1.1 Lkptomu.9 sidlis, m. "0.27 Diaptomus sicilis, CII 0.90 ad. Diaptomus sicilis, 1.17 Diaptomus shoshone, ad. 2.5 Daphnia (optimum) 0.9 Daphnia (minimum) 0.45 Daphnia (maximum) 1.20 Chaoborus americanus 5.0 Daphnia (optimum) 0.80 Daphnia (minimum) 0.65 Daphnia (maximum) 1.68 ad. Sagitta elegans, 17.0 PseudocaZwu.48 binutus 0.9 Oithona 8imilis 0.5 Acanthocyc&qx viridis 2.5 Varied 0.1 to 1.0 Cladocera B1.0 < --Con er 7--Smyly 1970.
2750 = = = =
1.6 6.8 1.6 3.9
2750 21 4.6 5
4.4 Fish's RD for predator a very large ratio.
unknown.
10.6 3.0 Extrapolation
suggests
Fish's RD for predator a very large ratio.
unknown.
Extrapolation
suggests
Ratio usually
(5)
This suggests that for various size categories of Daphnia, the ratio of EI is proportional to the ratio of their lengths to the 5.2 power. Consequently, as little as a 20% reduction in mean length of a Daphnia
6
7
large Ratio
where K is the Eactor for weight to energy conversion. Equations 2 and 3, incidentally, also draw attention to the major effect that prey size has on fish energy intake. For L. gibbosus preying on Daphnia, by combining Eq. 2 with 3, we have EI = m”Z2N CS K0.052 P2.
Ref.*
"2.
population retaining the same density would reduce EI by 68%. The rate of energy ingestion for L. gihbosus from those species-size categories subject to predation by zooplankton, if the predacious zooplankton were not present is EI 7b= RD2,&CS7Xwt~,
(6)
where h is herbivores of the species-size categories eaten by the predacious zooplankton. To estimate the change in EI due to predacious zooplankton, we need to estimate
Plankton
predation
577
the reduction in prey available to the fish because of the zooplankton. The maximum likely reduction is a 1 : 1 ratio of the number of prey eaten by the predacious zooplankton and the reduction in those available to the fish. Predation on a herbivore population limited by its own rate of grazing on algae might increase the reproduction of the remaining individuals. Then there would be less than a 1 : 1 ratio of the number eaten by the zooplankton and the reduction in number available to the fish. Modeling such population interactions is beyond our interest here. We will assume the 1 : 1 ratio here so as to maximize the estimate of the reduction of prey items available to the fish due to predacious zooplankton. For each predacious zooplankter present, the herbivore population will have been reduced by the product of the reciprocal of the efficiency of energy transfer from the herbivore to the predacious zooplankton times the ratio of the prcdator’s weight to herbivore’s weight. Efficicncy is defined as the weight gain of the predacious zooplankton for the weight of herbivore eaten. For the fish the rate of energy ingestion with both herbivore and predator present is
rocal of the energy transfer efficiency divided by the CS,. If we assume 10% as a representative efficiency and 80% as the CS,, the presence of the predacious zooplankton increases fish energy intake when the ratio RD2, : RD21bexceeds 12.5. We have summarized 19 observations of predacious zooplankton and their frequcntly eaten prey: those in Table 4, plus Anderson’s mean values for the size of prey eaten by D. shoshone, D. arcticus, D. neuadensis, C. hicuspiclatus, and C. UVnalis, but excluding Dodson’s values for D. shoshone because of Anderson’s observation that this species prefers copepods to cladocerans when both are present. Table 4 presents the square of the ratio of the fish’s RD to a predacious zooplankton to the RD to the preferred prey of the zooplankton. For 15 of thcsc observations, the ratio appears to exceed 12.5. In these instances, the model predicts that predacious zooplankton increase the energy intake of planktivorous fish. For half of all 19 predator-prey pairs the margin for error in the model is a factor of 16 or greater. It therefort seems clear that predacious zooplankton increase the food intake of planktivorous fish.
EIk, = RD2,, N, wtzj CS, K
Discussion
+ RD2rb NF,- N&
2)
h
wtF,KCSh . (7)
The ratio of the energy ingestion by the fish when predacious zooplankton are present to that without predacious zooplankton is EI ?Ih EII,1 + N1,wtlj [RD2, CS,, - 13D21,( l/eff ) CW, RDZ,, N/, CS,,wtk
(8)
Energy ingestion by the fish will be greater in the presence of predacious zooplankton when the solution to Eq. 8 is greater than anti; the solution will bc greater than one when the expression within the parentheses is greater than zero, and the parenthetical term will be greater than zero when the ratio RD2 p : RD2a is greater than the recip-
Starting from the work of Hrbacek (1962) and Brooks and Dodson ( 1965)) various studies of size-selective predation by planktivorous fish have established the generalization that intensive fish predation results in essentially complete elimination of larger zooplank ton. Our study confirms the suggestion of these and later investigators that the two major factors determining this sclective predation are visibility and case of capture of prey, but defines this more precisely by predicting the relative intensity of predation on each species-size category of P’CY * Lepomis gibbosus shows an increase in reaction distance with increase in prey size LIP to the largest Daphnia that we used, over 3.0 mm. This increase in reaction distance most likely corresponds with an increase in the intensity of predation, as Wer-
578
Confer and Blades
ner and Hall (1974) demonstrated for Lepomis macrochirus. Allan (1974) proposed that the intensity of fish predation on Cladocera increases as size increases up to some intermediate size and that any furthcr size increase does not increase predation intensity. The conflict between his model and ours of a continuous increase in predation intensity with increasing prey size merits examination. From analyses of prey size in fish stomachs (Galbraith 1967), Allan proposed that Salmo gairdineri shows maximum interest for prey 1.6 mm and larger, Perca f1a.vescen.s for prey 1.3 mm and larger, based on the frequency of occurrence of these prey sizes in the guts. However, to estimate predator interest one should also consider the frequency of occurrence of a prey in the environment. Ivlev’s ( 1961) electivity index does consider both frequencies, and the electivity indices can be estimated from Galbraith’s graphed results. For S. gairdineri, on all eight sampling dates the electivity indices increase with increasing prey size. Typical values from Stager Lake, August 1960, are 0.27 for 1.6mm Daphnia, 0.40 for 1.9-mm Daphnia, and 0.55 for 2.6-mm Daphnia. Galbraith’s data show that P. flavescens does not eat as many large Daphnia as the trout. Nonetheless, for perch in four of five samples the highest electivity indices are for the largest daphnids, which are larger than 1.3 mm. Galbraith’s data and the measurements of fish reaction distance reported here and by Werner and Hall (1974) thus support a model of continuous increase in fish predation intensity as prey size increases. There may be some zooplankton size beyond which a further increase evokes no increase in fish interest. For fish fry this size is less than the size of many zooplankton. But for larger planktivorous fish, prey at least as large as very large Daphnia do not exceed the size at which maximum interest is reached. The significance of predacious zooplankton to the energy intake of fish depends on the abundance of the predacious zooplankton and their influence on the rest of the
plankton. Patalas ( 1971) found the nearly strict carnivores, M. edax and C. bicuspidatus, to be among the six most widely distributed and abundant zooplankton in 45 lakes in northwestern Ontario. The abundance of these and other species of predacious zooplankton is affirmed by his and myriad other surveys. Predation rates by zoopIankton frequently are high. Anderson’s data (1970) show the predation rate by D. shoshow on prey less than 0.9 mm to average 12.3 prey per predator after 24 h. More typically laboratory values for other predacious zooplankton are around one to three prey per predator per day. Size selection for small prey is typical of most euplanktonic, predacious zooplankton. Dodson’s (1974) results suggest that predacious zooplankton not themselves exposed to fish predation can become sufficiently abundant to eliminate small species of zooplankton. Predacious zooplankton thus play a major role in the energy transformations of planktonic communities. The energy uptake of fish will be influenced by predacious zooplankton. Discussions of human food supply frequently stress that more energy can be obtained by shortening the food chain. However for planktivorous fish reducing the number of energy transfer steps by removing predacious zooplankton would not be advantageous. For planktivorous fish the small prey such as nauplii, rotifers, and early copepodites are difficult to see while large, predacious zooplankton are highly visible and the increased visibility of predacious zooplankton more than compensates for the energy lost by the respiration of the predacious zooplankton. References Allan,
J. D. 1974. Balancing predation and competition in cladocerans. Ecology 55 : 622-629. ANDERSON, R. S. 1970. Predator relationships and predation rates for crustacean zooplankters from some lakes in western Canada. Can. J, Zool. 48: 1229-1240. BROOKS, J. L., AND S. I. DODSON. 1965. Predation, body size, and composition of plankton. Science 150 : 28-35.
Plankton CONFER, J. L.. 1971. Intrazooplankton predation by Mesocyclops edax at natural prey densities. Limnol. Oceanogr. 16: 663-666. DODSON, S. I. 1974. Zooplankton competition and predation: an experimental test of the size-efficiency hypothesis. Ecology 55 : 605613. EDMONDSON, W. T., AND G. G. WINBERG [Eds.] 1971. Secondary productivity in fresh waters. IBP IIandbook 17. Blackwell. GALBRAITH, M. G., JR. 1967. Size-selective predation 011 Daphnia by rainbow trout and yellow perch. Trans. Am. Fish. Sot. 96: l10. HRBA~EK, J. 1962. Species composition and the amount of zooplankton in relation to fish stocks. Rozpr. Cesk. Akad. Ved 72: 1-116. HUTCIIINSON, B. B. 1971. The effect of fish predation on the zooplankton of ten adirondack lakes, with particular reference to the alewife, Abosa pseudoharengus. Trans. Am. Fish. Sot. 100: 325-335. IVLEV, v. S. 1961. Experimental ecology of the feeding of fishes. Yale Univ. MCLAREN, I. A. 1969. Population and production ecology of zooplankton in Ogac Lake, a
579
predation
landlocked fiord on Baffin Island. J. Fish. Res. Bd. Can. 26: 1485-1559. MCQUEEN, D. J. 1969. Reduction of zooplankton standing stocks by predaceous Cyclops bicuspidatus thomasi in Marion Lake, British Columbia. J. Fish. Res. Bd. Can. 26: 16051618, MAIN, R. 1962. The life history and food relations of Episcrura Zacustris Forbes (Copepoda: Calanoida). Ph.D. thesis, Univ. Michigan. PATALAS, K. 1971. Crustacean plankton communitics in forty-five lakes in the experimental lakes area, northwestern Ontario. J. Fish. Res. Bd. Can. 28: 231-244. SMYLY, W. J. P. 1970. Observations on the rate of development, longevity and fecundity of Acanthocyclops uiridis ( furine) ( Copeoda, Cyclopoida) in relation to type of prey. Crustaceana 18 : 21-36. WERNER, E. E., AND D. J. HALL. 1974. Optimal foraging and the size selection of prey by the bluegill sunfish ( Lepomis macrochirtcs ) . Ecology 55 : 1042-1052.
Submitted: Accepted:
26 September 1973 27 March 1975
