This dissertation has been 64-9560 microfilmed exactly as received
FABIAN, Michael William, 1931- A QUANTITATIVE STUDY OF THE PREDATION OF FRESH WATER AND TROPICAL FISH FRY BY CYCLOPOID COPEPODS.
The Ohio State University, Ph.D.,1964 Z oology
University Microfilms, Inc., Ann Arbor, Michigan i A QUANTITATIVE STUDY OF THE PREDATION OF
FRESH WATER AND TROPICAL FISH FRY
BY CYCLOPOID COPEPODS
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University
By
Michael William Fabian, B.S., M.S.
The Ohio State University 1964
Approved by /MS'* Adviser Department of Zoology and Entomology ACKNOWLEDGMENTS
The writer wishes to extend his thanks to Dr. A. G.
Broad, Department of Zoology and Entomology, The Ohio State
University, for his extremely helpful suggestions and guid
ance, and to whom the results are herewith dedicated.
Research was conducted at the Franz Theodore Stone
Laboratory of The Ohio State University at Put-in-Bay, Ohio, and the campus at Columbus, Ohio. Preliminary experiments were also completed at Geneva College, Beaver Falls, Penn
sylvania.
I wish to extend my sincere thanks to Dr. C. A.
Dambach, Director of the Natural Resources Institute; Dr.
L. S. Putnam, Director of the F. T. Stone Laboratory; and Dr. T. M. McMillion, Geneva College, Beaver Falls, Penn
sylvania, for the liberal use of materials and equipment.
The assistance is gratefully acknowledged of Mr. Harold
Wasko, former Director of the Toledo Aquarium, in providing bluegill fry, Dr. Jerry Hubschmann, in providing observa tional assistance, and Russell V. Skavaril for his advice on treatment of the statistical analysis. Special thanks are due Dr. C. C. Davis, Western Reserve University, Cleveland, Ohio, who originally stimulated my interest in the problem. This investigation was partially supported by funds furnished by the Ohio Division of Wildlife through the Ohio
Natural Resources Institute. CONTENTS
Page
ACKNOWLEDGMENTS...... ii
LIST OF T A B L E S ...... v
LIST OF ILLUSTRATIONS...... vii
INTRODUCTION ...... 1
MATERIAL AND M ET H O D S ...... 6
RESULTS ...... 10
DISCUSSION ...... 51
SUMMARY ...... 57
LITERATURE CITED ...... 59
AUTOBIOGRAPHY...... 63 LIST OF TABLLS
Table Page
1. Daily Mortality of Pumpkinseed Sur.fish Fry Exposed to Various Densities of Cyclopoid G o p e p o d s ...... 10
2. Mortality of 3.8-4.0 mm. Zebra Fry Exposed to Various Densities of Gyclopoids Collected from Lake E r i e ...... 12
3. Effect of Activities of Cultured Cyclops vernalis on the Hatch of Zebra Eggs and Survival of Y o u n g ...... 15
4. Effect of Length of Zebra Fry on Mortality when Exposed to Cultured Cyclops vernalis ...... 16
5. Effect of Cyclopoid Copepods on the Survival of 7-8 mm. Walleye F r y ...... 17
6. Effect of Cyclops vernalis on Survival of Newly Hatched Channel Cat F r y ...... 18
7. Mortality of Larvae of Unknown Species of Fresh Water Fish after Exposure to Cyclopoid C o p e p o d s ...... 19
8. Effect of Cyclopoid Copepods on Survival of 3.5-7.0 mm. Bluegill F r y ...... 21
9. Mortality of Gourami Fry Following Exposure to Cyclops in Large and Small Vessels ...... 24
10. Mortality of Gourami Fry after Three Days Exposure to Cyclopoid Copepods ...... 27
11. Cichlid Fry Mortality in the Presence of Mature and Copepodid Cyclops vernalis ...... 29
12. Effect of Separation of Cyclopoids from Cichlid F r y ...... 30
v Table Page
13. A Summary of the Effects of Cyclopoid Copepods on 5.5 mm. Convict Cichlid, Cichlasoma ttigrofasciatum...... 31
14. Relationship of Volume to Mortality after Three Days Exposure to Cyclops sp...... 33
15. Effect of Cyclopoids on Fry in Large Aquaria . . 34
16. Effect of Cyclops per Fry Ratio on Loss of Five Experimental Fish Species ...... 38
17. A Summary of Experiments Showing the Mortality Effect as the Number of Fry is Increased . . . 40
18. A Summary of the Effect of Variations in the Cyclops per Fry per Liter Ratio on M o r t a l i t y ...... 41
19. Analysis of Variance of Cyclops per Fry per Liter Ratio and Its Effect on Mortality . . . 42
20. Cyclops per Fry Ratio Effect on Per Cent M o r t a l i t y ...... 44
21. Analysis of Variance of Cyclops to Fry Ratio Its Effect on M o r t a l i t y ...... 45
22. Cyclops per Fry per Square Centimeter Effects on Mortality ...... 47
23. Analysis of Variance of Cyclops per Fry per Square Centimeter and Its Effect on Mortality. 48
24. Number of Cyclopoids Present during a Three Year Period of Stocking Walleye Fry in Lake Erie. . 49
vi LIST OF ILLUSTRATIONS
Figure
I. Cyclops per Fry Effects on Mortality
INTRODUCTION
For many years cyclopoids copepods were thought to occupy a position in the food chain intermediate between algae and small fish. This concept placed them, with
Daphnia and other primary consumers, at an intermediate en ergy transformation level. Jurine (1820) was perhaps the first to note that Cyclops was carnivorous. Birge (1897) stated that Cyclops feeds on rotifers, nauplii and other animals. Further evidence of the carnivorous nature of cyclopoid copepods was furnished by Klugh (1927), Roy
(1932), Dziuban (1937, 1939), Coker and Hayes (1940), and
Ruttner (1953). Lindberg (1949) and Hintz (1951) both found that cyclopoids captured and devoured mosquito larvae.
Dziuban (1937, 1939), after observation and anal ysis of the gut contents, maintained that Cyclops insignis,
C. fuscus, C. viridis, vernalis, C. albidus and C.
serrulatus were predominantly predaceous. Fryer (1957),
analyzed the gut contents of many cyclopoid species and
concluded that some were predominantly herbivorous while
others, (Acanthocyclops viridis, A. vernalis, and
Mesocyclops leuckarti), were mainly carnivorous. Fryer also suggested that certain species might be undesirable in lakes because they eat food that otherwise might be avail
1 able to fish. Schmalgauzen (1958) found that Macrocyclops sp. preyed upon Leptodora. Monakov (1959) reported that
Acanthocyclops viridis readily ate Cyclops sp. and small oligochaetes and consumed daphnids and tendipedids hardly at all. Dziuban (1937) found that certain cyclopoids con sumed Daphnia, copepod larvae, and planarians. Individual cyclopoids consumed from one to six planarians, 4-6 mm. in length, during a twenty-four hour period.
The ability of Cyclops to kill vertebrate animals was noted by Babak (1913) who observed the death of a lar val axolotl after attack by numerous Cyclops. A second re port of this predaceous activity on a vertebrate animal was given by Spandl (1926) who stated that small fishes were the victims of cyclopoid attacks, but that eventual death resulted from infection. Dziuban (1939) conducted experi ments with frozen eggs of perch and pike and fresh eggs of the orfy and carp. He noted that Cyclops did not eat the living eggs but actively and completely consumed
Saprolegnia growing on dead eggs and alevins. All of 48
Saprolegnia infected carp alevins were eaten by copepods, but only 23 of 60 non-infected alevins were consumed.
Cyclops evidently began to feed exclusively on the fry.
Oliva and Sladacek (1950) observed and reported attacks by
Cyclops strenus and Cyclops vicinius on axolotl larvae.
The larvae died several days later. In 1953 Fryer captured from a stream a recently hatched roach which was being 3 attacked by Cyclops viridis. The same paper ( F r y e r , 1953) includes a report of one aquarist who witnessed attacks by
Cyclops sp. on newly hatched tropical fish fry and attrib uted the death of two dozen young to this cause. A. van der
Werff (1953, 1956) observed and reported many instances of cyclopoids attacking young fish.
In 1959 C. C. Davis collected in Lake Erie some dead fish fry with several Cyclops bicuspidatus and Meso- cyclops edax attached to them. Further observations by
Davis (1959) indicated that 6.5 mm. long rock bass,
Ambloplites rupestris, could be damaged in the presence of
Mesocyclops edax at a copepod density equivalent to 500 per liter. Preliminary work by Fabian (1960) indicated that some cyclopoid copepods under laboratory conditions were
capable of killing the fry of bluegills, Lepomis macro- chirus, several species of sunfish, zebras, Brachydanio rerio, the gourami, Trichogaster trichopterus, and a cich
lid, Cichlasoma nigrofasciatum.
• Vladimirov (1960) stated that Cyclops vicinus,
Acanthocyclops vernalis and Mesocyclops crassus are pred
ators of shad eggs and larvae in the Dnieper River. In the
laboratory, eggs just released by female fish were opened
and the contents devoured by cyclopoids. A survey of eggs
from the Dnieper indicated that, in certain samples, up to
10 and 12 per cent of the eggs had been damaged by cyclo
poids . From 30 to 40 per cent of all the larval shad in 4 the samples had been partially eaten or attacked by co pepods. Later Vladimirov (1960) noted that shad eggs on the bottom of the estuary suffered a 21.5 to 49.2 per cent mortality and that the greatest loss of eggs occurred in the years of greatest abundance of Cyclops (1956, 1957).
Unfortunately Vladimirov did not consider the number of
Cyclops or fish used in his laboratory studies or report on the size of the copepod population in the river.
An Indian fisheries investigator, M. A. V. Laksh- manan (personal communication in 1960) confirmed the earlier work of Dziuban (1959) by observing the consumption of 5-7 mm. carp fry by cyclopoids. Kuznetsova (1962) found that survival of the embryos and larvae of the carp was highest in ponds with the smallest numbers of "water animals" which he identified as the consumers of the eggs and larvae. Although he gave no indication of number of animals involved, he stated that Cyclops was one of the predators.
Wickstead (1961) and others have investigated the diet of marine cyclopoid copepods. Some species are herbivorous, some carnivorous, and some are omnivorous.
Although many investigators have reported attacks of cyclopoids on young fishes, only Dziuban (1939) and
Vladimirov (1960, 1961) have considered the quantitative aspect of this predatory activity. This report presents the results of laboratory studies of the quantitative re- lationship that exists between copepod density, number larval fish, and the rate of predation by the copepods. 'MATERIALS AND METHODS
Adult and copepodid stages of cyclopoid copepods
used in the experiments were collected or cultured. The majority of the experiments utilized cyclopoids collected
in plankton nets from Hatchery Bay (Fishery Bay at Put
in-Bay, Ohio) in Lake Erie, usually on the day they were
used to start experiments. Whenever it was impossible to
obtain cyclopoid copepods in sufficient quantity, Cyclops vernalis from a laboratory culture were used. These had been cultured for several months in an algae medium (Coker,
1959) with the addition of finely ground boiled egg yolk.
No difference was noted between the behavior of freshly
caught animals and that of those maintained in laboratory
cultures.
Eggs and fry of undetermined species of sunfish were collected from nests in a small farm pond in Beaver
County, Pennsylvania, and in Terwilliger’s Pond on South
Bass Island, Ottawa County, Ohio. Bluegill fry, Lepomis macrochirus, were obtained from tanks at the Toledo Aquar
ium. Larvae of the walleye, Stizostedion vitreum, were
obtained from the Linesville Fish Hatchery, Linesville,
Pennsylvania. Eggs of the channel catfish, Ictalurus
6 7 punctatus, were obtained from a nesting site near South
Bass Island in Lake Erie.
Eggs and fry of the zebra fish, Brachydanio rerio, fry of the gourami, Trichogaster trichopterus, and eggs and fry of a cichlid, Gichiasoma nigrofasciatum, were ob tained by breeding these fish in thermostatically controlled aquaria at 25 degrees C.
An experiment consisted of exposing a known number of fish or eggs to a known number of adult or copepodid cyclopoids for an interval of at least several days. In
some experiments plankton organisms other than cyclopoid copepods were included and in others the copepods were
separated from the fish by a barrier of bolting silk. For each experiment a control, lacking copepods but otherwise treated as was the test, was provided. Results are ex
pressed as total mortality per day from all causes.
Water which was used in a typical experiment was either from the same vessel in which the eggs were hatched
or from the same geographical locality in which the fry were collected. Selected numbers of the desired fish or eggs were carefully placed in the vessels chosen for the
experiment and were observed for any damage due to trans
fer. On several occasions the fry were allowed to accli mate to the transfer for twenty-four hours before placing
cyclopoids in with them. Cyclopoids used in experiments were initially placed in either one liter or 200 ml. 8 fingerbowls. They were then removed by means of a wide mouth pipette, counted and placed in a second fingerbowl.
From this second fingerbowl the cyclopoids were then transferred to the experimental vessels. A check of the species composition of the cyclopoids was made either prior to or following an experiment.
Experiments were conducted in vessels of several sizes: 200 ml. fingerbowls, one, two and three liter battery jars, and ten gallon aquaria.
In most experiments a small number of fry was used in each vessel in order to minimize death of fry from physiological or behavioral activities. Aeration and con- commitant turbulence of the water was employed in those experiments that involved one through three liters of water.
For the sake of uniformity, results are presented in terms of copepod equivalents per liter. Fry in test groups were subjected to cyclopoid concentrations ranging from 2.6 to 500 individuals per liter. Most of the exper iments were conducted within the range of 20 to 100 co pepod equivalents per liter, which is the order of magni tude of the density of cyclopoid populations in many fresh water lakes and ponds. Wiebe (1930) in a study of several ponds found 503 and 505 Cyclops per liter. Kraatz (1941) reported from 57 to 100 cyclopoids present per liter of water from an Ohio lake. Pennak (1946) found in over half of his samples from 7 Colorado lakes up to 100 copepods per liter. Ten of these samples had lOO to 200 copepods per liter and twelve had up to 560 per liter. 2oen (1957) noted Cyclops viridis, Hacrocyclops fuscus and M. albidis in Danish lakes in concentrations from 127 to 245 per lite
Hubschmann (1960) reported that, in the latter part of
June and early July, daily sampling at the same Lake Erie station revealed a standing crop of cyclopoids that yielded from 58.3 to 150.3 individuals per liter.
Direct observation of the experimental group was maintained much of the time to verify that death of the fry actually was due to cyclopoid attacks. Observations and recording of resulting deaths were made daily in the mornings. Although most of the data presented are the re sults of three days of experimental procedure, records were maintained daily for a period of up to six days for most experiments.
The results of all experiments were subjected to a Chi square analysis of the test and control deaths in order to determine whether chance sampling could account for the variance in control and test deaths. All of the experimental tests were also subjected to analysis by the test for linear regression and deviation from regression. RESULTS
A summary of six experiments designed to test the hypothesis that mortality of fish is higher in vessels con taining copepods than in those without them is presented in Table 1. Sunfish eggs were pipetted from nesting sites and permitted to hatch in the laboratory. Control and test fry were placed in 200 ml. of filtered pond water.
TAB1E 1.
DAILY MORTALITY OF FUMPKLNSEED SUNFISH FRY EXPOSED TO VARIOUS DENSITIES OF CYCLOPOID COPEPODS
Cyclopoid Number of Mortality per Day Equivalents Fry ______per Liter______1_____ 2_____ 3
0 14 0 2 5
Cumulative 0 14.2 50.0 Per cent Loss t
25-50 11 3 5 2
75 10 4 0 4
150 7 2 1 3
500 3 2 1 -
Cumulative 35 .4 58.1 87.1 Per cent Loss for all Test Groups
Chi square 5.3, D. F. 1
10 11
Cyclops vernalis y Mesocyclops leuckarti, and Eu-
cyclops agilis collected from the same pond and cultured C. vernalis were used in these experiments. No attempt was made to feed the fry of either group. Although the yolk
sac was still visible at the end of the experiment the lack
of food may have contributed to the total deaths. The loss
of 50 per cent of the control fry in three days is inor
dinately high and may indicate a general weakened condition
that could also account for the high mortality among the
test fry. Nevertheless the test groups (with cyclopoids)
showed a higher daily mortality and a 37.1 per cent higher
total mortality than did the control groups. The Chi
square test in a two by two table of the data in Table 1
indicated in a comparison of all test and control groups
that the mortality of fry in the test groups was signif
icantly higher than that in controls.
The zebra aquarium fish, Brachydanio rerio, was
chosen as an experimental fish because it is easy to raise
and can produce large numbers of eggs and fry. Zebra fry
are about 3.8 ram. long immediately after hatching or about
the same size as the fry of many important fresh water fish,
uagler (1952) citing measurements by Carr (1942) indicated
that largemouth bass, one day after hatching were 4.1 mm. in
length. Davis (1959) states that the newly-hatched sheeps-
head fry were 4.2 mm. in length. Newly-hatched bluegills
are approximately 2.8 mm. long. 12
Experiments with the zebra fish are presented in
Tables 2, 3, and 4. Table 2 summarizes a series of 14 experiments involving recently hatched zebra fry from 3.3 to 4.0 mm. long. In these experiments both the test and control vessels contained filtered lake water. Immature and mature cladocerans and fine, dry tropical fish food were added to both control and test vessels, but cyclopoids were placed only in the test vessels. The copepods used were Cyclops vernalis, C. bicuspidatus, and Macrocyclops
sp. collected from Hatchery Bay, near Put-in-Bay , Ohio.
TABLE 2
MORTALITY OF 3.8 - 4.0 mm. ZEBRA FRY EXPOSED TO VARIOUS DENSITIES OF CYCLOPOIDS COLLECTED FROM LAKE ERIE
Cyclopoid Number Mortality per Day Equivalents of Fry per Liter 1 2 3
0 47 1 4 0
Cumulative 2.1 10.6 10.6 Per cent Loss
10-15 9 0 1 2
100-150 17 7 6 1
151-200 31 23 5 1
Cumulative 52.6 73.7 80.7 Per cent Loss for Test Groups
Chi square 48.1, D. F. 1 13
Table 2 appears to show a gradual increase in fry mortality with an increasing concentration of cyclopoids per liter. The Chi square test in a two by two table in dicates a highly significant difference between the test and control groups, (Chi square 48.1, D. F. 1).
An experiment was devised to test the hypothesis that physical or chemical conditions produced by the co pepods caused the death of the fry. All of the copepods from a test vessel were removed and placed in the water of a control vessel. The control fry were removed and placed in the water of the original test vessel. These control fry remained alive for three days, after which they were re moved. Five zebra fry were placed in the water which con
tained Cyclops vernalis. Within one minute, a Cyclops was
observed attacking a fry. A few seconds later another
Cyclops attacked and remained attached to the same fry.
Within fifteen minutes all five fry were under attack. At
the end of one hour all five fry were dead and had been al most completely devoured. Eight additional zebras were then
placed in the same vessel with identical results. All of
these eight were consumed in an hour.
Eggs of the zebra danio, Brachydanio rerio were
selected for studies of the effects of cyclopoids on their
survival. Results of these experiments are given in Table
3. The eggs used in these experiments were 1.0 mm. in 14 diameter or slightly larger than eggs of the white crappie and slightly smaller than bluegill eggs. Morgan (1951,
1954) reported that eggs of the bluegill, Lepomis macro- chorus, are about 1.26 mm. in diameter and those of the white crappie, Pomoxis annularis, from 0.82 to 0.9 mm. in diameter.
Table 3 represents a summary of 24 experiments in volving zebra eggs which had been laid the night before setting up the experiment. It was possible to separate, by color, healthy eggs from infertile and damaged ones.
Only healthy eggs were selected. These were placed in the experimental jars with great care in order to minimize damage to the outer membrane. The zebra eggs were placed in fingerbowls containing Cyclops vernalis which had been cultured in a battery jar on egg yolk-algae medium. Ob servations during several days showed no damage to the eggs, and all of the fry appeared to hatch in good condition.
Most of the fry hatched from the first through the fourth day. It is difficult to compare daily mortality between the groups tested because of this variation in hatching time. A noticeable increase in mortality in both the test and control groups during the experimental period of six days was observed. The cumulative per cent loss appears to be directly related to copepod density. The Chi square test indicates a significant difference between the controls and test groups of zebra eggs, (25.2, D. F. 1). 15
TABLE 3
EFFECT OF ACTIVITIES OF CULTURED CYCLOPS VERNALIS ON THE HATCH OF ZEBRA EGGS AND SURVIVAL OF YOU n G
Cyclops Eggs in Mortality per Day per Liter vessels ______I______2______3 4 5_____ 6_
0 70 031960
Cumulative 0.0 4.3 5.7 18.6 27.1 27.1 Per cent Loss
20 15 1 2 0 2 1 2
Cumulative 6.7 20.0 20.0 33.3 40.0 53.3 Per cent Loss
30 14 3 4 O 2 1 1
Cumulative 21.4 50.0 50.0 64.2 71.4 78.5 Per cent Loss
40 15 334300
Cumulative 33.3 40.0 66.7 86.7 86.7 86.7 Per cent Loss
50 15 5 4 0 1 1 1
Cumulative 33.3 60.0 60.0 66.7 73.3 80.0 Per cent Loss
70-100 10 0 0 2 4 3 1
Cumulative 0.0 0.0 20.0 60.0 90.0 100.0 Per cent Loss
Cumulative 69 17.4 36.2 44.9 62.3 71.0 78.3 Per cent Loss in All Experimental Vessels 16
Many observations were made on the survival of 8-10 mm. long sunfish fry in vessels with large numbers of cyclopoids. Dziuban (1939) stated that two day old carp fry were able to feed on Cyclops. Several experiments were devised to test for the size at which a fry might be resist ant to attacks. Since previous experiments seemed to show a relationship between cyclopoid concentration and mortality,
TABLE 4
EFFECT OF LENGTH OF ZEBRA FRY ON MORTALITY WHEN EXPOSED TO CULTURED CYCLOPS VERNALIS
Cyclopoid Zebra Length Number Mortality per Day Equivalents mm. of Fry per Liter 1 2 3
0 3.8 10 0 0 0 150 3.8 10 10 - -
0 4.1 10 0 0 0 50 4.1 10 1 0 1 100 4.1 10 0 1 0
0 5.0 10 0 2 0 50 5.0 10 0 7 1 150 5.0 10 1 9 -
0 7.0 10 0 0 0 50 7.0 10 0 0 0 150 7.0 10 0 0 0
0 8.0 10 0 0 0 150 8.0 10 0 0 0
0 9.0 10 0 0 0 150 9.0 10 0 0 0
0 10.0 10 0 0 0 150 10.0 10 0 0 0 17
large concentrations of cultured Cyclops vernalis were used
in all these experiments. Table 4 presents the results of
- a series of experiments with selected sizes of zebra fry in
vessels with density equivalents of over fifty Cyclops per
liter. It should be noted that death of fry occurred in all
test vessels that contained fry belov; 7.0 mm. in length.
No mortality of fry was noted in the vessels with fry 7.0
mru. long or longer.
Several experiments were devised which involved
recently-hatched fry of the walleye, Stizostideurn vitreum
and Cyclops vernalis. Water, fry and cyclopoids used in
these experiments were taken from hatching tanks of the
Linesville Fish Hatchery, at Linesville, Pennsylvania.
The results of these experiments are presented in Table 5.
TABLE 5
EFFECT OF CYCLOPOID COPEPODS ON THE SURVIVAL OF 7-8 mm. WALLEYE FRY
Cyclopoid Number Mortality per Day Per cent Equivalents of Fry Loss Per Liter ______1______2______3
0 24 0 O 1 4.2
50 36 3 2 1 16.7
51-100 29 3 7 lO 34.4
Chi square, 11.3, D. F. 1.
The walleye fry were physically active, free
swimming, and ranged from 7 to 8 mm. in length. Mortality
of these fry was very low after a three days exposure to 18 copepods at a density of 50 per liter. As the cyclopoid concentration was doubled the mortality of fish also dou bled; however, the total mortality was not as high as that noted with other experimental fish. The Ghi square test, in a two by two table and utilizing Yates* correction in dicates a significant difference between the test and con trol groups of walleyes.
Eggs of the channel catfish, Ictalurus punctatus, were collected from Lake Erie and placed in vessels con taining two liters of water. At hatching, the fry were approximately 13 mm. in length and physically active.
These facts, coupled with the relatively low concentration of Cyclops vernalis to which they were exposed probably account for the low mortality in the test vessels after three days exposure to cyclopoids. Results of this experi ment are presented in Table 6. The Ghi square test fails to show that differences between the test and control groups are significant.
TABLE 6
EFFECT OF CYCLOPS VERNALIS ON SURVIVAL OF NEWLY-HATCHED CHANNEL CAT FRY
Number Length Cyclopoids Mortality per Day Per cent of Fry mm.______per Liter 1 2 3 4 5 Loss
18 13 0 0 0 2 1 1 22.2
17 13 27 1 3 1 2 0 41.2
Chi square 0.7, D. F. 1 19
Actively swimming fry, believed to be rock bass, were collected near the shore of Hatchery Bay in Lake Erie.
Sixteen of these fry, 6-8 mm. in length, were placed in several test vessels containing Cyclops vernalis and Cyclops bicuspidatus. Table 7 shows that the mortality was only
12.5 per cent in the vessels with 6-8 mm. long actively swimming fry. The same mortality was also noted in the con trol vessels.
TABLE 7
MORTALITY OF LARVAE OF UNKNOWN SPECIES OF FRESHWATER FISH AFTER EXPOSURE TO CYCLOPOID COPE PODS
Cyclopoid Number Length Deaths per Day Equivalents of Fry mm. per Liter 1 2 3
0 24 4.5-10.0 O 0 3
Cumulative 0.0 0.0 12.5 Per cent Loss
20 5 4.5 4 1 0
25-50 16 6.0-8.0 0 1 1
Cumulative 19.0 28.6 33.3 Per cent Loss for Test Groups
On two occasions observations were made of attacks by cyclopoids on relatively large, active fry. A cyclopoid was observed attached to the pectoral fin of an 8.0 mm. gourami. Twney-four hours later, the fry appeared normal and alive with no apparent damage. On another occasion a 20 late yolk sac fish of unknown species, 7-8 mm. in length was caught in a #20 plankton net. A Cyclops sp. was at tached to the belly of the fry. It remained attached for approximately one minute. The fry was then placed in an eight inch finger bowl to determine whether any permanent damage had resulted from the attack. After two days the fry was still alive and apparently not seriously injured.
Two 8.0 mm. gourami fry and onej 7.0 mm. undetermined species of fry from Lake Erie were plac-ed in a finger bowl containing 27 cyclopoids. This is equivalent to 135 cyclopoids per liter. No mortalities occurred during a five day period of observation.
It is believed that when many species of fry reach a length of about 7.0 to 8.0 mm., they can withstand cyclopoid attacks. The size at which fry can withstand these attacks probably varies with each species. Activity or motility of the fry may be of prime importance to sur vival.
During the course of the preceeding experiments, the question of whether Cyclops is capable of killing fry of the bluegill, Lepomis macrochirus, one of the most com mon freshwater game fish in the United States, arose.
Bluegill eggs and fry were collected from nesting sites in the Toledo Aquarium and transported to the laboratory where some additional hatching occurred. The fry were exposed to 21 various densities of cyclopoid copepods in a series of experiments summarized in Table 8. Size of the bluegill fry used in these experiments varied from 3.5 to 7.0 mm.
Morgan (1951) stated that the bluegills which he studied were approximately 3.5 mm. in length 16 hours after hatch ing.
TABLE 8
EFFECT OF CYCLOPOID COPEPODS ON SURVIVAL OF 3.5-7.0 mm. BLUEGILL FRY
Cyclopoid Number Mortality Per Day Equivalents of Fry per Liter 1 2 3
0 69 1 3 1
Cumulative 1.5 5.8 7.2 Per cent Loss
10-50 30 7 2 8
Cumulative 23.3 30.0 56.7 Per cent Loss
51-100 19 6 3 6
Cumulative 31.6 47.8 78.9 Per cent Loss
101-150 32 19 1 3
Cumulative 59.6 62.5 71.9 Per cent Loss
Chi square, 38.5, D. F. 1.
Cyclops vernalis and Cyclops bicuspidatus were the two most abundant cyclopoids encountered during these experiments although Mesocyclops leuckarti, Eucyclops sp., 22
Ectocyclops phaleratus, Ectocyclops sp., and Cyclops exilis were also present and apparently are capable of killing young fish. Once, two cyclopoids were observed chewing or biting on the tail of one of the bluegill fry. Within ten minutes the fry ceased moving and additional cyclopoids be gan feeding on all parts of the body. Concentrated formalin was poured over the fish, killing the cyclopoids while they were still attached to the fry. The four cyclopoids on the body were identified: one male Eucyclops sp.; one female
Ectocyclops sp.; and two male Cyclops bicuspidatus.
In spite of the fact that the bluegill larvae were active swimmers at the beginning of the experiments, mortality was relatively high in all test groups during the initial twenty-four hours of exposure. Mortality increased with an increase of cyclopoid concentration after one and two days exposure but appeared to decrease with higher con centrations in the second and third days. This might be due to the fact that the copepods in vessels containing 101-150 per liter consumed a large number of fish (19) on the first day. The hunger of the cyclopoids possibly was eased init ially and this may have been a factor in the reduced pre dation of the second and third days. Dziuban (1937) found that Cyclops consumed more planarians at the start of his experiments than they did later. Table 8 shows signif icant differences between control and test vessel mortality of the bluegill, (Chi square, 38.5). The Chi square 23 statistical test, in special selected experiments, in dicated no significant difference in mortality between bluegills and tropical fish used experimentally, (Ghi square, 2.1, 1.3, and .0004).
Most of the freshwater fish lay their eggs on the lake or stream bottom, on aquatic plants or in or on ob jects. Pelagic eggs and fry, which are common in marine habitats, are infrequent in freshwater situations, accord ing to Davis (1959). The gourami, Trichogaster trichop- terus, was chosen to provide eggs and fry of a pelagic nature. These fish place their eggs in a bubble nest very close to the surface of the water. After the eggs hatch, the larvae, which are approximately 4.0 mm. in length, swim freely near the top of the awuarium. There is no initial period of resting on the bottom as is common with many freshwater fish larvae. It was postulated that, since many of the carnivorous cyclopoids are found on the bottom or in the regions of aquatic vegetation, mortality would be low in experimental tests with pelagic fry of this type.
Table 9 represents one series of experiments with the gourami. Table 10 is a summary of all the experiments performed with the gourami. Table 9 shows that as the av erage number of cyclopoids increased the percentage loss of fish increased slightly. It should be noted, however, that during the course of the experiment the number of cyclop oids in each vessel decreased. This could be due to their TABLE 9
MORTALITY OF GOURAMI FRY FOLLOWING EXPOSURE TO CYCLOPS IN LARGE AND SMALL VESSELS
Number of Fry Length Volume of Number of Cyclops Mortality per Day Water Live Cyclops per Liter 1 2 3 OO-O n o o OOOOr-IO 2 4 mm. - 6 mm. 200 cc. 0 0 0 0 2 If ii II 0 0 0 0 IT 2 11 it 0 0 0 0 2 M it It 6 30 1 0 2 tl it tt 8 40 0 0 2 If tt It 10 50 0 0
II 10 11 ti 0 . 0 0 0 10 It it IT 5 25 1 0 10 11 ti If 5 25 1 0
_ 20 4 ram. - 5 mm. 2 liters 0 0 20 It IT 11 100 50 - - being eaten by the fry or even to the predatory activity of one cyclopoid upon another. The death of five fry in the ten and twenty fry tests apparently was caused by the equivalent of two and five Cyclops per fry per liter respec tively. Ten fry were destroyed by the equivalent of 7.5
Cyclops per fry per liter. Mortality of the gourami fry in the two volumes of water (Table 9) was compared by the Chi square test using the Yates correction. The difference be tween mortalities observed in vessels containing 200 and
2,000 ml. of water was without significance, (Chi square,
0.38, D. F. 1). A Chi square test of Table 9 showed a highly significant difference between the control vessels and vessels with Cyclops, (Chi square 10.2).
It would appear that the mortality of the gourami fry (Table 10) was less than that of the bluegill (Table 8) in comparable cyclopoid concentrations and after three days1 exposure. Mortality of 4.0-6.0 mm. long gourami fry by 5 to 50 cyclopoids per liter was 30.3 per cent whereas the loss of bluegills of comparable size was 56.7 per cent over the same period of time. Mortality of the gourami con trol fry was extremely low with the death of only one in dividual during a three day period. Five bluegill controls died during the same interval.
An attempt was made to test the effect of an alter nate or competitive food supply on predation of fish fry by cyclopoid copepods. Several four-liter culture vessels of 26
Chlorella sp. and a small quantity of egg yolk were pre pared in which Cyclops vernalis was introduced. Over a period of several months the cyclopoids increased in num ber, and most of the females produced eggs. Apparently from the large number of animals present after several months the nauplii and copepodid stages also grew and reproduced. It would appear that C. vernalis was living, growing, and re producing in this culture medium.
Five 5.0 mm. gourami fry were placed in one culture vessel that contained at least several hundred Cyclops vernalis. Within an hour I observed attacks ori and death of three of the fry. No trace of the other two fry could be found, probably because of the large size of the con tainer and the density of the algae. Five control fry sur vived for three days in the same type of culture medium from which the cyclopoids had been removed.
Three fry were placed in a Chilomonas and Cyclops vernalis culture. The three fry were alive at the end of eight hours but were destroyed within eighteen hours.
Three control fry survived at least three days in the cul ture medium without Cyclops. It appears from the observed activities that Cyclops vernalis attack fry in the presence of other food materials.
The convict cichlid, Cichlasoma nigrofasciatum, was selected as an experimental fish because of the resemblance of its spawning activity to that of the bluegill. After 27
TABLE lO
MORTALITY OF GOURAMI FRY AFTER THREE DAYS EXPOSURE TO CYCLOPOID COPEPODS
Number Cyclopoid Mortality Per cent of Fry Equivalents Loss ______per Liter______
51 O 1 2.1
56 5-50 17 30.3
20 75 15 75.0
10 125-250 5 50.0
Chi square 38.1, D. F. 1
preparation of a nest site and laying of the eggs, the male
and female guard and fan the eggs until they hatch. Early
and middle yolk sac cichlid fry, which were used in experi ments, ranged from 5.0 to 5.5 mm. in length. These fry were initially larger and much more active than the bluegill
fry used, but remained on the bottom of the experimental
vessels for a longer period of time. This presumably would
increase the duration of their exposure to the carnivorous
cyclopoids.
Fryer (1957b) and others have mentioned that the re mains of copepodid stages of copepods were found in the gut
of mature carnivorous cyclopoids. Since almost all of the
prvious experiments were conducted with mature copepods, a
series of experiments was conducted in which both mature
animals and copepodid stages were counted. The primary
criterion of an adult copepod, given by Yeatman (1959) as 28 the relative size of the last urosomal segment and the presence of egg sacs on the female, was used to distinguish
copepodid stages. If the presence of copepodid stages pro vides an alternate food supply then we might expect a reduc
tion in mortality of fry.
Table 11 shows that 23 out of 40 cichlid fry were
destroyed within three days with the equivalent of forty
cyclopoids per liter in two liter vessels. Twenty to 70
copepodids were present in each experimental vessel. In
the presence of 80 and 161 mature copepods, even with in
creased numbers of copepodid stages, the fry were
destroyed within two days. The presence of copepodid
stages does not appear to diminish destruction of cichlid
fry at relatively low adult concentrations per liter. To
the contrary, the data in Table 11 suggest that copepodids
also prey on fish.
In spite of numerous observations of copepod attacks
on young fish in the laboratory and in the field, no such
observations accompanied most of the deaths reported in
experiments. Accordingly, an effort was made to determine
if metabolic products produced by cyclopoids could cause
the death of fry. One experiment was devised, as seen in
Table 12, in which one hundred mature Cyclops vernalis and
Mesocyclops leuckarti were placed in the same container with the fry but physically separated from them by a net of
#20 bolting silk. Each vessel contained three liters of TABLE 11
CICHLID FRY MORTALITY IN THE PRESENCE OF MATURE AND COPEPODID CYCLOPS VERNALIS
Number of Number of Total Number Copepods Number Mortality Per Day Total Mature Copepods Copepodids of Cyclopoids Per Liter of Fry 1 2 3
0 0 0 0 10 0000
40 20 60 30 10 0 0 5 5
40 27 67 33.5 10 0 ? 9 9
40 57 97 48.5 10 0 0 1 1
40 70 110 55 10 1 2 5 8
60 60 120 60 10 2 2 5 9
80 128 208 104 10 5 5 - 10
161 184 345 178 10 6 4 - 10
KJ VO 30 water which was aerated continuously. The airstone was placed beneath the cyclopoid retainer net to facilitate turbulence and transfer of metabolic byproducts through the net. The Chi square test of the data in Table 12 fails to show a significant difference between control and test groups. This is possibly due to the small number of fry used. Gourami control fry in other experiments lived for periods up to three days in water that had previously held several hundred cyclopoids. Other experiments (Table 9) clearly show that the gourami fry do not survive when ex posed directly to copepods.
TABLE 12
EFFECT OF SEPARATION OF CYCLOPOIDS FROM CICHLID FRY
Number of Cyclops Number Deaths per Day Cyclops______per Lifer______of Fry______1______2_____ 3_
100 Cyclops 10 0 0 1 physically separated from the fry
100 33 15 3 1 0
100 33 7 2 0 2
Chi square 0.77, D. F. 1
A summary of all tests with the convict cichlid
(Table 13) shows that there was a relatively rapid rise in mortality of the control fry between the second and third day. The control mortality increased by 15 per cent with 31 TABLE 13
A SUMMARY OF THE EFFECTS OF CYCLOPOID COPEPODS ON 5.5 mm. CONVICT CICHLID, CICHLASOMA NIGROFASCIATUM
Cyclopoid Number Mortality per Day Equivalents of Fry per Liter 1 2 3
73 1 1 11
Cumulative 1.3 2.7 17.8 Per cent Loss
3-20 51 9 14 18
Cumulative 17.6 45 .1 80 .4 Per cent Loss
30-40 40 12 9 4
Cumulative 30.0 52.5 62.5 Per cent Loss
50-80 24 8 8 7
Cumulative 33.3 66.7 95. 8 Per cent Loss
100-300 15 5 2 5
Cumulative 33.3 46.7 80.0 Per cent Loss
Chi square 9.4, D. F. 1 the death of 11 fry. The four other experimental classes had mortality increases which varied from 10 to 35 per cent from the second to the third day. It would appear that other factors, possibly lack of food or disease, may have affected some fry between the second and third day. During the first and second days, mortality of fry in vessels that contained cyclopoids was significantly greater than that in control 32 vessels, (Ghi square, 18.8). A significant difference was also noted between control vessels and cyclopoid concentra tions as low as 20 per liter.
Table 14 is a summary of experiments carried out in vessels of various sizes. Over half of the experiments in fingerbowls were conducted with 50 or fewer cyclopoid equiv alents per liter. Although volumes of water were varied and ranged from 200 ml. to 37,000 ml. there were three basic bottom areas involved in the tests. The 200 ml. finger- bowls had a bottom area of 56.7 sq. cm. Vessels with 500 ml. to 3,000 ml. of water were 226.6 sq. cm. in bottom area.
The 37,000 ml. aquarium had a bottom area of 1,258 sq. cm.
The mortality during a three-day period was 56.8 per cent in the 200 ml. vessels and, although the volumes of the other vessels ranged from two to 15 times more and the bottom area was increased approximately four times, the over-all mortal ty in these vessels was only 57.3 per cent.
This test in a two by two table failed to show that the minor differences between mortality in the 200 ml. and 500 ml. vessels differed significantly, (Ghi square, .001;
D. F., 1). Similarly, no significant difference was found between the mortality of fry in 200 ml. vessels and the com bined mortalities in 500, 1,000, and 2,000 ml. vessels,
(Chi square, 0.0008; D. F., 1). i 33
TABLE 14
RELATIONSHIP OF VOLUME TO MORTALITY AFTER THREE DAYS EXPOSURE TO CYCLOPS SP.
Number Volume of Cyclops Mortality Per cent of Fry Experimental per Liter Loss Vessels (ml.)
221 200 O 19 8.6 80 200 10-30 30 37.5 71 200 40-50 35 49.3 41 200 60-125 24 58.5 58 200 150-500 53 91.4
Test 250 142 56. 8 Totals
48 500 O 6 12.5 30 500 50 10 33.3 10 500 loo 1 10.0 38 500 150 26 68.4
Test 78 37 47.4 Totals
48 1000 o 5 10.4 55 1000 50 24 43.6 33 1000 150-500 30 90.9
Test 88 54 61.4 Totals
72 2000 O 7 9.7 72 2000 10-30 44 61.1 30 2000 40-50 15 50.0 40 2000 60-125 34 85.0
Test 142 93 65.5 Totals
lO 3000 O 1 10.0 21 3000 10-30 5 22.7
31 37,000 2-7 24 77.4 34
Three experiments were designed to assess the mortality of fry in a large volume of water with a compar
atively low cyclopoid density. The results of these experi ments are presented in Table 15.
The specified species of fry and less than ten
cyclopoids per liter were placed in 37 liters of water in
large aquaria. Feeding conditions in control and test
aquaria were maintained at equal levels daily by placing
Daphnia sp., protozoons, and tropical fish food in both
containers.
TABLE 15
EFFECT OF CYCLOPOIDS ON FRY IN LARGE AQUARIA
Type Number Cyclops Mortalities per Day of Fry of Fry per Liter 1 2 3 4 5 6 7 8
Bluegill 20 0 ------2
Gourami 10 0 0 0 0 0 0
Cichlid 21 0 0 0 5
Bluegill 20 2.6 ------9
Gourami 10 6.6 0 0 0 0 0
Cichlid 21 3.2 8 7 4
Overall mortalities in test and control groups seem
to indicate a relationship between species behavior and
total mortality. The gourami, which was very active and
pelagic had no losses in either test or control groups over
a five day period. The cichlid, also an active fry, re- mained on the bottom of the aquarium and had a 90.5 per
cent mortality in a three day period in test aquaria. Blue
gill larvae were active swimmers which made it impossible to
conduct daily counts of surviving fish. It was decided in
stead to continue the experiment for a period of eight days
and then remove and count the surviving fry. If we can
assume an equal natural mortality for the test and control
groups, the mortality due to cyclopoids was 35 per cent. A
highly significant difference in cichlid mortality was
noted between the control and test aquaria, (Ghi square,
16.6; D. F., 1). The difference in mortality of test and
control groups of bluegills in ten gallon aquaria showed a
significant difference, (Ghi square, 4.5; D. F., 1). Both
of these statistical tests were by use of the Ghi square
analysis in a two by two table, using the correction for
small numbers. These results indicate that activity of the
fry may be of prime importance in survival of fry exposed
to cyclopoids.
Fryer (1957a) mentions that one cyclopoid devoured
a 3.0 mm. chironomid larva in approximately 30 minutes and
on one occasion a 2.0 mm. larva was ingested within nine minutes. Oligochaetes were very quickly devoured. Spec
imens of Nais sp. 4.0 mm. in length were eaten in as little
as 3.5 minutes. Dziuban (1937) found that a fasting
Cyclops consumed a 4-6 mm. planarian in fifteen to twenty m i n u t e s . 36
Several pertinent observations of the speed with which predation occurs were made. In one experiment 59
Cyclops vernalis consumed four cichlid fry, 5.5 mm. in length, in two hours. One hundred sixty-nine cyclopoids consumed 13 zebra fry within two hours. In another experi ment, 181 cyclopoids consumed six gourami fry in 3.2 hours.
On a fourth occasion 89 cyclopoids devoured five gourami fry in 4.25 hours.
Chance or apparently fortuitous contact seemed to determine which part of the fry was attacked initially. If the head were the initial region of contact, chewing ac tivities generally killed the fry within several minutes.
On many occasions the yolk sac was ruptured and the yolk would flow out and appear to coagulate. Death of the fry followed within minutes. On many occasions the cyclopoids attacked the fry in the tail region. For many fry this initiated muscular activity which threw the cyclopoid off and often removed the copepod from the immediate vicinity of the fry. If a cyclopoid were successful in remaining attached to the tail, however, the activities of the fry often ceased within 20 minutes. During this period, the posterior third of the animal had been consumed. Attach ment of the cyclopoid, however, did not always result in death of the fry. Cyclopoids were also observed attached to and chewing on the tail or other region of a fry when, for 37 no apparent reason, the cyclopoid disengaged itself and swam a w a y .
A s u m m a r y of experimental tests is presented in
Table 16. This compilation shows the percentage loss and the total Cyclops per fry ratio for five species of fish studied. In all of these groups tested by means of the Ghi square formula, it was assumed that the natural deaths in the test groups, were proportional to the natural deaths in the control groups. The corrected death total was computed by simple proportion and substracted from the test totals to give an assumed death rate attributed to Cyclops. The statistical tests of Tables 16 and 17 are based on this assumption.
Table 16 shows that the Cyclops to fry ratio ranged from 2.4 for the zebra eggs to 9.8 for the zebra fry. The ratios differed slightly between fish species, but four of the seven categories were exposed to copepods in a Cyclops to fry ratio of 4.7 to 6.5. Mortalities in these species varied from 40 to 62 per cent. The Chi square tests showed highly significant differences between comparisons of deaths of the gourami and zebra, (Chi square, 10.4), and the gour ami and cichlid, (Chi square, 16.6). Comparisons between the bluegill and cichlid, zebra, and gourami failed to show significant differences between total mortalities, (Chi square, 2.1, 1.3, 0 . 0 0 0 4 ) . 38
TABLE 16
EFFECT OF CYCLOPS PER FRY RATIO ON LOSS OF FIVEHEx PERTMENTAL FISH SPECIES
Species Volume Number Number Mortality Cyclops Percent (ml.) of Fry of Cyclops per Pry Loss
Gourami 200 36 109 12 3.0 33.3 Gourami 2000 40 250 20 6.3 50.0
Totals 76 359 32 4.7 42.2 Loss attributed to cyclopoids 40.7
Bluegill 200 61 443 43 7.4 71.0 Bluegill 2000 20 80 12 4.0 60.0
Totals 81 523 55 6.5 68.0 Loss attributed to cyclopoids 60.5
Zebra 200 26 203 17 7.8 65.4 Zebra 500 8 89 6 11.1 75.0 Zebra 1000 23 169 23 8.2 100.0
Totals 57 561 46 9.8 80.0 Loss attributed to cyclopoids 70.1
Zebra 500 51 250 30 5.0 59.0
Loss attributed to cyclopoids 54.0
Zebra 200 69 140 54 2.4 78.3* Eggs Loss attributed to cyclopoids 50.0
Sunfish 200 36 210 32 5.8 89.0 Loss attributed to cyclopoids 62.7
Cichlid 200 17 107 16 6.3 94.0 Cichlid 1000 10 186 7 18.6 70.0 Cichlid 2000 60 481 58 8.0 96.5 Cichlid 3000 22 200 8 9.1 36.4
Totals 109 974 89 8.9 81.6 Loss attributed to cyclopoids 70.5
*Exposure for 6 days 39
Table 17 is a summary of the loss of fish from experimental and control vessels which are grouped in four classes: Those experiments that involved two, four or five, ten, and twenty fry. Experiments with groups of twenty fry showed the lowest percentage loss at a concentration of
1-30 Cyclops per liter. Experiments with combined totals of the 20 fry groups showed the lowest Cyclops to fry ratio and the lowest percentage loss. At the 100 or more cyclopoids per liter concentration the 10 fry group had a mortality of
85 per cent compared to 56 per cent in the five fry groups.
It should be noted that the Cyclops per fry ratio was also higher in the 10 fry groups. Comparisons by means of the
Chi square test were made of all groups. These tests showed that there was a significant difference at the 1 to 2 per cent level between mortality in the ten fry groups and the four or five groups, at concentrations in excess of 100 copepods per liter, (Chi square, 5.8).
On the basis of all experiments with each group of fish, the Cyclops per fry ratio was lowest in the 20 fry groups and highest in the five and ten fry experiments.
Two fry and five fry groups, with an almost identical
Cyclops to fry ratio, had losses of 55.3 and 56.5 per cent respectively, (Table 17). TABLE 17
A SUMMARY OF EXPERIMENTS SHOVING THE MORTALITY EFFECT AS THE NUMBER OF FRY IS INCREASED
Cyclopoids Jrlsh per Number of Cyclops Percent Percent l o s s per Liter Experiment Experiments per Fry Loss Assumed from 6yclops
1-50 2 21 3.7 59.5 42.8 0 21 15.5
51-100 2 3 7-u 100.0 0 3 0.0
101-500 2 5 14.5 80.0 0 5 0.0
Test Totals 5*8 67.2 55*5
1-50 5 13 2.8 57.5 47.5 0 13 9-7
51-100 5 4 4.1 94.0 68.7 0 4 22.2
101-500 5 15 9.7 68.5 65.2 o 13 3.1
Test Totals 5.9 66.5 56.5
1-50 10 lo 3.6 53.0 45.5 0 10 7.5
51-100 10 3 11.0 66.7 66.7 0 3 0.0
101-500 10 4 13.0 93.0 85.8 O 4 7.2
Test Totals 6.9 65.0 56.5
1-50 20 3 4.5 44.3 32.5 0 3
51-100 20 2 5.6 75.0 75*0 0 2 0.0
Test Totals 4.9 56.5 4-7.0 41
Table 16 and 17 represented arrangements of data by type of fish and number of fry per experimental vessel.
Table 18 represents the arrangement of all data to dem onstrate the Cyclops per fry per liter ratio effect on per cent mortality of fry.
TABLE 18
A SUMMARY OF THE EFFECT OF VARIATIONS IN THE CYCLOPS PER FRY PER LITER RATIO ON MORTALITY
Cyclops per fry Per cent Loss per LiTer______after Three Days
3.0 20.0 21.8 76.2 77.3 29.0
8.0 80.0 69.3 77.6 50.0
13.0 91.0 17.8 85.0
18.0 100.0 87.5 70.0 76.0
25.0 100.0 100.0 71.5
35.0 20.0 50.0 100.0
50.0 73.0 4 75.0 93.0 TABLE 19
ANALYSIS OF VARIANCE OF CYCLOPS PER FRY PER LITER RATIO AND ITS EFFECT ON MORTALITY
SOURCE OF VARIATION SUM OF SQUARES DEGREES OF MEAN SQUARE F FREEDOM
Regression 1,506.6358 1 1,506.6358 2.02 ns
Deviation 16,340.260 22 742.73907 -
Array Means from Regression 4,710.33 (6) 785.05 1.08 ns
Observed values from Array Means 11,629.93 (16) 726.87 -
Total 17,846.895 23- _
Conclusions — data are linear, no relationship between Y and X.
-p- 43
The analysis of variance test (Table 19) of the data presented in Table 18 indicates that the F test of regres
sion is not significant. This indicates that within the limits of these experiments there is no relationship be tween volume and percent mortality.
Table 20 represents a summary in which the number of fry remains relatively constant but the number of
Cyclops per fry is increased. It is suspected that at con
centrations higher than those used, mortalities would de
crease or level off with a plateau. The analysis of vari
ance test of the data presented in Table 20 is found in
Table 21. The F test for linearity is not significant, in
dicating that the data are linear. The F test of regres
sion mean square divided by deviation mean square provides
a test for the hypothesis that the slope of the least
squares line equals zero. A significant F-value here in
dicates that the slope does not equal zero and that there
is a significant relationship between percent loss and the
number of Cyclops per fry. The equation for slope of the
line and the 95 per cent confidence intervals of the line,
for each group, are also presented in Table 21. TABLE 20
CYCLOPS PER FRY RATIO EFFECT ON PER CENT MORTALITY
Cyclops per Fry Per cent Mortality
1.0 3 3.3 28.6 50.0 20.0
2.0 52.2
3.0 89.0 66.6 34.3 47.7 90.4
4.0 73.4 80.0 33.8 33.0
5.0 49.6
7.0 100.0 100.0 74.1 80.0
8.0 83.4
9.5 96.0
11.0 75.0
Data presented in Table 22 represent a summary of the effects of the Cyclops per fry per square centimeter effects on mortality of all experimental fish. The statis tical treatment of this data was the same as in the preced ing analysis, and is presented in Table 23. TABLE 21
ANALYSIS OF VARIANCE OF CYCLOPS TO FRY RATIO AND ITS EFFECTS ON MORTALITY
SOURCE OF VARIATION SUM OF SQUARES DEGREES OF MEAN SQUARE FREEDOM
Regression 5,885.1274 1 5,885.1274 14.82**
Deviation 7,940.3140 20 397.0157
Array Means from Regression 2,558.07 (7) 365.43 0.88ns
Observed values from Array Means 5,382.2457 (13) 414.01
Total 13,825.441 21
The equation of the line estimated from these data is Y = 37.029 + 5.84 X
The confidence intervals (95%) of this line are
• Number ± Limits
1 14.143 2 11.828 3 10.013 4 8.991 5 9.012 6 11.932 7 14.247 8 18.211 9 22.529 46
FIGURE 1. Cyclops per Fry Effects on Mortality
oo 50
& 40
4 6 8 CYCLOPS PER FRY 47
TABLE 22
CYCLOPS PER FRY PER SQUARE CENTIMETER EFFECTS ON MORTALITY
Cyclops per Fry Per cent Loss per Square Centimeter______After Three Days
.008 25.0 17.4 37. 8
.017 59.6 34.2
.036 42.5 89.6 58.4
.054 82.5
.066 79.0
.082 86.0 50.0
.120 83.4
.210 70.9 54.8
.357 72.0
The analysis of these data (Table 23) indicates a linear relationship for the range of .008 to .120 Cyclops per fry per square centimeter. A significant F value also indicates that the slope of the line is not zero. The equa tion for the slope is given in Table 23 and also the 95 per cent confidence intervals of the line for the first seven g r o u p s . TABIE 23
ANALYSIS OF VARIANCE OF CYCLOPS PER FRY PER cm2 AND ITS EFFECTS ON MORTALITY
SOURCE OF VARIATION SUM OF SQUARES DEGREES OF MEAN SQUARE F FREEDOM
Regression 3,545.4288 1 3,545.4288 9.907**
Deviation 3,936.6635 11 357.8785
Array Means from Regression 1,605.2771 (5) 321.05542 0.826ns
Observed values from Array Means 2,331.3867 (6) 388.5644
Total 7,482.0923 12 _
The equation of the line estimated from these data is Y = 35.87 + 489.68 X
The confidence intervals (95%) of this line are
Group Number * Limits
1 16.860 2 14.769 3 11.863 4 12.061 5 13.822 6 17.432 7 28.613
-P' oo 49
As a test of the hypothesis that cyclopoids are ca pable of killing various fry under natural conditions,
plankton samples were taken at the time of stocking walleye
fry, Stizostideon vitreum in Lake Erie. The samples were
collected by a Juday plankton trap and preserved in 70 per
cent alcohol and subsequently counted. Samples at each
stocking site were taken just under the surface of the water,
at an intermediate depth and near the bottom. Bottom depths
ranged from six feet to 25 feet. Due to the nature of the
Juday plankton trap the "bottom samples" are taken 12 to 24
inches above the bottom. This gives very inadequate in
formation of the number of copepods swimming close to the
substrate.
TABLE 24
NUMBER OF CYCLOPOIDS PRESENT DURING A THREE YEAR PERIOD OF STOCKING WALLEYE FRY IN LAKE ERIE
Date Station Cyclopoids per Liter ______Top Middle Bottom
1960 1 1.5 1.9 2.3 1960 2 1.9 2.5 3.6 1960 3 0.9 0.2 0.9
1961 4 5.2 2.7 1.8 1961 5 2.8 1.9 2.8 1961 6 1.1 4.3 2.4 1961 7 0.3 1.2 5.5 1961 8 0.5 2.5 0.4
1962 9 1.0 2.2 2.0 1962 10 1.1 1.6 0.7 1962 11 2.9 2.3 1.6 1962 12 1.4 1.0 1.1 50
Table 24 indicates that a very small number of co- pepods were at each collection site. Although the activity of a fry exposes it to larger numbers of copepods than may be evident from a single sample, the cyclopoid density at the time of stocking was not believed to affect the survival
of walleyes in Lake Erie in 1960, 1961 and 1962. DISCUSSION
Of all the species of fry used in the experimental phase of the work, the zebra showed the least physical ac tivity after emergence from the egg. These fish rest on the bottom or sides of the experimental vessels for con siderable periods of time before attaining the free swim ming stage. This swimming and resting behavior was noted for several days after hatching. Although this behavior is also seen in some freshwater fish, the over-all muscular ac tivity of zebra fry was much less than any of the other experimental fish. This type of behavior could result in a high mortality from cyclopoid predation. The zebra fry had the highest first day mortality, especially in large con centrations of cyclopoids.
Observations and records of the experiments in which cyclopoids were placed with zebra eggs showed that there was no apparent damage to the egg. After the eggs hatched, however, the number of living and swimming fry declined rap idly. If the cumulative mortality for the last three days of the zebra egg data be compared to that of the zebra fry data it can be seen that they are similar, even to similar percentage losses at the three day and six day intervals. Dziuban (1939) stated that cyclopoids do not eat live eggs 51 52 but Vladimirov (1960) observed in laboratory tests that 10 to
12 per cent of the shad eggs from the Dnieper River were damaged by copepod attacks. This suggests the possibility that cyclopoid copepods may select certain species of fish eggs over others.
Experiments with zebra fry of various lengths, 7-8 mm. walleyes, 6-8 mm. fry of undetermined species, and 13 mm.
channel cat fry indicate that as each fry becomes older, and
presumably stronger, they become better able to resist
cyclopoid attacks. The zebra fry in controlled experiments
appeared to be resistant above 7.0 ram. in length. Walleye
fry, which essentially were twice the size of the zebra, had
some mortality but basically less than other experimental
tests. The channel cat fry, which are twice the size of a
walleye, had very low mortality during the first several
days. Statistical tests indicated no significant difference
in mortality between control and experimental tests.
Dziuban (1939) mentioned that in his tests Cyclops
readily consumed the alevins of carp and orfy up to an age
of two days after hatching. After this period the situation was reversed and the carp fed on the Cyclops. Unfortunately,
Dziuban gave no indication of the number of Cyclops in
volved or the size of the larvae.
On several occasions (Table 7) experiments and ob
servations indicated that fry which had survived in the lake
and reached lengths of 6.0 mm. or longer were capable of 53
resisting cyclopoid attacks. This resistance was noted even
under conditions where cyclopoids were very numerous.
Bluegill larvae were chosen for experimental purposes
as typical inhabitants of freshwater lake and pond habitats within the United States. The fact that Cyclops can kill bluegills possibly has implications in the management of
ponds in which fish are stocked. Ponds with large numbers
of small fish probably remove the Cyclops which in its turn
could limit the bluegill population by its carnivorous ac
tivity. If statements by Vladimirov (I960, 1961) are cor
rect then large numbers of cyclopoids could also reduce sur
vival of certain species of eggs.
Cyclopoid attacks on the gourami presented an interest
ing problem since gourami are pelagic from the time, of
hatching. Consequently they were seen infrequently on the
bottom of experimental vessels. The over-all mortality of
gourami fry was less than any other fish. It is possible
that this may be an artifact because the total Cyclops to
fry ratio was also the least.
Experiments with the cichlid, Cichlasoma nigro-
fasciatum, were probably the most fascinating of all exper
iments performed. Although the cichlid, after hatching is
extremely large, its tail and body moved vigorously and
continuously. The cichlid, in contrast to the gourami re
mained on the bottom of the experimental vessels. Agitation
of the water immediately over the fry was maintained by means 54 of an air stone to simulate the fanning activities of the adults. Results shown in Table 11 indicate a close relation ship between the total number of cyclopoids present and mortality of the fry. In all previous experiments, mortal ities of fry in concentrations less than 60 adult Cyclops per liter were moderate but experiments with adult and co- pepodid stages showed mortalities of 64 per cent over a three day period. Many investigators have reported that cyclopoid copepods will devour adult copepods, nauplii and copepodid stages. It would appear from this series of experiments that rather than decrease mortality the co- pepodids may have contributed to the mortality. Mortality of cichlid fry was the highest of all the experimental fish.
This lends support to the supposition that location of the fry on the bottom gave Cyclops more opportunity for chance contact.
Data which were accumulated on the various exper iments made it possible to arrange and compare results from several viewpoints. The experimental data were rearranged on the basis of size or volume of the experimental vessels and presented in Table 14 and Table 18, on the assumption that experiments in vessels with larger volumes would result in less mortality. Statistical tests, however, indicated no significant difference in mortality between the various volumes. 55
A significant linear relationship does exist as the number of Cyclops per fry is increased and also as the
Cyclops per fry per square centimeter ratio is increased, the mortality increases.
Comparative mortalities between the different types of fish showed few significant differences between fish. In selected experiments significant differences were noted be tween the gourami and zebra, (Chi square, 10.1). This is interesting in view of the fact that both the cichlid and zebra remained much of the time on the bottom whereas the gourami was a free swimming fish from the time of hatching.
The true significance and importance of carnivorous cyclopoid copepods in lakes, rivers, or oceans still remains to be ascertained. In all probability, the presence of carnivorous cyclopoid copepods among littoral communities gives them an unusual role in energy transformation. They have been observed feeding on Peritricha, Rotifera, planar- ians, juvenile and large cladocerans, nauplii and adult co pepods, oligochaetes, marine worms, and dipterous larvae.
Observations have also been made of attacks on certain species of fish eggs and salamander larvae. Cyclopoids have been observed attacking larval tropical fish species, larval carp, shad, sturgeon, rockbass and bluegills.
In view of the fact that they often occur in enormous numbers in the littoral region of lakes, cyclopoid crusta ceans possibly play an important role in regulating fish 56 populations. Not only do they possibly attack fish eggs but also compete with young fish for potential food. The impor tance of any predator depends on a series of factors, among which is physical contact with the prey. In nature carniv orous cyclopoid copepods are closely confined to the sub stratum and it is their habit to swim over the bottom or over the surface of plants. This activity favors a close and in timate contact with potential fish food, eggs and fry. A second predator prey relationship involves the relative abundance of predator or prey. Since cyclopoids may some times appear in massive numbers, evidence from the present investigation indicates that cyclopoid concentrations equivalent to or less than those found in many natural lake situations may significantly reduce the fish population.
Since cyclopoids may act as predators at one time they also hold the unusual position of being preyed upon as the fish become older. It is suggested that past investigations on food requirements of fry be re-evaluated in the light that the -presence of large numbers of cyclopoids may have been a factor in destruction of the fry. SUMMARY
1. By experimental means it was established that
Cyclops vernalis, C. bicuspidatus and Mesocyclops leuckarti
were capable of killing recently hatched fry of Lepomis
macrochirus, Stizostedion vitreum, Brachydanio rerio, Tricho-
gaster trichopterus, Cichlasoma nigrofasciatum, and un
identified species of sunfish.
2. Mortality of the fry in these experiments in
creased as the Cyclops per fry ratio was increased.
3. Mortality of the fry in these experiments in
creased as the Cyclops per fry per square centimeter ratio was increased.
4. Cultured Cyclops vernalis in experimental tests apparently did not damage zebra eggs.
5. Most experimental fish above 7.0 mm. in length were resistant to cyclopoid attacks and showed less mortal
ity.
6. The presence of copepodid cyclopoids apparently
does not reduce mortality of fry.
7. In these experiments tests failed to show signif icance of mortality between different volumes of water used.
Significant differences of mortality were noted within each volume of water used. 57 8. No significant difference in mortality was noted between tropical aquarium fish species used and fresh water species.
9. Mortality was higher with those fry which re mained on the bottom of the experimental vessels and less with actively swimming or pelagic fry.
10. Quantitative plankton samples which were taken during the stocking of walleyes in Lake Erie in 1960, 1961, and 1962 indicated that very low numbers of cyclopoids were present at those times. The predatory activity of such low numbers of cyclopoids probably did not affect walleye sur vival in those years.
11. It is suspected that whenever predatory cyclopoid copepods are present in large concentrations which can be found in various habitats, that they are an important factor in survival of freshwater fish species. LITERATURE CITED
Babak, E. 1913. Axolotl (Amblystoma). Akvaristicky obzor, 3 (2): 17-21.
Birge, E. A. 1897. Plankton studies on Lake Mendota. II. The crustacea of the plankton, July, 1894-December, 1896. Trans. Wise. Acad. Sci. Arts and Letters. 11: 274-448.
Coker, R. E. and W. J. Hayes. 1940. Biological observations in Mountain Lake, Virginia. Ecology, 21: 192-198.
Coker, R. E. 1959. Culture methods for cyclopoid copepods and their forage organisms, p. 226. In Culture Methods for Invertebrate Animals edited by P. S. Galtsoff, F. E. Lutz, P. S. Welch and J. G. Needham. Dover Publications Inc., N. Y.
Davis, Charles C. 1959a. Damage to fish fry by cyclopoid copepods. Ohio. Jour. Sci., 59 (2): 101-102.
1959b. Should we love or hate Cyclops? The Aquarium, 28 (7): 200-203
1959c. A planktonic fish egg from freshwater. Limnology and Oceanography, 4 (3): 352-355.
Dziuban, N. A. 1937. On the feeding of some Cyclopidae. Trans. R. E. Foerster. Doklady Akad. Nauk. SSSR. 17 (6): 319-322.
1939. New data on the feeding of some Cyclopidae. Trans. R. E. Foerster. Trudy Moskovskogo Tekhnich- eskogo Instituta Rybnogo Khoziaistva i Promyshlen- nosti im A. I. Mikoyana, Mosrybvtuz, Moskva. 163-172.
Fabian, M. W. 1960. Mortality of freshwater and tropical fish fry by cyclopoid copepods. Ohio Jour. Sc., 60 (5): 268-270.
Fryer, C. G. 1953. Notes on certain British freshwater Crustaceans. Naturalist, London. 101-109.
1957a. The feeding mechanism of some freshwater cyclopoid copepods. Proc. Zool. Soc. London, 129: 1-27.
59 60
______. 1957b. The food of some freshwater cyclopoid copepods and its ecological significance. Jour. Animal Ecology, 26 (2): 261-283.
Hintz, Howard W. 1951. The role of certain arthropods in reducing mosquito populations of permanent ponds in Ohio. Ohio Jour. Sc., 51 (5): 277-279.
Hubschmann, Jerry H. 1960. Relative daily abundance of planktonic Crustacea in the island region of Western Lake Erie. Ohio Jour. Sci., 60 (6): 335-340.
Jurine, L. 1820. Histoire des Monacles qui se trouvant aux environs de Geneve. Paris. klugh, A. B. 1927. The ecology, food-relationships and culture of freshwater Entomostraca. Trans. R. Can. Inst., 16: 15-98.
Kraatz, W. C. 1941. Quantitative plankton studies of Turkeyfoot Lake, near Akron, Ohio. Ohio Jour. Sci., 41: 1-22.
Kuznetsova, I. I. 1962. Povyshenie vyzhivaniya potomstva u sazana, (Cyprinus carpio L.). (Increasing the survival rate of the progeny of Cyprinus carpio L„). Trans, by P. Narbut. Zool. ZhurTJ 41 (9): 1367- 1373.
Lagler, Karl F. 1952. Freshwater Fishery Biology. 84-90. Wm. C. Brown Co., Dubuque, Iowa.
Lindberg, K. 1949. Crustaces copepodes comme ennemis naturels de larvae d ;Anopheles. Bull. Soc. Pathol. Exot., 42: 178-179.
Monakov, A. V. 1959. (Predatory feeding of Acanthocyclops viridis (Jur) Copepoda, Cyclopoid.). Trans, by P. Isiarbut. Trudy Inst. Boil. Vodokhranlishch. Akad. Nauk. SSSR, 2 (5): 117-127.
Morgan, George D. 1951. The life history of the bluegill sunfish, Lepomis macrochirus, of Buckeye Lake (Ohio). Denison University Bulletin, Jour, of the Scien tific Laboratories, 42 (4): 21-59.
______. 1954. The life history of the white crappie, Pomoxis annularis of Buckeye Lake, Ohio. Denison University Bulletin, Jour, of the Scientific Laboratories, 43 (6) (7) (8): 113-144. 61
Oliva, O. and V. Sladecek. 1950. Mohou buchanky poskodit poter rybek? ( H a r m s caused by Cyclops on young fishes.) Akvaristicke listy., " 7 1 (1): 13. (In Czech, English summary).
Pennak, R. W. 1946. The dynamics of freshwater plankton populations. Ecol. M o n o g r . , 16: 340-355.
Plew, W. F. and R. W. Pennak. 1949. A seasonal investi gation of the vertical movements of zooplankters in an Indiana lake. Ecology, 30: 93-100.
Roen, U. 1957. Contributions to the biology of some Danish free living freshwater copepods. Biol. Skr. Dan. Vid. Selsk., 9 (2).
Roy, J. 1932. Copepodes et Cladoceres de l'ouest de la France. Recherches biologiques faunistiques sur le plancton d'eau douce des Vallees du Loird et de la Sarthe. Gap.
Ruttner, F. 1953. Fundamentals of Limnology. 2nd (Eng lish) ed. Toronto, Canada.
Schmal'gauzen, I. I. 1958. Preliminary notes on feeding of carnivorous cladocerans, Leptodora kindtii and Bythothrephes. Doklady Akad. Nauk. SSSR, 122 (1-6): 828-830.
Spandl, H. 1926. Copepoda, Ruderfusskrebuse. In Biologie der Tiere Deutschlands. Lief 19- Teil 15, 1-82. van der Werff, A. 1953. Cyclops de vernietiger van jonge vis. (Cyclops, the destroyer of young fish?) Over- druk, Het Aquarium. 24 Jaargang No. 2. Juli, 1953.
1956. Cyclops de vernietiger van jonge vis (II). Overdruk, Het Aquarium. 26 e Jaargang n r 9, Februari, 1956.
Vladimirov, V. I. 1960. (The role of invertebrates in the dynamics of abundance of migratory fishes.) Trans, by P. Narbut. Voprosz. Ikhtiol, 16: 56-66.
1961. (Reproduction of shad and sturgeon under conditions of regulated river flow. Trans, by P. Narbut. Soveshch Ikhtiol. Komissii. Akad. Nauk. SSSR, 13: 277-282.
Wickstead, J. 1959. A predatory copepod. Jour. Anim. Ecology, 28 (1): 69-72. 62
______. 1961. Food and feeding in pelagic copepods. Proc. Zool. Soc., London, 139 (4): 545-55.
Wiebe, A. H. 1930. Investigations on plankton production in fish ponds. Bull. U. S. Bur. Fish., 46; 137- 176.
Yeatman, Harry C. 1959. Gyclopoida, p. 795. In Ward, H. B. and G. G. Whipple, Freshwater Biology edited by W. T. Edmondson. 2nd Edition. John Wiley and Sons, Inc., N. Y. AUTOBIOGRAPHY
I, Michael William Fabian, was born in Mercer County,
Pennsylvania, September 27, 1931. I received my secondary
school education in the public schools of Mercer, Pennsylva
nia, and my undergraduate training at Grove City College,
which granted me the Bachelor of Science degree in 1952.
From the Michigan State University, I received the Master
of Science degree in 1954. While in residence there I held
a half-time teaching assistantship in the Zoology Depart
ment .
I taught in the Cleveland Public Schools for two
years, following which I was an instructor at Arizona State
University for one year and assistant professor at Geneva
College for three years. During two summers I attended the
F. T. Stone Laboratory of The Ohio State University and
subsequently received a National Science Foundation Science
Faculty Fellowship for 15 months. This grant enabled me to
complete language, course, and residence requirements for
the Doctor of Philosophy degree.
I have been an Assistant Professor of biology at
Westminster College, New Wilmington, Pennsylvania since
1961.
63