IBIS 129 159-167 Survival of Mallard ducklings Anus plufyvhynchos and com- petition with fish for invertebrates on a flooded gravel quarry in England DAVID HILL," ROSALIND WRIGHT? & MICHAEL STREET+ "Edward Grey Institute of Field Ornithology, Department of Zoology, South Parks Road, Oxford, OX1 3PS t. The Game Conservancy, A.R.C. Wildfowl Centre, Great Linford, Buckinghamshire Accepted 15 January 1986 The survival of Mallard Anasplatyrhynchos ducklings was studied on a complex of flooded gravel quarries in England and their weight changes in response to invertebrate predation by fish were studied by experiment. The majority of duckling loss occurred during the first 12 days after hatching but those feeding on the river where fish were scarce survived better than those feeding on lakes where fish density was high. Duckling mortality was higher in broods with large home ranges. In the experimental trials an increase in fish density was related to a reduction in both the number of emerging invertebrates and the biomass of aquatic macrophytes. Ducklings feeding in these ponds travelled further and gained less weight than on the pond with a low fish density where the number of emerging invertebrates was higher. Duckling mortality has been shown to be the key factor causing the main fluctuations in autumn mallard populations (Hill 1984a) and is higher on flooded gravel quarries than on more natural lakes and river systems (Hill 1982). Mallard Anas platyrhyn- chos ducklings feed on emerging adult chironomids and their larvae during the first two weeks of life (Lees & Street 1973), the abundance of which varies during spring and summer (Hill 1982). Inter-brood competition for food during the early stages of life has been shown to regulate the spacing of broods (Haland 1983). However, there are marked individual differences in home range size. Recent work has shown that competition with fish for invertebrates could affect brood home ranges (Eriksson 1979, Andersson 1981, McAllister Eadie & Keast 1982, Pehrsson 1984). Fish density could therefore affect duckling survival, particularly on flooded quarries which have been stocked with fish for sport. This paper aims to relate duckling survival to the areas used by broods during the rearing period and investigates the role of fish density on invertebrate abundance and duckling survival by experiment. Study area Mallard broods were censused and radio-tracked and fish density estimates were obtained from a 300 ha complex of 12 flooded gravel quarries adjacent to the upper reaches of the Great Ouse at Great Linford in Buckinghamshire. The lakes vary in age from 1 to 37 years, the majority having been excavated in the last 15 years. Only one lake, the reserve lake, has an area with much vegetation around the shoreline. Generally, the lakes have steep banks and only submerged macrophytes are present. For a more detailed description of habitat refer to Hill (1984b). * Present address: The Wildfowl Centre, Game Conservancy, ARC, Great Linford, Milton Keynes, Buckinghmshire.
Transcript
I B I S 129 159-167
Survival of Mallard ducklings Anus plufyvhynchos and com- petition with fish for invertebrates on a flooded gravel quarry in
England
DAVID H I L L , " R O S A L I N D W R I G H T ? & M I C H A E L S T R E E T + "Edward Grey Institute of Field Ornithology, Department of Zoology, South Parks
Road, Oxford, OX1 3 P S t. The Game Conservancy, A.R.C. Wildfowl Centre, Great Linford,
Buckinghamshire
Accepted 15 January 1986
The survival of Mallard Anasplatyrhynchos ducklings was studied on a complex of flooded gravel quarries in England and their weight changes in response to invertebrate predation by fish were studied by experiment. The majority of duckling loss occurred during the first 12 days after hatching but those feeding on the river where fish were scarce survived better than those feeding on lakes where fish density was high. Duckling mortality was higher in broods with large home ranges. In the experimental trials an increase in fish density was related to a reduction in both the number of emerging invertebrates and the biomass of aquatic macrophytes. Ducklings feeding in these ponds travelled further and gained less weight than on the pond with a low fish density where the number of emerging invertebrates was higher.
Duckling mortality has been shown to be the key factor causing the main fluctuations in autumn mallard populations (Hill 1984a) and is higher on flooded gravel quarries than on more natural lakes and river systems (Hill 1982). Mallard Anas platyrhyn- chos ducklings feed on emerging adult chironomids and their larvae during the first two weeks of life (Lees & Street 1973), the abundance of which varies during spring and summer (Hill 1982). Inter-brood competition for food during the early stages of life has been shown to regulate the spacing of broods (Haland 1983). However, there are marked individual differences in home range size. Recent work has shown that competition with fish for invertebrates could affect brood home ranges (Eriksson 1979, Andersson 1981, McAllister Eadie & Keast 1982, Pehrsson 1984). Fish density could therefore affect duckling survival, particularly on flooded quarries which have been stocked with fish for sport.
This paper aims to relate duckling survival to the areas used by broods during the rearing period and investigates the role of fish density on invertebrate abundance and duckling survival by experiment.
Study area
Mallard broods were censused and radio-tracked and fish density estimates were obtained from a 300 ha complex of 12 flooded gravel quarries adjacent to the upper reaches of the Great Ouse at Great Linford in Buckinghamshire. The lakes vary in age from 1 to 37 years, the majority having been excavated in the last 15 years. Only one lake, the reserve lake, has an area with much vegetation around the shoreline. Generally, the lakes have steep banks and only submerged macrophytes are present. For a more detailed description of habitat refer to Hill (1984b). * Present address: The Wildfowl Centre, Game Conservancy, ARC, Great Linford, Milton Keynes, Buckinghmshire.
I 60 D . H I L L , R . W R I G H T A N D M . STREET
Methods
I B I S 129
Estimates of duckling survival During May and June 1980, 26 broods were located on six flooded quarries within the Great Linford complex and on the river Ouse. Broods were censused once every four days and age was estimated by reference to broods of known age accompanied by marked females. Within-brood mortality, equated with the decline in brood size, was calculated. This assumes that duckling dispersal from individual broods was negligible and that brood mixing did not occur. On no occasion were lone ducklings or broods of mixed ages encountered. Calculations included broods which disap- peared from their normal range and were not seen again, presumed dead. Home range data from radio-tagged broods suggested that they should have been seen if they were still alive.
Radio-tracking Fourteen female Mallard were caught on the nest approximately seven days prior to hatching, determined by calculating mean egg density (Hill 1984c) and radio- transmitters were fitted to them. Two or three radio fixes were obtained daily by triangulation and careful approach at random times during the first 21 days after hatching. The location of individual broods which hatched from the original 14 females was recorded as either on land, less than 1 m from land on water, or more than 1 m from land. In some cases where the brood was not observed their position was taken as the same as that of the female.
Mortality of ducklings was calculated from successive radio-locations of each brood. Mortality is expressed as the percentage of a brood dying per day in order to compare mortality in different sized broods which dies over a different number of days.
Analysis of home range
Data were collected on eight broods giving a total of 482 radio-locations during the summers of 1981 and 1982. Home range was determined by the reciprocal mean distance deviation model developed by Dixon & Chapman (1980). The contour enveloping 95% of fixes was taken as the home range. Where the home range was ‘linear’, for example on a river, the Dixon & Chapman method proved inappropriate, and therefore the maximum polygon area method was used. These data were not used with the census data collected in 1980.
Estimates of fish density
Coarse fish density (kg ha-’) excluding pike Esox lucius was calculated for the river Ouse from two periods of electro-fishing along 128 m of water (8 m wide), by the Anglian Water Authority on 25 July 1984. Fish density within the reserve and for four other lakes within the Linford complex was estimated by mark-recapture techniques over a two-year period, 1983 and 1984. No physical differences occurred in the river and lakes which would have made the estimates unrepresentative of fish densities in 1980 when the duckling census was conducted. Further, we suggest that estimates obtained from mark-recapture were as reliable as possible using this well- tested technique (Youngs & Robson 1978).
I987 D U C K L I N G S U R V I V A L A N D FISH C O M P E T I T I O N 161
Experimental ponds In order to investigate the effect of fish density on insect food supplies for ducklings, and subsequently duckling foraging activity and weight changes, two ponds previously without fish were divided by netting into eight experimental sectors with an average surface area of 340 m2. The ponds had shallow sloping shores with sparse stands of reedmace Typha latifolia and some submerged macrophytic vegetation, mainly Potamogeton pectinatis and Elodea canadensis.
Coarse fish (roach Rutilus rutilus and bream Abramis brama), caught in seine nets in one lake within the Linford complex, were used to stock six of the experimental sectors. These species were chosen because they were the most abundant within the Linford lakes and were consequently likely to have been most representative of the effects on invertebrate numbers. The remaining two experimental sectors contained no fish and acted as the control. Fish stocking densities were classified as low (50 kg ha-'), medium (150 kg ha-') and high (250 kg ha-').
The emergence of Chironomidae and Ephemeroptera was monitored by floating traps containing glass plates, the lower surfaces of which were coated with tree- banding grease. Each trap sampled an area of 0.1 m2 for a period of ten days during 18 June to 28 June 1984. Insect emergence was also monitored in the same way on three other lakes within the Linford complex. Data are presented as the mean number of insects emerging per m2 per day.
Ten sample units of submerged aquatic macrophytes, each of 0.07 m2, were taken from each of the experimental ponds containing the four fish density categories using a standard cylinder sampler in order to investigate change in plant biomass in relation to fish density.
Duckling feeding trials Nine mallard ducklings were fed ad libitum on turkey starter crumbs from the day of hatching. In the mornings during the 10-day period 18 June, when the ducklings were 2 days old, to 28 June 1984, food was withdrawn one hour prior to the experiment. The ducklings were imprinted at hatching onto the observer who weighed them and then placed them within one of the experimental sectors selected at random. The ducklings fed independently of the observer along the waters' edge. They ingested chironomids and Ephemeroptera as these prey items reached the shoreline. The movements of the ducklings were mapped with reference to the shoreline and the total distance travelled during the half hour feeding period was calculated. At the end of the feeding period the ducklings were removed and re- weighed. Weight change was calculated. This procedure was repeated after an initial period of one hour by placing the brood on another experimental sector selected at random. A total of 40 feeding periods was recorded in the four sectors (ten in each sector) over the ten-day period.
All statistical procedures were taken from Sokal & Rohlf (1969).
Duckling survival Brood censusing in 1980 revealed the decline in mean (It s.e.) brood size from 8.7 0.8 to 3.2 f 1.2 during the first 12 days after hatching. Thereafter, there was little change in brood size and mean brood size at fledging was 2.4 f. 1.2 (Fig. 1).
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Figure 1. T h e decline in mean brood size ( rt s.e.) with age of Mallard ducklings on the Reserve study area in 1980, including broods lost entirely.
Table 1. Comparison of Mallard brood density, fish density and duckling survival between the riwer Ouse and reserve at Great Linford in 1980
River Ouse Reserve ___
Broods (n ) 6 20 Brood density (ha-') 0.47 0.22
Duckling survival (yo) 47 10 Fish density (kg ha -') 19.0 249.0
Consequently, the majority of duckling loss occurred during the first 12 days after hatching. Nine of the 20 censused broods were assumed to have lost all ducklings.
Brood density in 1980 on the river was more than twice as high as on the reserve study area and survival to three weeks of age was significantly higher on the river than on the reserve (test of equality of proportions, P < 0.05), (Table 1). Fish density was thirteen times higher in the reserve than in the river Ouse (Table 1).
Movements of radio-tagged broods
Mean ( k s.e.) home range size of eight radio-tagged Mallard broods was 9.9 4.7 ha (Table 2). Six of these eight broods used the adjacent river at some time during the rearing period. Only one brood used the river exclusively. The percentage of a brood
163 I987 D U C K L I N G S U R V I V A L A N D FISH C O M P E T I T I O N
Table 2. Home range size (95% activity contours) of eight radio-tagged Mallard broods in relation to brood mortality
Brood mortality Brood Home range size (ha) (yo dying per day)
1* 2 3 4 5 6* 7 8
0.8 1.2 2.3 4.4 7.1 9.8
12.1 41.5
0.9 6 5 0 0.8 2.8 7.2 9.1
14.3
Note: * Broods which fed on river almost exclusi- vely; Brood 6 , however, travelled 350 m from the nest-site to river, during which time most mortality occurred.
Table 3. Foraging distance of nine Mallard ducklings, their weight change and insect emergence on four experimental ponds stocked with dzyerent densities of coarse fish ( f s.e.)
Low Medium High Control
Fish density (kg ha-') 50 150 250 0 Insect emergence (n m-' day -') 79 f 8 5 6 + 2 5 1 k 3 4 3 + 1 3 Meanforagingdistance(m0.5h-') 22+ 5 5425 6 2 f 9 48+15 Meanweightchange(g0.5 h- ') 0.35k0.12 0.01+0.11 -0.29k0.14 -0.13+0.12 Biomass (dry wt) of aquatic
lost per day during the first 12 days increased with an increase in home range area ( rS(6 , = 0.74, P < 0.05).
Using the method of Neu et al. (1 974) broods were located foraging at the waters' edge ( < 1 m from the shore n = 227) significantly more frequently than would have been expected (Exp = 8) on the basis of the area of that habitat ( P < 0.001). Land tended to be avoided n = 207, Exp = 276; P < 0.01) as did water further than 1 m from shore (n = 48, Exp = 198; P < 0.001).
Effect of fish density on insect abundance to ducklings Emergence of chironomids and Ephemeroptera in the experimental sectors with the lowest density of fish was higher than that from the other sectors ( P < 0.05) (Table 3). However, the control sectors which contained no fish had the lowest insect emergence.
In a between-site analysis incorporating data from the experimental ponds and those for four sites within the quarry complex, the numbers of emerging chironomids and Ephemeroptera were inversely correlated with fish density (Y = - 0.81, slope = - 0.71, P < 0.01).
D . H I L L , R . W R I G H T A N D M . STREET
280-
240
- c - f 200- I )I 0 73
N
'E 160 I
a, C
g 120- E
I B I S 1 2 9
0 0
- 0
- 0
0
0
0
0
0 0
I I I 1 I I 0 50 100 I50 200 250 300
Flsh densty (kg ha- ' )
Figure 2. T h e emergence of Chironomidae and Ephemeroptera in relation to fish density from six experimental ponds stocked with fish, one control pond (open symbol) and four lakes within the Llnford cnmplex of flooded gravel quarries.
The distance over which the ducklings foraged in the feeding trials was significantly different in the sectors stocked with fish (one-way analysis of variance, F(z.12) = 9.94, P < 0.01). Ducklings travelled further during the feeding bout on the sector stocked at high fish density than on the sectors stocked at low fish density ( t g = 4.1, P < 0.01, Table 3).
Weight gains of ducklings were significantly different between sectors (one-way analysis of variance, F(3,130)= 6.32, P < 0.01, Table 3). Weight gains were in accordance with fish density and mean foraging distance. Whilst feeding in the low fish density sectors ducklings gained more weight than whilst feeding in any other sector. Ducklings actually lost weight in the high fish density sectors and in the controls with no fish, although insect emergence was also lower here.
The biomass of aquatic macrophytes was significantly higher in the sectors with low fish density than in the sectors with high fish density (one-way analysis of variance, F(3,36) = 13.5, P < 0.01, Table 3). The controls contained the highest biomass of aquatic macrophytes.
Discussion
The majority of ducklings which disappear do so during the first 12 days after hatching, which corresponds to the time when they are most dependent on aquatic invertebrates for food (Chura 1961, Lees & Street 1973, Bengtson 1975, Street 1977). Starvation, chilling and predation have been implicated as the main causes of loss (Solman 1945, Koskimies & Lahti 1964, Hill 1982, Talent et al. 1983), although the role of pike predation at Linford has yet to be studied.
I987 DtJCICLINCI S U R V I V A L A K D F I S H C O M P E T I T I O N 165
Those broods which had large home ranges suffered heavier mortality than those with smaller ones, and movements could have been in response to food shortage. Talent et al. (1982) found the average size of the cumulative home range ofa Mallard brood to be 11 ha, similar to that in this study of nearly 13 ha. This similarity is interesting considering the radically different types of hab.tat in the two studies. Talent et al. (1982) studied brood home ranges in prairie habitat containing mainly temporary and seasonal wetlands, in contrast to the relatively immature flooded gravel quarry in this study.
When food is in short supply, competition from other predators that prey on the same invertebrate species is likely to increase unless the r,dmbers of competitors decline as a consequence. This competition will come from other broods or from coarse fish. Haland (1983) found that interactions between mallard broods were infrequent, however, and brood spacing was brought about by mutual avoidance. Brood density has been shown to increase in response to an increase in food supplies (Godin & Joyner 1981, Talent et al. 1982) and is higher on lakes with a large amount of shoreline in relation to water area (Patterson 1976, Mack & Flake 1980), since shoreline is the main feeding habitat of Mallard ducklings (Pehrsson 1979). In the United States ponds experimentally sprayed with a pesticide used to control spruce budworm Choristoneura fumiferana produced significantly fewer invertebrates, which had dramatic effects on the survival of Black Duck Anus rubrzpes and hlallard ducklings (Hunter et ul. 1984). Ducklings on sprayed ponds spent more time searching for food and less time resting, and their rate of movement around ponds was greater than for ducklings on unsprayed ponds.
Some coarse fish species eat similar invertebrates to Mallard (Eriksson 1979, Pehrsson 1984) and Goldeneye Bucephala clangula ducklings (McAllister Eadie & Keast 1982) and fish removal can result in an increase of certain benthic macroinvertebrates such as some crustacea and members of the family Chironomi- dae (Eriksson 1979, Gilinsky 1984, Morin 1984).
Broods in the present study occurred at twice the density on the river Ouse, which had fewer fish than on the reserve, where fish density was much higher. Eriksson (1979) showed that broods preferred lakes without fish and that their use of an experimental lake increased after fish were removed. Pehrsson (1 979) showed that the highest rate of food intake in Mallard ducklings occurred on lakes where fish had been removed, and further, by experiment Pehrsson (1984) showed that ducklings obtained more food in lakes where fish had been removed. T h e experiment in the present study showed that high fish density reduced the abundance of emerging chironomids and Ephemeroptera. Ducklings foraged over a greater distance where food was less abundant and, in the high density fish pond, lost weight. 'I'hey were able to gain weight in the pond with a low fish stocking density from which the most invertebrates emerged. Hunter et al. (1984) showed a similar disparity in weight change between broods feeding on ponds sprayed with a pesticide which reduced invertebrate food, and broods feeding on ponds which had not been sprayed.
There was less submerged macrophytic vegetation in the sectors with high tish density, and most in those with no fish. Consequently, the effect of fish density could operate in three inter-related ways. First, the fish appear to physically break up macrophytic vegetation when they are at a high density, simply by swimming amongst it (King & Hunt 1967). Second, some of the vegetation is eaten by the fish. Third, because bream feed to a large extent on the bottom of the ponds, the water becomes clouded which reduces light penetration and subsequently the growth of macropytic vegetation. T h e reduction in submerged vegetation therefore probably results in a decline in colonization by invertebrates.
T h e control pond, however, gave a particularly interesting result. With no fish at
I 66 D . H I L L , R. W R I G H T A N D M . STREET I B I S 129
all insect emergence was the lowest of the four categories. We suggest that one plausible explanation could be that fish also remove predators of chironomids such as diving beetle larvae Dytiscus marginalis and members of the Corixidae. In the absence of fish these predators have been shown to reduce the numbers of other macroinvertebrates such as chironomids (Benke, 1978, Thorp & Bergey 1981). This clearly requires further investigation at Linford.
The implications of this study are that fish predation can affect the biomass of invertebrates available to Mallard ducklings and this needs consideration where the two are managed together. Mallard ducklings are seasonal residents and require invertebrates at a specific time of the year for a relatively short period. To some extent some inter-specific competition with fish is avoided, because ducklings feed mainly at the water’s edge, but they also feed on invertebrates washed ashore which have emerged from areas where they are more susceptible to fish predation. There is as yet no experimental evidence to suggest that duckling survival is affected by fish density. The role of fish predation on macroinvertebrates is complex (Gilinsky 1984) and further controlled experiments are needed to determine the effects of competi- tion with ducklings.
This work was carried out as part of a research studentship awarded to the senior author by the Amey Roadstone Corporation, a member of the Gold Fields Group of Companies. We wish to thank N. Gray for logistical help, Robert Kenward for radio transmitters and Steve Tapper for help with home range analysis programs. Chris Perrins and Dick Potts supervised the project and, with Alan Allison and Myrfyn Owen, commented on earlier drafts.
References
ANDERSON, G. 1981. Influence of fish on waterfowl and lakes. Anser 20: 21-34. HENGTSON, S-A. 1975. Food of ducklings of surface feeding ducks at Lake Myvatn, Iceland. Ornis Fenn.
BENKE, A.C. 1978. Interactions among coexisting predators-a field experiment with dragonfly larvae. J .
CHURA, N.J. 1961. Food availability and preferences of juvenile Mallards. Trans. 26th N. Am. Wildl.
DIXON, K.R. &CHAPMAN, J.A. 1980. Harmonic mean measureofanimal activity. Ecology 61: 1040-1044. ERIKSSON, M. 1979. Competition between freshwater fish and Goldeneyes Bucephala clongula for
common prey. Oecologia 41: 99-107. GILINSKY, E. 1984. The role of fish predation and spatial heterogeneity in determining benthic
community structure. Ecology 65: 455468. GODIN, P.R. & JOYNER, D.E. 1981. Pond ecology and its influence on mallard use in Ontario, Canada.
Wildfowl 32: 28-34. HAI.AND, A. 1983. Home range and spacing in Mallard Anas platyrhynchos broods. Ornis Scand. 14: 24-
35. HILL, D.A. 1982. The comparative population ecology of Mallard and Tufted Duck. Unpubl. D.Phi1.
thesis. University of Oxford. HILL, D.A. 1984a. Population regulation in the Mallard (AnasplatyrhynchosL.). J . Anim. Ecol. 53: 191-
202. HILL, D.A. 1984b. Factors affecting nest success in the Mallard and Tufted Duck. Ornis Scand. 15: 11 5-
122. HILL, D.A. 1984~ . Laying date, clutch size and egg size in the Mallard and Tufted Duck. Ibis 126: 48+
495. HUNTER, M.L., WITHAM, J.W. & DOW, H. 1984. Effects of a carbaryl induced depression in invertebrate
abundance on the growth and behaviour of American Black Duck and Mallard ducklings. Can. J . Zool. 62: 452-456.
52: 1 4 .
Anim. Ecol. 47: 335-350.
Conf. 2: 121-134.
1987 D U C K L I N G S U R V I V A L A N D F I S H C O M P E T I T I O N ' 6 7 KING, D.S. & HUNT, G.S. 1967. Effect of carp on vegetation in a Lake Erie marsh. J . Wildl. Manage. 3 I :
KOSKIMIES, J . & LAHTI, L. 1964. Cold-hardiness o f the newly haxhed young in relation to ccology and distribution in ten species of European ducks. Auk 81: 281-307.
LEES, P. & STREET, M. 1973. The feeding ecology of young Mallard and Tufted Duck in wet gravel quarries. I n Population ecology of game species. IV Proc. Int. Union of Game biologists Conf. Stockholm.
MCAILISTER EADIE, J. & KEAST, A. 1982. Do Goldeneye and Perch compete for food? Oectilogia 55: 225- 230.
MACK, G.D. & FLAKE, L.D. 1980. Habitat relationships of waterfowl broods on South Dakota stock ponds. J . Wildl. Manage. 44: 695-700.
MORIN, P.J. 1984. The impact of fish exclusion on the abundance and species composition of larval odonates: results of short term experiments in a north Carolina farm pond. Ecology
NEW, C.W., BYERS, C.R. & PEEK, J.M. 1974. A technique for analysis of utilisation-avail Wildl. Manage. 38: 541-545.
PATTERSON, J.H. 1976. The role of environmental heterogeneity in the regulation of duck populations. J . Wildl. Manage. 40: 22-32.
PEHRSSON, 0. 1979. Feeding behaviour, feeding habitat utilisation and feeding efficiency of Mallard ducklings (Anus plutyrhynchos) as guided by a domestic duck. Viltrevy 10: 193-218.
PEHRSSON, 0. 1984. Relationship of food to spatial and temporal breeding strategies of Mallards in Sweden. J. Wildl. Manage. 48: 322-339.
SOKAI., R.R. & ROHLF, F.J. 1969. Biometry. Pub. Freeman & Co. San Fransisco. pp 776. SOLMAN, V.E.F. 1945. The ecological relations of pile (Esox lucius L.) and waterfowl. Ecology 26: 157-
170. STREET, M. 1977. The food of Mallard ducklings in a wet gravel quarry and its relation to duckling
survival. Wildfowl 28: 113-125. TALENT, L.G., JARVIS, R.L. & KRAPU, G.L. 1983. Survival of Mallard broods in South-Central North
Dakota. Condor 85: 74-78. TALENT, L.G., KRAPU, G.L. 7 JARVIS, R.L. 1982. Habitat use by Mallard broods in South-Central North
Dakota. J . Wildl. Manage. 46: 629-635. THORP, J.H. & BERGEY, E.A. 1981. Field experiments on responses of a freshwater benthic macroinverte-
brate community to vertebrate predators. Ecology 62: 365-375. YOUNGS, W.D. & ROBSON, D.S. 1978. Estimation of population number and mortality rates. I n Bagenal,
T.B. (ed.), Methods for assessment of fish production in freshwaters. 3rd edition: 137-164. IHP Handbook No. 3. Oxford: Blackwell Scientific Publications.
