Pike predation affects breeding success and habitatselection of ducks

LISA DESSBORN* , †, JOHAN ELMBERG* AND GORAN ENGLUND ‡

*Aquatic Biology and Chemistry, Kristianstad University, Kristianstad, Sweden

†Department of Wildlife, Fish, and Environmental Studies, Swedish University of Agricultural Sciences, Umea, Sweden‡Department of Ecology and Environmental Science, Umea University, Umea, Sweden

SUMMARY

1. Fish and ducks often belong to the same local food web, and several studies indicate that

there is a general negative effect of fish on breeding ducks. This pattern has so far been

addressed mainly within the framework of competition for common invertebrate prey,

while predation by large fish as a force behind settlement and abundance patterns in ducks

remains largely unknown. This is the first study to address the effect of fish predation on

breeding ducks, isolated from that of competition, and the first experiment to explore the

ability of ducks to identify and avoid lakes with high risk of fish predation.

2. We used a before–after control–impact design and 11 naturally fishless lakes. Waterfowl

on the lakes were surveyed during the breeding season of 2005. Large adult pike (Esox

lucius) were added to two lakes in early spring 2008, and waterfowl surveys were repeated

on all 11 lakes.

3. Pike introduction did not affect the number of pairs on lakes during the nesting season

in any of three focal duck species (mallard Anas platyrhynchos, teal Anas crecca, and

goldeneye Bucephala clangula). During the brood-rearing season, however, there was a

decrease in duck days in teal and goldeneye in lakes with pike, with similar trends

observed in mallard. The number of goldeneye ducklings was also significantly lower in

lakes with pike. We were unable to determine whether the response was attributable to

direct pike predation or to broods leaving experimental lakes, but in either case, our study

demonstrates high fitness costs for ducks breeding on lakes with pike.

4. The apparent inability of nesting ducks to detect pike and the clear fitness implications

may influence the annual recruitment of ducks on a larger scale as pike are both common

and widespread. Vegetation complexity and food abundance are likely to be of overriding

importance when breeding ducks are choosing a nesting site. As pike have a strong

influence on breeding birds, relying on vegetation and cues of food abundance, while

ignoring indicators of predation risk from fish, could lead to lakes with pike acting as an

ecological trap.

Keywords: duck, ecological trap, non-adaptive response, pike, predator avoidance

Introduction

Trophic interactions in freshwaters are often complex

and variable in time and space. Two important groups

in these systems – fish and waterbirds – are of great

value in terms of biodiversity, recreation and economic

revenue. They also include several species that have

become models in ecological research. In many cases, it

is evident that co-occurring fish and ducks are part of

the same food web. Fish can have a strong impact on

aquatic invertebrates and, in fish removal experiments,

Phillip & Wright (1993) and Giles (1994) showed the

Correspondence: Lisa Dessborn, Aquatic Biology and Chemis-

try, Kristianstad University, SE-291 88 Kristianstad, Sweden.

E-mail: lisa.dessborn@hkr.se

Freshwater Biology (2011) 56, 579–589 doi:10.1111/j.1365-2427.2010.02525.x

� 2010 Blackwell Publishing Ltd 579

negative impacts of fish on invertebrate numbers.

Overlapping feeding niches of fish and ducks, partic-

ularly during the breeding season of the birds, have

prompted a number of studies relating lake choice and

breeding success of ducks to the presence and density

of fish (e.g. Andersson, 1981; Eadie & Keast, 1982;

DesGranges & Rodrigue, 1986; Bouffard & Hanson,

1997). Others have investigated the effects of competi-

tion from fish on feeding and growth in ducklings

(Pehrsson, 1984; Hunter et al., 1986; Phillip & Wright,

1993) In general, the presence of fish correlates nega-

tively with the breeding success of ducks (though see

Paszkowski & Tonn, 2000). Several studies suggest that

fish are superior competitors for common invertebrate

prey, although the study designs used thus far gener-

ally do not exclude other interactions. For example, fish

can affect turbidity and macrophyte cover, which in

turn may affect foraging conditions for ducks (Bouffard

& Hanson, 1997; Ward & Newman, 2006; Hansson

et al., 2010). Although many fish species represent no

direct threat to ducklings, some large predators such as

pike may also affect ducks directly by taking both

ducklings and adults (Solman, 1945; Lagler, 1956;

Pehrsson, 1977).

The relative importance of competition and preda-

tion in duck–fish interactions is difficult to quantify in

descriptive studies. Even in lakes where the only fish

species is a potential predator on birds, there may be a

combination of competition and predation affecting

ducks, as some fish go through ontogenetic niche

shifts from invertivore (potentially competing with

ducks) to taking larger prey (Nordberg, 1971). The

predation aspect has not been investigated in previous

duck–fish studies (e.g. Eriksson, 1983) although there

were two early attempts to quantify its impact by

analysing the gut contents of large predatory fish

(Solman, 1945; Lagler, 1956). Both studies suggested

that fish predation may reduce annual recruitment in

duck populations. In addition, Solman (1945) found

evidence that fish predation is size-dependent, affect-

ing small ducks (both ducklings and small-bodied

species overall) rather than large ones, and that diving

ducks are more susceptible than dabbling ducks

because of their foraging habits.

Goldeneye (Bucephala clangula L.), Eurasian teal

(Anas crecca L., hereafter ‘teal’), and mallard (Anas

platyrhynchos L.) are widespread and common ducks,

whose life histories and general ecology are well

documented. Food abundance, terrestrial predators,

and inclement weather are known to affect duckling

survival in these species (e.g. Seargeant & Raveling,

1992; Gunnarsson et al., 2004; Chouinard & Arnold,

2007), although the impact of fish competition and

predation on the distribution and abundance of

waterbirds is still poorly understood. For example,

many lakes and wetlands within the very large

geographical ranges of these species do not support

broods despite their supposed suitability as nesting

habitats (Toft, Trauger & Murdy, 1982; Gunnarsson

et al., 2004). An unexplored hypothesis to explain this

pattern of ‘many empty lakes’ is the presence of fish.

Predatory fish, such as pike (Esox lucius L.) and its

congeners, are widespread within the ranges of these

ducks and known to feed on ducklings and occasion-

ally even on adult teal (Solman, 1945; Pehrsson, 1977).

The extensive distribution of pike reflects its ability to

colonise and use a wide range of aquatic systems. It has

also been stocked in many previously fishless systems

(Filipsson, 1994). Accordingly, introduction of pike

may reduce habitat suitability for breeding ducks by

increasing predation risk and, possibly, competition

with ducklings for invertebrate prey (Beaudoin et al.,

1999). If pike have strong negative effects on the fitness

of breeding ducks, natural selection should favour the

ability to detect and avoid such lakes.

Our aims were to find out whether ducks avoid

breeding on lakes with large pike and, if not, to assess

the impact of large pike on duck breeding success.

There is no previous study in which the direct effect of

fish predation has been isolated from competition, nor

any experimental study exploring the ability of ducks

to identify and avoid lakes with high risk of fish

predation (cf. Paasivaara & Poysa, 2004). We intro-

duced large pike to naturally fishless lakes, using a

before–after control–impact design, thereby avoiding

the confounding effects of young fish as competitors

with ducks. We predicted that i) the number of

nesting pairs of mallard, teal, and goldeneye should

be lower after pike introduction and ii) duckling

production in these ducks should be lower after pike

introduction.

Methods

Study system

The study was carried out in the coastal lowland

along the Baltic Sea in north-central Sweden at 63�30¢–

580 L. Dessborn et al.

� 2010 Blackwell Publishing Ltd, Freshwater Biology, 56, 579–589

64�30¢N (Angermanland and Vasterbotten provinces),

within the middle boreal biotic zone (Ahti, Hamet-

Ahti & Jalas, 1968). Most local lakes are oligo- to

mesotrophic and surrounded by mixed coniferous

forest and bogs. Eutrophic lakes are sparse and found

in river valleys and near the sea. The majority of lakes

in the study region have permanent fish populations,

but some lakes without connecting streams are natu-

rally fishless because of winterkill during near-anoxic

conditions under the ice, episodes of low pH, or the

combination of small area and isolation (Ohman et al.,

2006; Englund et al., 2009). Fish species composition of

235 of the �400 lakes in the region was known prior to

this study (G. Englund, unpubl. data). The three most

common fish species assemblages are pike + perch

(Perca fluviatilis L.), perch only, and pike + per-

ch + roach (Rutilus rutilus L.). Lakes where pike is

the only species are rare on a regional scale (a few per

cent), but the frequency can be as high as 30% in

samples of lakes apparently suitable for ducks (i.e.

small and shallow coastal lakes; G. Englund, unpubl.

data). In such lakes, the diet of pike is dominated by

invertebrates and small pike (Beaudoin et al., 1999).

Experimental design and lake selection

Within an area of roughly 500 km2 and comprising of

many catchments, we selected 11 naturally fishless

lakes that had been part of a previous descriptive

study (Elmberg, Dessborn & Englund, 2010). A

before–after control–impact design was used to quan-

tify the effects of pike on the occurrence and breeding

success of ducks. In short, all study lakes were

surveyed in 2005, adult pike were introduced to two

of the lakes in early May 2008, and ducks were

subsequently surveyed with the same methods at all

lakes during the 2008 breeding season. The strongly

unbalanced design (nine control and two manipulated

lakes) was necessitated by practical difficulties asso-

ciated with the pike introduction.

To minimise variation among lakes not related to

pike predation, we used the following criteria to select

study sites: (i) small to moderate surface area (1.1–

3.9 hectares; Table 1), (ii) circular, rectangular, or

oblong outline to facilitate waterfowl surveys and to

reduce variation in shoreline length ⁄surface area ratio,

(iii) an easily accessible vantage point from which

ducks could be surveyed without disturbing them,

and (iv) regional extremes in trophic status were Ta

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Pike predation on breeding ducks 581

� 2010 Blackwell Publishing Ltd, Freshwater Biology, 56, 579–589

avoided. Human disturbance was low during the

study period (traffic, boating, hiking, fishing, etc.),

and the distance between study lakes was sufficient

(3–15 km; mean = 8.5 km) to treat them as indepen-

dent units in terms of waterfowl use. All lakes were in

different catchments or lacked a permanent stream

connection.

Environmental variables

Maximum lake depth was measured with an echoso-

under (Eagle Cuda 168; http://eaglenav.com/), while

pH was measured with a portable pH meter (Mettler

Toledo 1120; http://us.mt.com/us/en/home.html) on

one occasion between mid-June to mid-August. Water

samples for analyses of total phosphorus concentration

were taken in mid-June. Lake area and altitude were

estimated from digital topographical maps.

Baseline fish data

Each lake was surveyed twice (in the period May and

August) between 1999 and 2005 as a part of a study by

Elmberg et al. (2010). On each occasion, we used

minnow traps, spinning, and two to four multi-mesh

gill nets (bottom set, monofilament nylon and mea-

sured 30 · 1.5 m with mesh size ranging between 5

and 55 mm knot to knot) (Appelberg, 2000). All lakes

were also surveyed by electrofishing and small det-

onations. Electrofishing was performed for 45 min

from a boat in the shallow water close to the shore

(depth 0.1–0.5 m). Ten small detonators were used in

each lake, each stunning fish within a radius of about

2 m. As an additional validation of fishless conditions,

we interviewed local fishermen and, once before and

once after the waterfowl baseline survey in 2005,

walked the perimeter of all study lakes searching the

littoral zone for fish. These final procedures confirmed

the fishless status of lakes presumed to be fish free

with the one exception of Lake Trekanten (Table 1),

where we observed a perch from the shore. Therefore,

this lake was subjected to a renewed fish survey after

the waterbird survey in 2005. Despite intensive efforts

with gill nets, minnow traps, electrofishing and by

spinning, and further visual observations around the

lake, no further fish were caught or seen. Hence, we

consider Trekanten effectively fishless during this

study.

Waterbird data

All study lakes freeze over in winter, and water-

birds breeding on them are long-distance migrants

(Fransson & Pettersson, 2001). To quantify lake use

and breeding success of ducks, we carried out seven

surveys between May and the third week of July in

each of the years 2005 and 2008. The first three

surveys in each year were conducted before most

eggs hatched, and these were collectively termed

‘pair surveys’. These data were used to test the

prediction about the number of pairs settling to

breed. The last of these first three surveys was

carried out in the second week of June. The

remaining four surveys (starting in the third week

of June) covered the period when the vast majority

of duck broods hatch and were collectively termed

‘brood surveys’. Data from these surveys were used

to test predictions about breeding success. On each

visit to a lake, we did two point count scans (the

entire lake is slowly scanned twice from a fixed

location) separated by a period of still observation

(all observed moving birds are noted: 20 min in pair

surveys, 30 min in brood surveys; Koskimies &

Vaisanen, 1991). Considerable effort was made not

to alarm the birds during surveys. To further reduce

the chance of missing broods hiding or resting in

shoreline vegetation, we covered the entire perim-

eter of each lake by foot after the standard count

procedure on the third pair survey and the fourth

brood survey. Most surveys were conducted in the

morning or in the evening, when duckling foraging

normally peaks (Ringelman & Flake, 1980). In the

case of bad weather (very windy, cold, or rainy

weather dramatically reduces foraging activity and

detection probability of ducklings), surveys were

postponed until the weather improved.

We recorded all waterbirds on each lake on each

visit and used all observations of mallard, teal, and

goldeneye for our analyses. We expressed lake use as

‘[species name] days’, which is the daily number of

individuals of that species seen on a lake summed

over both pair and brood surveys. These numbers

include adults and ducklings, as well as groups of

adult birds that may or may not be local breeders.

Thus, a female duck with six ducklings produced

‘7[species name] days’ for a certain lake on each of the

days they were observed there.

582 L. Dessborn et al.

� 2010 Blackwell Publishing Ltd, Freshwater Biology, 56, 579–589

Pike introduction

Pike were caught in a nearby estuary in the beginning

of May 2008 when they were coming to shallow

waters to spawn. They were trapped in nets, and

males with a body length of 60–72 cm were used for

the experiment. Using pike of only one sex assured

that the experimental lakes would return to a fishless

state after the experiment, and males were chosen

simply because they were more numerous. Trapped

pike were transported in fish tanks to the experimen-

tal lakes, where 22 individuals were introduced into

Lake Bussjon ⁄ Innertavle and 25 to Lake Oviken.

Measured as pike biomass ha)1, we introduced

15 kg ha)1, which is higher than the density reported

by Rask & Arvola (1985) (4 kg ha)1) but lower than

that reported by Venturelli & Tonn (2005) (35 kg

ha)1).

To confirm that our introduction treatment was

successful, we used nets to recapture introduced pike

in autumn 2008. In Bussjon ⁄ Innertavle, we recaptured

12 out of 22 pike and before that one individual had

been seen to be taken by a white-tailed eagle (Haliae-

etus albicilla L.) during the first brood survey. Unex-

pected early winter conditions prevented efficient

recapture fishing in Oviken, and only five out of 25

individuals were caught in 2008. However, at ice-out

in early May 2009, we found three dead individuals

that had probably been killed by low oxygen concen-

tration during the winter. Thus, we conclude that the

operational pike density was 3.6–6.7 individuals ha)1

in Bussjon ⁄ Innertavle and 2.2–6.7 ha)1 in Oviken. The

pike caught in the autumn were in good condition

and most of them contained a large quantity of

invertebrates.

Statistics

The effect of pike introduction was analysed with a

repeated-measures ANOVAANOVA using the difference in the

number of duck days in 2005 to 2008 as response

variable, and the pike treatment and the time period

of sampling (pair surveys versus brood surveys) as

factors.

A similar analysis was performed using the change

in the number of duckling days as response variable.

However, because ducklings occurred only during the

last pair count and the brood period, we used

sampling occasion (p3, b1–b4) as within-subject factor.

Results

Goldeneye was the most abundant and frequent of the

three duck species, being observed on all 11 lakes

(Table 1). Teal was the second most ubiquitous

species and was missing in only one lake (Buss-

jon ⁄ Taftebole). Mallard was the least abundant spe-

cies and was absent in two lakes (Middagstjarnen and

Orrmyrtjarnen). The number of adults and ducklings

varied greatly between lakes (Table 1). Some lakes

supported all three species in both years, including

broods of two or three species (e.g. Holmsjon).

However, Middagstjarnen did not apparently support

ducklings of any species on any visit in either year.

Such a general pattern of uneven occurrence and

‘empty lakes’ is common for breeding waterfowl in

temperate zone landscapes with an abundance of

lakes and ponds (cf. Eriksson, 1983; Gunnarsson et al.,

2004; Elmberg et al., 2005).

The main effect of pike was significant for both

goldeneye and teal, the number of duck days being

lower in lakes to which pike had been added (Table 2;

Fig. 1a,b). There were also significant interaction

effects between pike presence and sampling period

for both species, indicating that the negative effects of

pike were more pronounced during the brood survey

period (Table 2; Fig. 1a,b). To study this pattern in

more detail, we tested the effect of pike introduction

Table 2 Result of a repeated-measures A N O V AA N O V A on the change in

number of duck days per lake from 2005 to 2008. ‘Treatment’

refers to the addition of pike (control lakes versus experimental

lakes) and ‘time’ to the annual sampling period (pair surveys

versus brood surveys)

Source df MS F P

Goldeneye

Between subjects

Treatment 1 1893.8 49.3 <0.001

Error 9 38.8

Within subjects

Time 1 989.6 23.6 <0.001

Time · treat 1 1258.7 30.1 <0.001

Error 9 41.9

Teal

Between subjects

Treatment 1 861.2 5.8 <0.05

Error 9 102.2

Within subjects

Time 1 861.2 13.9 <0.005

Time · treat 1 921.2 14.9 <0.005

Error 9 61.8

Pike predation on breeding ducks 583

� 2010 Blackwell Publishing Ltd, Freshwater Biology, 56, 579–589

for each sampling period. For both duck species, we

found a significant effect for the brood period (t-test:

goldeneye, t = 7.31, P < 0.001; teal, t = 3.09, P < 0.05),

but not for the pair period (t-test: t < 1.53, P > 0.05 for

both species). The significant main effects of survey

period in the models simply reflect a larger decrease

in duck days from 2005 to 2008 in the treatment lakes

for the brood survey period when compared to the

pair survey period (Table 2; Fig. 1a,b). The number of

lakes with mallard observations was too small for a

satisfactory statistical analysis (Fig. 1c).

Pike introduction also had an impact on the number

of goldeneye duckling days measured as the differ-

ences between 2005 and 2008 (effect of treatment in

Table 3; Fig. 2a), and the strength of this effect

increased over the season, as indicated by a significant

treatment · count interaction (Table 3, Fig. 2a). The

number of lakes with teal and mallard ducklings was

too small for statistical analysis. Both the number of

broods and ducklings appear to be affected by pike

introductions (Fig. 3), and only one goldeneye brood

was observed after the last pair count in 2008. It may

be noted that no mallard broods were observed on

experimental lakes after pike introduction.

Discussion

Contrary to our first prediction, we did not find an

effect of pike introduction on lake use in any of the

three duck species during the pair surveys, which

would have been expected if ducks are able to

recognise the danger and thereby avoid fish predation

on ducklings. In contrast, other aquatic vertebrates,

such as fish and amphibians, have elaborate adapta-

tions that reduce fish predation, including reduced

activity, increased shoaling behaviour, and altered

morphology in response to the presence of predatory

fish (Power, 1984; Bronmark & Miner, 1992; Kats &

Pair survey Brood survey–50

–40

–30

–20

–10

0

10

20

30

Lake

-leve

l diff

eren

ces

in u

tilis

atio

n(d

uck

days

)

Goldeneye

Pair survey Brood survey–50

–40

–30

–20

–10

0

10

20

30 Teal

Pair survey Brood survey–50

–40

–30

–20

–10

0

10

20

30 Mallard

(a)

(b)

(c)

Fig. 1 Each symbol represents the lake-level difference in use

between 2005 and 2008 (sum of duck days) by (a) goldeneye, (b)

teal, and (c) mallard for pair surveys and brood surveys, respec-

tively. Experimental lakes are in black and control lakes in white.

Table 3 Result of a repeated-measures A N O V AA N O V A on the change in

number of duckling days per lake from 2005 to 2008. ‘Treatment’

refers to the addition of pike (control lakes versus experimental

lakes) and ‘count’ to the different occasions when ducklings

were counted (p3, b1–b4)

Source df MS F P

Goldeneye

Between subjects

Treatment 1 315.6 24.5 <0.001

Error 9 12.8

Within subjects

Count 1 66.6 5.5 <0.05

Count · treat 1 136.0 11.2 <0.01

Error 9 12.2

584 L. Dessborn et al.

� 2010 Blackwell Publishing Ltd, Freshwater Biology, 56, 579–589

Sih, 1992; Brown, 2003). Both visual and olfactory cues

may elicit such responses (Brown, Chivers & Smith,

1997; Ferrari, Messier & Chivers, 2006; Johansson &

Andersson, 2009).

During the main brood-rearing period (brood sur-

veys), however, lake use in goldeneye and teal

decreased significantly after the introduction of pike.

The effect was most evident in goldeneye, probably a

consequence of more encounters with pike, as the

ducklings forage under water (Cramp et al., 2006).

Solman (1945) demonstrated that diving ducks were

more frequent in pike guts than dabbling ducks.

Although our mallard sample was limited, our data

indicate that this species may also be negatively

affected by pike during the brood-rearing period.

A few broods of goldeneye and teal were observed

on experimental lakes in 2008, but many fewer than in

the control year (Fig. 3), and the number of duckling

–15

–10

–5

0

5

10

15La

ke-le

vel d

iffer

ence

s in

util

isat

ion

(duc

klin

gs d

ays)

–15

–10

–5

0

5

10

15

Lake

-leve

l diff

eren

ces

in u

tlisa

tion

(duc

klin

g da

ys)

–15

–10

–5

0

5

10

15(c)

(b)

(a)

p3 b1 b2 b3 b4

p3 b1 b2 b3 b4

p3 b1 b2 b3 b4

Lake

-leve

l diff

eren

ces

in u

tilis

atio

n(d

uckl

ing

days

)Goldeneye

Teal

Mallard

Fig. 2 Each symbol represents the lake-level difference in use

between 2005 and 2008 (sum of duckling days) for (a) goldeneye,

(b) teal, and (c) mallard for the last pair survey (p3) and the

brood surveys (b1–b4), respectively. Experimental lakes are in

black and control lakes in white.

0

5

10

15

Duc

klin

gs in

bro

od

Goldeneye

Teal

Mallard

2005

0

5

10

15

p3 b1 b2 b3 b4

p3 b1 b2 b3 b4

Duc

klin

gs in

bro

od Goldeneye

Teal

2008

(a)

(b)

Fig. 3 Size of individual broods on experimental lakes (N = 2) in

(a) the control year (2005) and (b) the experimental year (2008;

pike introduction). Lines connect observations that are highly

likely to be the same brood (based on age and duckling number).

Broods that were observed only once are illustrated with a single

dot. ‘p3’ = pair survey three; ‘b1–b4’ are brood surveys one

through four.

Pike predation on breeding ducks 585

� 2010 Blackwell Publishing Ltd, Freshwater Biology, 56, 579–589

days was significantly reduced by the pike treatment

for goldeneye (Table 3). The experimental design

imposes limitations on establishing causes of the

observed effects on duck days and duckling days;

thus, we cannot tell whether lower brood use and

smaller broods after pike introduction were attribut-

able to pike predation per se or whether broods

abandoned experimental lakes after finally detecting

pike or experiencing pike attacks. If the reduction in

duck days is a direct result of pike predation, it clearly

has an effect on breeding success although overland

travel to change lakes would also probably lower

duckling survival (Ball et al., 1975). Moreover, females

have no guarantee of finding a better brood-rearing

lake, since most lakes in the area hold pike.

We argue that our second prediction was generally

met; lake use and duckling production were nega-

tively affected by pike. We demonstrated high fitness

costs for ducks breeding on lakes with pike. How-

ever, the three duck species did not show the

expected adaptive response of avoiding nesting on

lakes with introduced pike. We see three possible

explanations for this counter-intuitive result: (i)

ducks do not detect pike presence, (ii) ducks detect

the presence of pike, but nevertheless use the lake

because it provides good nesting habitat, and take the

brood to another lake directly after hatching or only

after a pike attack has occurred, and (iii) ducks detect

pike but nest at the lake and remain there with the

brood because the other qualities of the lake out-

weigh predation risk.

It seems extremely unlikely that female ducks

would not be able to detect pike, as it usually takes

very little time even for a human observer to see

(breeding) pike in spring when waterbirds scout

potential breeding sites. Ducks may not, however,

identify pike as a potential predator. There are many

other large fish in European waters, such as common

bream (Abramis brama L.), pikeperch (Sander lucioperca

L.), and common carp (Cyprinus carpio L.), and the

ability of ducks to discriminate among fish may be

limited. Avoiding lakes with large fish could therefore

lead to the rejection of good breeding habitat. Paas-

ivaara & Poysa (2004) argued that lake choice by

goldeneye females is correlated only with food avail-

ability and not with either vegetation structure or risk

of pike predation. In their study, goldeneye duckling

mortality was higher in lakes with high pike density,

indicating that females were unable to identify pike

and avoid predation. Similar conclusions were drawn

by Beattie & Nudds (1989), who tested the behavio-

ural response of goldeneye ducklings to a model of a

predatory fish. Group cohesion increased and diving

activity decreased when exposed to predatory as well

as non-predatory fish models. Ducklings did not

avoid fish models, however. Thus, there is no current

evidence for efficient instinctive anti-predator adap-

tations specific for pike in ducklings.

The study design does not tell us whether females

nesting at the pike lakes stayed there with the brood

(which was subsequently eaten by pike) or whether

they took the ducklings elsewhere after hatching, but

previous research offers some clues. The abundance of

aquatic macrophytes and other aspects of habitat

complexity generally increase preference by breeding

ducks (Nilsson & Nilsson, 1978; Kaminski & Prince,

1984; Elmberg et al., 1993). Upland vegetation, which

offers concealment and reduces the risk of predation

on the nest, is another important cue for ducks in

choosing breeding habitat. Lake choice may be influ-

enced by overriding factors such as experience

(female ducks are often philopatric and tend to return

to the same lake each year) or food availability.

Experience might attract the females to the experi-

mental lakes as they were fishless the previous year,

although relying too heavily on experience of fish

presence may not be adaptive as pike are excellent

colonisers and the fish status of a lake can change

from one breeding season to the next. There is a well-

documented general positive association between

abundance of invertebrate prey and duckling survival

(e.g. Street, 1977; Johnson, Nichols & Schwarz, 1992;

Cox et al., 1998), and invertebrate abundance in turn

influences lake choice (e.g. Dessborn et al., 2009).

Despite the large effects of introduction of large pike

in our experiment, it is possible that food abundance

and vegetation cover are overriding factors governing

lake choice. If pike lakes are favoured over pike-free

ones with less invertebrate prey or vegetation cover,

even though reproductive output is lower, the effect is

essentially that of an ‘ecological trap’. Coined by

Dwernychuk & Boag (1972), this concept is used to

describe a mismatch between habitat preference and

actual habitat suitability. In this context, our study

system may offer an additional twist of complexity;

heterospecific attraction may operate among dabbling

ducks (Elmberg et al., 1997), as it does in many other

bird assemblages where later-arriving species use

586 L. Dessborn et al.

� 2010 Blackwell Publishing Ltd, Freshwater Biology, 56, 579–589

established pairs of other species as a cue to select

breeding sites profitable in terms of nest sites or food

abundance (Monkkonen & Forsman, 2002). We spec-

ulate that later-arriving teal are attracted to lakes

already being used by mallard, which arrive a few

days earlier. In doing so, the larger species less

vulnerable to pike predation (mallard; cf. results in

this study and Elmberg et al., 2010) might ‘set the

trap’ for the smaller and more vulnerable species

(teal).

A main objective of our study was to create an

experimental situation that excluded competition

between fish and ducks to assess more directly the

effect of predation. Thus, we did not attempt to mimic

a natural system, but a system in which predatory

effects could be isolated. This approach was success-

ful, but we cannot readily extrapolate the effects to a

natural system where individuals have different sizes

and trophic levels. Most boreal lakes also have a more

diverse fish fauna, and it is important to consider how

other types of fish–duck interactions may influence

population dynamics and distribution patterns in

ducks. Natural pike-only lakes typically have many

small individuals that rely primarily on a diet of

invertebrates, and it has been shown that densities of

some invertebrate taxa can be depressed by pike

predation (Beaudoin et al., 1999). Although the latter

finding points towards a possible competitive inter-

action, it should be noted that ducklings primarily

rely on relatively small invertebrates, such as chiron-

omids (Danell & Sjoberg, 1980), whereas fish often

selectively forage on large invertebrates (Hanson &

Butler, 1994; Bouffard & Hanson, 1997), thus reducing

niche overlap and the strength of competition.

The most common type of fish assemblage in the

study region is a combination of pike and invertivores

(mainly perch and roach), creating a food web type

that may have several interesting consequences for the

relationship between pike and ducks. Invertivorous

fish and ducks are potential competitors for inverte-

brates (Eriksson, 1979; Eadie & Keast, 1982), and pike

can reduce the density of invertivores such as perch

(Wahlstrom et al., 2000). Thus, there is a prospective

indirect positive interaction between pike and ducks

caused by pike predation on invertivorous fish.

However, adding invertivorous fish to a pike–duck

system may also introduce apparent competition

(Holt, 1977), which means that the presence of another

prey species increases the density of pike and thus

predation pressure on breeding ducks. Whether the

net effect of pike on ducks is stronger or weaker in

lakes with invertivorous fish depends on the relative

strength of the different indirect interactions.

Although the negative effects of pike on breeding

ducks may be alleviated in food webs with further fish

species, the lack of predator recognition or avoidance

in ducks in the present study is intriguing in proxi-

mate as well as evolutionary terms. Long-distance

migrating ducks are textbook examples of organisms

capable of tracking food resources in unpredictable

and variable landscapes and of making adaptive

choices of breeding habitat accordingly (Krapu et al.,

1997; Nummi & Hahtola, 2008). In the light of this

suite of adaptations, it would be remarkable if they

were not able to make adaptive choices with respect to

predation risk, particularly to pike, which are easy to

detect. Further experiments are needed to understand

whether adult ducks and ducklings actually lack

instinctive abilities to assess predation risks from

predatory fish and whether such abilities can be

learned.

Acknowledgments

We sincerely thank Per Wedholm, Stina Gustavsson,

Fredrik Engdahl, Christer Olsson, Christian Otto, and

Daniel Lussetti for skilful field work. The study was

supported by grants V-162-05 and V-98-04 from the

Swedish Environmental Protection Agency to JE and a

grant from the Swedish Research Council for Envi-

ronment and Agricultural Sciences and Spatial Plan-

ning to GE.

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