Vigilant Behaviour and Shoal Size in Minnows

Z. Tierpsychol., 67,167-178 (1985) @ 1985 Verlag Paul Parey, llerlin und Hamburg ISSN 0044-3573 / Intercode: ZETIAG

School of Animal Biology, University College of Nor th Wales, Bangor, Gwynedd

Vigilant Behaviowr and Shoal Size in Minnows

By A. E. MAGURRAN, W. J. OULTON and T. J. PITCHER

With 7 figures

Received: December 12, 1983

Accepted: March 6, 1984

Abstract and Summary

The aim of this work was to investigate the relationship between shoal size and vigilance. The behaviour of minnows (Phoxinus phoxinns) foraging on an artificial food patch during the simulated stalking approach of a model predator (pike: Esox lucius) was recorded for shoals of 20, 121, 6 and 3 fish. Minnows in large shoals reduced their foraging sooner but remained feeding on the patch longer than in small shoals. The relatively late reaction of small shoals to the model and the rapid cessation of feeding once the predator was detected, indicates that small shoals were less vigilant than large shoals. The gradual reduction of foraging in large shoals was accompanied by an increasing number of investigative approaches in which individuals monitored the model’s approa&. This enabled minnows in larger shoals to balance more efficiently the conflicting demands of feeding and watching for predators.

Introduction

Fish in small shoals spend less time foraging and are more timid than fish in large shoals (MAGURRAN and PITCHER 1983; PITCHER and MAGURRAN 1983). One possible explanation for this behaviour is that fish in small shoals are less efficient a t detecting approaching predators and must as a consequence allocate more time to being vigilant. An extensive literature on birds, where vigilant behaviours are relatively easy to recognise, shows that the time individuals spend watching for predators decreases with increasing flock size, and that birds in large flocks respond to predators sooner (for example BERTRAM 1980; HOOGLAND and SHERMAN 1976; KENWARD 1978; POWELL 1974). Unlike birds, fish do not exhibit unequivocal vigilant postures but in

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168 A. E. MAGURRAN, W. J. OULTON and T. J. PITCHER

the presence of predators fish shoals become more cohesive (CERRI 1983; PITCHER et al. 1983; RUPPELL and GOSSWEIN 1972) and may display character- istic evasive tactics if attacked (PITCHER 1979). Although a number of workers have suggested that the detection of predators is also an important function of shoaling behaviour (EIBL-EIBESFELDT 1962; KEENLEYSIDE 1979; RADAKOV 1973) evidence that group vigilance does increase with shoal size has not been presented until now.

These experiments examine the behaviour of shoals of foraging minnows during the standardised approach of a model pike. Pike are important pisci- vores, sympatric with minnows in many European freshwater systems (VARLEY 1967). MANN (1982) found that minnows comprised up to 50 "/o of the'diet of young pike in southern England.

Methods

Shoals of 20, 12, 6 and 3 minnows (Phoxinus phoxinus) were compared in the experiments. Solitary minnows were not used since pilot work had shown them to behave aber- rantly. The largest shoal size was set a t 20 fish, a number that ensured no overcrowding in the tank and no depletion of the food patch during a trial. Fish, which measured 5-6 cm, were randomly allocated to each shoal size from a large pool of minnows taken from a stream where pike (Esox Iucius) are the main predators.

The experiment tank measured 180 X 50 X 35 cm deep and was lit by 2 X 85 W fluorescent tubes. Artificial weed beds (20 X 15 cm in area) were placed at the centre and ends of the tank and the tank floor was covered with a homogeneous layer of aquarium gravel.

The model predator, which was constructed in resin from the Fast of a young dead pike, measured 20 cm and was realistically painted (Fig. 1). In order to avoid disturbance at the beginning of a trial the model was housed in a darkened tubular hide at one end of the tank (Fig. 1). During each trial the model was pulled manually out of the hide and along the tank with a monofiIament fishing line. This line ran through small plastic tubes

Fig. 1: The 20 cm model pike emerging from its hide

Vigilant Behaviour and Shoal Size in Minnows 169

clamped to the two ends of the tank and lubricated with petroleum jelly. With practice it was possible to move the model slowly and steadily along the tank in a simulated stalk. In this stalk, which was based on our observations of the behaviour of a live pike of similar size, the pike moved forwards 10 cm (at the rate of 0.75 cm/s) and then paused for 30 s before recommencing its journey. In total this gave 13 pauses and 12 periods of movc- ment. The stalk covered 120 cm and lasted 10 min.

Each trial began when a vertical food patch consisting of a gravel filled ice-cube tray loaded with Tetrafin flake food was introduced into the tank at the far end from the model (see MACURRAN and PITCHER 1983 for more details of patch). Test fish had spent 10 X 15- min periods feeding on such a patch in the week prior to the first trial with the model. After the test shoal had been allowed to forage undisturbed for 5 min the pike model was manaeuvred along the tank in the simulated stalk (Fig. 2). At the maximum distance (120 cm) the pike model was still within the hide and at the minimum (0 cm) it was directly along- side the patch. Once the model reached the patdi it remained there for 1 min before being manceuvred back into the hide.

Fig. 2: The model pike approaching the feeding patch. Note the minnows which have ceased to forage and arc leaving the patch in a school

Five trials (four days apart) with the model were completed for each shoal size. O n the days between the trials, when the pike model remained in its hide, the minnows were allowed to forage undisturbed on the patch for 15 min.

During each trial the behaviour of minnows at the patch was recorded continuously on videotape. Events out-of-view were described on the sound channel of the videotape.

In order to provide a baseline against which to compare the responses of the minnows during the stalk, three randomly selected 15-min bouts of undisturbed foraging on the patch were also recorded. Finally, two trials, in which the minnows foraged on the patch but where the line was pulled through the tank without the model, were carried out as controls for friction.

In summary data were collected for each shoal size as follows: five trials with the model, two line-only controls and three bouts of undisturbed foraging.

Analysis

Cyprinid fish do not exhibit any obvious vigilant behaviour. Much of the existing work on vigilance has employed animals which stop feeding and scan for predators by


Interaction (shoal size x distance)

adopting a head-up posture (for example UNDERWOOD 1982, mammals; BERTRAM 1980, birds). In this experiment vigilance has been inferred from changes in behaviour. Two approaches were therefore adopted in analysing the videotape record. First, the feeding behaviour of the fish a t the patch was measured and secondly, behaviours which have been described for minnows in response to real predators were recorded.

* * * * * *

Feeding behaviour Four minnows were randomly selected during each 30-s pause in a stalk, including

the line-only controls. (In the case of the shoals of 3 all fish were monitored.) The time (out of each pause of 30s) that these fish spent foraging on the patch and the number of visits that they made to the patch were scored. These values were recorded in a similar protocol for undisturbed foraging and line-only controls.

Anti-preclator behaviour One behaviour which minnows often perform during the stalk of a live pike is “skit-

tering” (MAGURRAN and PITCHER, in prep.). A skitter is a rapid movement where the fish darts 1-5 cm and returns to the same place, like a “boomerang”. Skitters occur most frequently when a group of minnows have just detected a pike and are changing from dis- persed shoaling to compact schooling. Skittering may spread from one fish through the shoal and is frequently a precursor of evasive manceuvres.

In addition to skittering, minnows often make investigative approaches towards a predator. Here one or a few fish swim up to within 30 cm of the pike and then retreat. The function appears to be to monitor the distance of the predator and its current behav-

The frequency of skitters and the number of fish involved in investigative approaches were recorded for each 30-s pause of each simulated stalk (or the equivalent period for controls).

. iour.

Results

A two-way analysis of variance carried out separately on the two measures of foraging (that is time on patch and visits to patch) showed that there were

Tuble 1: Results of two-way analysis of variance with replicates (SOKAL and ROHLF 1981) carried out separately on two measures of foraging behaviour (time on patch and visits to patch) during (A) the simulated stalk and (B) undisturbed foraging. Pike distances are the 30-s pauses in the stalk, time into trial equals the equivalent 30-s periods in the undisturbed foraging. Variance ratios in (A) were adjusted to take account of significant interactions

(SOKAL and ROHLF 1981). Data were transformed where necessary

/ A ) During simulated stalk with model predator

Time on patch 1 Visits to patch 1 Between shoal sizes Between pike distances

Between shoal sizes Time into tr ia l Interaction (shoal size x time into trial1

+ * * * * * NS 1 NS NS NS

~~ ~

‘m* p < 0.001; NS: not significant.

Vigilmt Behaviour and Shoal Size in Minnows 171

T I M E on P A T C H

$ $ $ $ $ $ $ $ $ loo- %

TIME on PATCH: NO P I K E

r

20

100 100

% x

-

- 6

100 100

K %

Time i n t o Tria l ( 8 ) Pike Distance (cm)

-

- 3

-

1 , I I I I

Fig. 3 (left): Median and interquartile time that shoals of minnows spent on the patch during the approach of the model pike. Each nicdian based on 20 observations (4 fish X 5 ieplicates) in shoals of 20, 12 and 6 , and 15 observations (3 fish X 5 replicates) in shoal of 3. Significant differences between the initial 100% and successive medians shown as J: < 0.05, :>:> < 0.01, :>;>:> < 0.001 Fig. 4 (right): Median and interquartile time rhat minnows spent on the patch during undisturbed foraging. N = 12 for shoals of 20, 12 and 6 , N = 9 for shoal of 3. The trial lengths were matched to the time taken for the stalk and foraging was measured over 30-s bouts separated by intervals of 15 s (equivalent

to pike travel time)

1 72 A. E. MAGURRAN, W. J. OULTON and T. J. PITCHER

significant differences in foraging between shoal sizes (p < O.001) and that feeding behaviour changed during the pike stalk (p < 0.001, Table 1). Further- more there was in both cases a significant interaction (p < 0.001) between model distance from the patch and shoal size indicating that the different- sized shoals did not respond to the stalk in the same way. Part of the difference between shoal sizes can be attributed to the fact that small shoals spent significantly less time foraging (confirming previous work, MAGURRAN and PITCHER 1983). This occurred both during the stalk of the model pike (p < 0.001, Table 1 A) and in the undisturbed feeding trials where the model was absent (p < 0.001, Table 1 B).

In order to examine the different responses of the 4 shoal sizes, irrespec- tive of this overall difference in foraging time, the median times spent foraging on the patch were expressed as a percentage of the median value at the start of the stalk when the predator model was still hidden (Fig. 3). None of these initial medians were significantly different from the median foraging times in the undisturbed trials (p > 0.05, 2-tailed Mann Whitney U test). The median time on the patch a t each successive distance of the pike stalk was tested against the initial (100%) median using a 1-tailed Mann Whitney U test. Significant reductions are shown in Fig. 3. (Strictly speaking these repeated Mann Whitney U tests are not truly independent but they do provide a good measure of reduction in foraging effort.) There was a clear link between response to the model and shoal size: fish in smaller shoals reduced their foraging later and ceased to feed sooner. Minnows in shoal size 20 made the first significant decrease (p < 0.05) in foraging when the pike was still 90 cm from the patch. By shoal size 3 minnows did not significantly reduce foraging until the pike was 30 cm away but stopped completely when it had advanced another 10 cm. Once the fish stopped feeding they either swam as a school around the tank or hid in the weed beds.

It could be argued that this result is a response to depletion of food on the patch or to varying amounts of food competition in the different shoal sizes. This is not the case because Fig. 4, in which time on the patch in the

TIME on PATCH: CONTROL

12

100 -

%

I Fin. I: Median and interauartile I Ibo iOo i0 do i0 i0 time that minnows in shoal size

12 spent on the patch durinp;

- 1

Line Distanoe (om) the line-only controls (N = 8)


Fig . 6: Mean frequency (f) ccf skitters per individual per stalk during the approach of the model pike. Dashed lines indicate median lier in larger skittering shoals. rate Based which on occurred 5 trials ear- in :yh SKITTERS (f/fish/atalk)

each shoal size .2 20

"r

6

3

120 80 40 0 Pike Distance (cm)

undisturbed feeding trials is plotted and tested as in Fig. 3, shows no significant decline in foraging in any shoal size (p > 0.05, I-tailed Mann Whit- ney U test). Nor is the effect due to friction created by the line moving through the tank. In none of the control trials where the fish were allowed to feed normally, and the line without model attached was pulled through the tank, did any significant decline in foraging time occur (p > 0.05, I-tailed Mann Whitney U test, for example see shoal size 12 in Fig. 5).

The relationship between the mean number of, skitters per stalk during the model's approach and shoal size was tested using a 2 way G test (SOKAL and ROHLF 1981). A greater frequency of skitters was observed in the larger shoal sizes (G = 36.4, df == 3, p < 0.001). This result is not surprising since there were more individuals there to skitter. When the data were recalculated as frequency of skitters per fish per stalk (Fig. 6 ) and tested in the same way, it was found that skittering rate varied with pike distance (G = 58.6, df = 12, p < O.OO1) but that individual fish in different shoal sizes skittered equally

1 74 A. E. MAGURRAN, W. J. OULTON and T. J. PITCHER

(G = 2.99, df = 3, p > 0.05) and that there was a significant interaction between shoal size and pike distance (G = 290, df = 36, p < 0.001). In other words although the total amount of skittering done by individual fish was the same in all shoal sizes, minnows in smaller shoals concentrated their skittering in the latter portion of the trial when the model pike was closer. The fact that skittering started earlier in large shoals is strong evidence that the fish in them did indeed see the model sooner.

Fig. 7 shows that fish in large shoals also made more investigative approaches towards the model (2-way G test: G = 230.1, df = 3, p < 0.001). As with skittering behaviour, the number of fish involved in these investigative approaches varied significantly with pike distance (G = 119.0, df = 12,

APPROACHES TO PIKE (+talk) 30

2o r f

10

120 100 80 60 40 20 0

Pike Distance (cm) Fig. 7: Frequency ( f ) of minnows engaged in investigative approaches towards the model pike during its stalk

(mean of 5 trials) Schooling or Hiding


Shoal size 20 12 6 6* 3

Shoal size 20 12 6 6* 3

TaGle2: Two-way analysis of variance with replicates between the five trials with the model in each shoal size (carried out separately for each behaviour and each shoal size)

l i m e on patch

Between trials Between distances Interaction

NS NS NS * * * NS

Visits to patch

Between distances Interaction

* *

p < 0.001) and there was furthermore a significant interaction between shoal size and model distance ((2 = 48.3, df = 36, p < 0.05).

Skittering behaviour and investigative approaches were completely absent in line-only and undisturbed foraging controls.

One potential problem in using model predators which appear in a standardised fashion is habituation (MANNING 1979; SHALTER 1978). HURLEY and HARTLINE (1974) presented models to damselfish (Chromis cyanea) living in their natural coral reef: habitat and found that habituation did not occur if the inter-trial interval was greater than 30 s. Table 2 shows that habituation was not a problem in our experiment with the model pike. With only one exception foraging behaviour (time on patch and visits to patch) did not differ sig:iificantly (p > 0.05) in the five trials carried out on the same shoal size. The anomaly of shoal size 6 can be accounted for if trial 3, in which the fish foraged significantly more (p < 0.05, LSR test, SOKAL and ROHLF 1981), is excluded from the analysis of variance.

Discussion

In this experiment minnows in the large shoals detected the stalk of the model pike sooner (as evidenced by the skittering behaviour) and made more and earlier investigative :rpproaches to the model but foraged on the patch for longer. The normal feeding and line-only controls confirmed that these results were a true response to the model and not caused by food depletion or competition. Habituation was also ruled out. In addition, the existence of skittering behaviour and investigative approaches, which did not occur when the model was hidden, is convincing evidence that the fish responded to the model as they would to a live stalking pike. By contrast, SEGHERS (1981)


found that shoals of spottail shiners (Notropis hudsonius) took longer to react to a model predator than did solitary fish and that the reaction distance of shoals was not contingent on shoal size. H e did however suggest that the larger shoals might be detecting the model sooner but delaying their response. Reaction distance, (when fish employ evasive manoevers) is, as SEGHERS (1981) points out, not necessarily the same as detection distance. In our study the early skittering and investigative approaches indicate that the detection distance of the larger shoals was greater even though their reaction distance (when they ceased to forage and left the patch) was less.

If fish in larger shoals are more vigilant (that is, see the model sooner) why do they remain feeding on the patch for longer? Ostensibly, a good anti- predator strategy is to keep as far as possible from the predator and a number of studies of bird vigilance have shown that larger flocks do take flight when a predator is further away (for example KENWARD 1978). However, in the real world prey are often intimately associated with their predators (for instance PITCHER [1980] showed that a shoal of roach [Rutilus rutilus] in one English river would be on average only 2 m from the nearest pike) so that maximising predator distance could mean that little or no time is available for feeding. The best strategy may therefore lie in balancing the two conflicting demands of foraging and avoiding predators.

One way to do this is to feed in a place with lower profitability but a reduced chance of predation. SIH (1980) found that first and second instars of the backswimmer Notonecta hoffmanni selectively avoided foraging in areas with a high food reward but a greater chance of being eaten by large conspecifics. There is also evidence that fish will forage in less rewarding places if by doing so they reduce the threat of predation. In a classic study by MILINSKI and HELLER (1978) sticklebacks (Gastevosteus aculeatus) attacked less dense swarms of Daphnia when shown the silhouette of a bird predator. This increased their vigilance since they did not need to invest so much effort in overcoming the confusion effect of the densest swarms.

An alternative solution, and the one demonstrated in this paper, is to make use of the increased vigilance of a larger group to forage for as long as possible. A pike will normally stalk right up to a shoal and only strike when it is 10-20 cm away. It has been shown that predators are less successful when attacking large shoals (NEILL and CULLEN 1974) and so fish in large shoals can afford to forage for longer. The investigative approaches, which are initiated early in the stalk, allow the fish to monitor the behaviour of the pike. Fish in small shoals by contrast are a t the double disadvantage of being less vigilant and less able to survive predator attack. Their late response to the model, their hasty departure from the patch and subsequent hiding or schooling are a consequence of this.

It is possible, however, that the delay in response by minnows in small shoals reflects a different compromise between foraging and predator avoidance. This would be the case if small shoals detected the model early in its approach but maintained their level of foraging and only left the patch when the predator was close enough to attack. Minnows in small shoals spend less time


feeding (MAGURRAN and PITCHER 1983) so it could be disadvantageous for them to reduce their foraging even further when the predator is out of its normal striking range. In our experiment the minnows in the shoals of 3 only began investigative approaches when the pike model was 50cm away and started skittering when it was 40 cm from the patch. It is therefore very un- likely that they detected the model as early in the trial as did the larger shoals. The precise nature of the predator avoidance/optimal foraging compromise in different shoal sizes represents a fruitful area for future research.

LAZARUS (1979) obtained analogous results in a study of flocks of weaver- birds (Quelea quelea) presented with an artificial alarm stimulus. In small flocks the birds took flight as soon as the stimulus was presented. As flock size increased they changed to showing flight intention behaviour or orienting towards the stimulus. LAZARUS argues that since large flocks, like large shoals, are less at risk from predation they can delay flying off. This has the important advantage of avoiding costly false alarms. It is likely that large shoals as well as large flocks benefit from this further advantage of increased group size.

Zusammenfassung

Das Ziel dieser Arbeit war es, die Beziehungen zwischen SchwarmgroBe und der Wachsamkeit vcin Elritzen (Phoxinus phoxinus) zu untersuchen. Schwarme unterschiedlicher GroBe (20, 12, 6 und 3 Fische), die an kunstlichen Futterstellen fraBen, wurden wahrend des simulierten Anschleichens einer Raubfischattrappe (Hecht, Esox lucius) beobachtet.

Elritzen in groflen Schwarmen horten eher auf zu fressen als kleine Schwarme, blieben jedoch liinger an der Futterstelle. Die verhaltnismaflig spate Reaktion kleiner Schwarme auf die Attrappe und der sofortige Abbruch der Nahrungsaufnahme, sobald die Attrappe entdeckt worden war, deuten darauf hin, daB kleine Schwarme weniger wachsam sind als groBe. Das allmahliche Verringern der Nahrungsaufnahme in grogen Schwarmen war von einer Zu- nahme von Erkundungsanriaherungen begleitet, wahrend derer einzelne Elrit- Zen das Naherkommen der Attrappe beobachteten. So konnen Elritzen in groi3en Schwarmen die widerstreitenden Anforderungen von Nahrungsauf- nahme und Wachsamkeit wirkungsvoller vereinen.

Acknowledgements We would like to thank J. R. ALLAN and M. SMITH for assistance with running the ex-

periments, E. PRITCHARD for caking the photographs of the model pike, H. HOFER for trans- lating the abstract and two referees for their helpful comments on the manuscript. The work was supported by a grant from the Natural Environment Research Council (U.K.).

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Authors’ address: A. E. MAGURRAN, W. J. OULTON and T. J. PITCHER, School of Animal Biology, University College of North Wales, Bangor, Gwynedd LL57 2UW, U.K.

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Vigilant Behaviour and Shoal Size in Minnows