."

This paper not to be cited without prior reference to the author

r.C.E.S. C.M. 1979/B: 5 Fishing Technology Committee Ref. Pelagic Fish Committee

THE ROLE OF SCHOOLING IN FISH CAPTURE

by

T.J. PITCHER

Department of Zoology, University College of North Wales, Bangor, Gwynedd, United Kingdom.

•

(presented in draft to the "Working Group on Reaction of Fish to Fishing Operations" May 10th, 1979)

International Council for the Exploration of the Sea, Charlottenlund Slot, Denmark 1

THE HOLE OF SCHOOLING IN"FISH CAPTUHE by T;J;PITCHER

ABSTRACT

This paper is a preliminary review of the links between shoaling behaviour, and the design of gear for fish capture, in the light of reeent experimental work on fish sehools. It examines possible roles for the repertoire of anti-predator taetics shown by shoals, for the details of the strueture and dynamies of cruising schools, for the ~ volumesand shapes exhibited by schools in the wild, and for the sensory basis of schooling behaviour.

HESUME

A la lumi~re des travaux reeents sur le sujet, cet article etablit la relation existant entre le eomportement des banes de poissons et le type d'appareil utilise pQur la capture. Le rdle ~ventuel du repertoire de taetiques utilisees par les banes de poissons eontre les predateurs, du volume et des formes de banes de poissons dans la nature ainsi que de la base sensorielle de leur comportement a ete examine • • 2 Man acts as a predator on fish and so, like natural predators, he can attempt ~o catch his prey in one of two general ways. He can rely on the fish not detecting his fishing gear, or not recognising it as a threatening stimulus, in which case the gear willbest be designed to capture fish which are behaving in anormal undisturbed way. On the other hand, since a.fish which detects a potential predator will employ its active defences, he can expectto be countered by a repertoire of anti-predator defences. Through a detailed knowledge of these behaviours, we can attempt to design gear which·exploits alarm and confusion to increase the probability of successful fish capture without at the same time scattering the social'groups in which the fish are iiving. 1tany fish of commercial importance live in shoals, and indeed this is one of the reasons why they have become food fish for man because more fish of a shoaling species can be caught at one time. This paper is a preliminary review of the links between shoaling behaviour and fish capture, in the light of recent experimental work on fish shoals. The long-term aim of this line of enquiry is toincrease the efficiency of the gear used in fish capture, although I should state that I see improvements to gear as desirable only when used to reduce the profligate energy consumption of modern fisheries (Leach 1975, Pitcher 1977), and not as way of increasing exploitation on alreädy hard-pressed fish stocks.

Although the terms have until recently been considered synonymous, I think it is useful to distinguish "shoal" from "school" (Pitcher 1978, 1979a). A "shoai" is the general term, equivalent to "flock" in birds, and refers to any social group of fish irrespective of their orientation or organisation. A "school" is one of the behaviours shown by a fish shoal and describes the • characteristically polarised and synchronised behaviour usually seen in travelling shoals. Por example, cod live in very loosely organised shoals for most of the time, and large cod are almost solitary. However, there are reports from divers that this species does form compact travelling schools at certain times, perhaps connected with migrations to the spawning ground. Large cod, which behaved in a highly individualistic manner in aquarium tanks, were induced to school continuously in the 10m diameter moving gantry tank at D.A.F.S. Aberdeen (Partridge, Pitcher, Cullen and Wilson, 1979). In fact, once the cod had schooled for a few days in this way, they proved virtually unstoppable, cruising at a remarkably constant speed even when the rotating gantry was halted. 3

Shoaling behaviour, although it ean of eourse have many other funetions, seems to aet as an anti-predator defenee in two ways (Piteher and Partridge, 1979b). It ean operate both as a 'primary' and as a 'seeondary' defenee (Robinson, 1969, Edmunds, 1974). First, under the eonditions of restrieted visibi1ity in relation tospeed of movement whieh obtain under water, prey whieh are elumped are less

likely to be found by asearehing predator (Triesman, 1975), and so i: shoaling is seleeted for by virtue of its being an aggregation. Unfortunately, this "strategie" advantage in shoaling may not be so widespread as·was onee thought (Cushing and Harden Jones 1968, and see diseussion in Piteher and Partridge~ 1979b), sinee there is now evidence that many shoals, 1ike big game herds, may be virtually continuously in attaek range of a predator (Seghers, 1974, Major, 1976, Pitcher, 1979b). Certainly, shoaling does not protect the f~sh.from modern fishing fleet~equipped with sophistieated sonar and aceompanied by spotter planes. Clark (1976) has gone so far as to suggest that shoaling has a seriously destabilising . influence on fish stocks at high exploitation rates. So despite shoaling having evolved as strategie 'primary' defence against natural predators (and even here the evidence is not as strong aswas onee thought), the behaviour aetively helps the fisherman "predator" armed with fast ships and long range deteetion gear, just as Triesman's theory prediets.

~eeondly, shoaling b~haviour, and more speeifieally sehooling itself, ' proteets fish which are already under attaek, thereby aeting as a 'seeondary': • taetieal defenee. There is now eonsiderable experimental evidence in favour of this point both from fish (Neill and Cullen, 1974) and from mammalian . ~ piseivores (PooIe and Dunstone, 1976), as weIl as supporting evidence from quantified field observations (Major, 1976, Nursall. 1973). The ways in which members of sehools are protected against attacking predators bear eloser examination. because of the possibility of designing gear to take advantage by antieipating the schools tactics.

A passive way in which school members are protected under attack is through the "eonfusion" effeet (seePitcher and Partridge, 1979b for discussion). Predators when eonfronted by a multiplicity of targets may become confused and therefore perform their attack sequence less effectively. Some pursuit predators 1ike tuna and perch appear to have "lock-on" mechanisms which enable them to avoid this effect, but lurking predators 1ike pike or squid attack 4 schools less efficiently than they do individual targets. Such predators tend to take periphera1 fish from thc schoo1 (Pitcher, unpub1ished observations; Mi1insky, 1977).. Thc neurophysio1ogica1 basis of the confusion effect may be periphera1 or centra1 in the eNS, and it cou1d act in the perceptua1 channe1·or cou1d be cognitive in action (Pitcher 1979b). It is possib1e that the dynamic organisation of the schoo1.under attack may change to enhance active1y the confusion effect. However, since we are dea1ing here with what is essentia1ly a passive defence depending upon causing a change in the predator behaviour, it is difficu1t to see how this cou1d'be relevant to the'capture of fish by man's gear.

A more like1y candidate for utility in the design of gear is the repertoire of active anti-predator tactics performed by schools under att~ck~ • These work.in a number of different tact1cal ways, such as by maximising the velocity relative to the predator ("fountain"), or by actua1ly increasing predator confusion. Figure 1 i11ustrates the repertoire of minnow schools which I have observed when they are under attack by a pike; simi1ar behaviours have been reported by Potts (1970), Major (1976), Nursall (1973) and Radakov (1973). During fishing operations one wou1d hope to encourage certain of these behaviours and inhibit others. For examp1e, fish may take some time to regroup after the "flash expansion" manocvre. whereas "compaction" might be encouraged as 1ess inimica1 to capture of severa1 fish at a time. The "fountain" tactic cou1d perhaps be exp10ited in gear designed to catch fish on their way back ~fter splitting. One problem here. is thntthere are as yet n~ Quantitativ9lt descriptions of these behav10urs, a1though such work is in progress. The major problem though is that we do not c1enr1y know whnt fnctors influence fish in deciding to perform one manoevre or another. Apriori, one wou1d expect f1sh to have evo1ved to se1ect behaviours from the repertoire at random in order to prevent anticipation by the predator. and there is some evidence that this is the case wlth minnows. However, sometimes the performance of one tnctic increases the probability of another occuring, for examp1e compaction may preceed flash expansion. Fish may respond to dif~erent predators, who may present characteristica11y different combinations of frightening stimuli. For example. Jacobson and Jarvi (1977) describe the different defensive behaviour emp10yed by juvenile salmon against burbot and against pike.. We require more detai1ed research on the factors which e1icit the various components of shoa1ing f1shes anti-predator repertoire before this know1edgc cnn be used cffective1y in the design of new fishing gear.

., ... 5

Can we usewhat is known of the structure and dynamies of cruising schools? In attempting to answer this question I wi1l give abrief summary of some recent findings on school structure, since much of the critical work is still in the process of publication. Experiments on schooling saithe, eod, and herring on the 10m diameter gantry tank at DAFS Aberdeen, have provided some of the new information. Figure 2 illustrates the apparatus and protocol employed: full details may be found in Pitcher et al. (1975) and in Partridge et al. (1979). Additional findings come from work on freshwater bream schools (Pitcher 1979a and in prep.).

Structure within schools can be shown to be non-random, and there is evidence for saithe and herring that the positions of fish tend statistically • towards the optimal packing of tetrahedra suggested by Pitcher (1973a) and by Breder (1976). The positio~at which saithe chose to join a swimming school also appear to fit the predictions of such a packing. The mean distance of nearest neighbours (NND) are between 0.6 and 0.9 body lengths, and these values appear to be relatively constant irrespective of fish lengthor school size. Neighbour fish are not found on the same horizontal level but are displaced above and bclow. The bubble of "free spacc" around a typical fish in the school approximates to a flattened sphere (Pitcher and Partridge 1979a), which becomes slightly smaller at higher swimming speeds, an~ with increases in arousal (bream: Pitcher in prep.). The observed structure of schools is not a direct consequence of'their 4Ia swimming hydrodynamies (Partridge and Pitcher 1979a), contrary to a number of detailed predictions (Weihs, 1975, Brcder, 1965). Squid schools appear to adopt a similar structure to fish (Hurley 1978), providing further evidence that hydrodynamies are probably not involved.

The most important factor affecting school structurc and nearest neighbour distances appear to be the Specics of fish: in general longer and more rigid-bodied fish cruise faster (Wardie 1977) and tend topack more tightly in the school, but other species differences can be equally important along the facultative - obligate schooling axis. For example, herring had a mean NND greater than those of saithe or cod and consequently swamin less tightly packed schools. But the herring schools were more organised, since the ratio of 1st to 2nd and 2nd to 3rd ncarcst-neighbours was consistently lower than in the other two species. Cod were much more 'likely to approach each other closely than saithe or herring so that although> the NND for cod was not very different from saithe, the variability of the NNDs was much 6 greater. Further details are given in Partridge et al,- (1979). Bream and minnow schools are more like those of saithe than herring or cod (Pitcher, 1979a, .~artri.dee,~l979f-·Pitcher,~1973a) •. : -It iS.worth stressing however, that these detailed differences in school structure-are far from Breder's (1959) prediction that obligate and'facultative schooling species would have very different structures: in general terms it seems that when fish schoo1 they do so in broadly similar ways.

The organisation and synchrony of behaviour within a schoo1 can also be examined. As might be expected, the velocities and headings of fish correlate most closely with those of their n~arest neighbour, and next most ~ closely with their 2nd nearest neighbour and so on. Nevertheless, it is interesting to note that the best predictor of velocity was an index which summed contributions'from the velocity of each fish in the school weighed by its rank order in proximity to the reference fish, and divided by the square of the distance (Partridge et al. 1979). This means that large but remote events can ?e responded to in the same way as small close events. In normal cruising schools this is the main mechanism keeping fish together. However, it tends to "damp out" major course changes, which does not fit very weIl with the observed behaviour of schools. There is now some evidence for a further mechanism, or at least a modification of the normal reactions,

(Pitcher, in prep.). In schools of bream responding to a submersible pump, and in saithe schools deflected from ,their path by a transparent perspex barrier, large changes in direction of swimming and large coordinated accelerations occur. The effect becomes apparent when large path changes by • a few fish in the schoo1 are accompanied by stimuli which frighten all the fish directly, so that at these times the usual nearest neighbour adjustments .. may be overidden, and a large, remote change in path followed, imbueing the school with its impressive synchrony in major manoeuvers. During rapid direction changes when saithe met a transparent barrier, a rapid turn, which appeared almost synchronus to the naked eye, secmed to start at a number of "seeding centres" from fish whose normal neighbour';'fo1lowing behaviour ceased. Other fish near to these "seeding centres" soon responded to the big changes near them presumab1y in the normal way (Partridge, 1978, Pitcher, in prep.). Support for this second kind of mechanism at work in addition to neighbour­ following comes from examination of the work of Bulyakas et al_ (1978), who have had some encouraging success in simulating minor school manoeuvers from 7

a mathematical model of neighbour reactions, but whose method does not appear to be able to predict major manoeuvres (Pitcher, work in progress). Hunter's work (1969) on the reactions of jack mackeral to one of their school fellows stimulated to accelerate with an electrode similarly failed to cover these larger scale actions, and was confined to the neighbour adjustment mechanism. These findings clearly have relevance to the reactions of fish to gear. They imply that gear which shepherds fish with aseries of small changes to direction will encourage them to continue to use the normal neighbour-adjustment mechanisms. One might expect.this tactic to be more successful than gear which brings the second mechanism into play by angendering violent.path~deflectionsin'periphernl fish. Such large changes'can spread very rapidly to the rest of the school and can take the whole body of fish unpredictably off in any direction. Nevertheless, it is not yet clear exactly how one could design gear to take account of this, and I should emphasise that these findings are at a.preliminary ~tage, requlring confirmation.

Subgroups of flsh in schoo1s, whose partially independent movements have the effect of openlng.up "lacanae", have been noted severnl times (Pitcher, 1973a, 1979a and b, Pitcher and Partridge, 1979a and b). Large schools are often made up of these sub-groups (see for example sonar plots presented in (Cushing(1977). Cluster analysis of the headings and velocities of saithe' (Partrldge, 1978 and in prep.), indicates a trend towards sUb-groups composed of 10-15 individuals. It is encouraging that this number accords weIl with • the prediction of clusters of 12-13 fish from the theoretical packing model (Pltcher, 1973a). The relevance of this finding to fishing gear is that the subgroup is probably the smallest number of fish which will act fully as a .. school, and so gear which was designed to avoid splitting off small numbers from the main body should exploit schooling more effectively.

The volume of water occupied by such a sub-group, or indeed by the whole school of N fish, can be predicted reasonably accurately by NL3 where L is

t~e mean body length of the fish. Pitcher and Partridge (1979a) give full details of the two independent methods used to arriveat this conclusion, which allows for outlying fish and for lacunae. More accurnte predictions can be obtained if swimming speed and arousel level are known, but these figures are unlikely to be available in the wild. In the original paper, the prediction of NL4 is shown to be compatable wlth many detailed photographic and visual reports, but generally predicts fish densities an order of magnitude greater 8

than published values from scanning sonar work. Tbe discrepancy is probably because the sonar sweeps are averaged and include gaps and edges of,the school, as of course would a trawl following the same path. Tbe consequences ' of actual school volumes and densities calculated by this methodhave been explored for freshwater fish (Pitcher et al., 1979, Pitcher, 1979b): for example a roach shoal occupies about "95ppm" of the water in i ts horne range of 800m length, and its volume is such that it is almost continually within attach range of one of the predatory pike population spaced out in the same river. Life tables and accurate population densities are not generally available for marine fish and so it is not'yet possible to extend this kind ofanalysis to marine habitats. Nevertheless, the volume predicti~n could be useful in ~ determining the maximum number of fish likely to be encountered by a particular type of gea!"', especial1y if we had further information on school compaction as the fish became frightened.

Observations on the shapes of schools in thc wild (e.g. Squire, 1977) and in large lnboratory tanks (Pitcher and Partridge, 1979a, Pitcher, 1973a), confirm the impression of most observers that shapes are extremely variable. On a theoretical basis, if fish are to minimise their chances of being seen, one would expect schools to adopt a flattened discoid shape (Pitcher and Partridge, 1979a). In practice schools tend to split and reform (probably as a result of sub-group movements), can send out and retract 'psuedopodia' (Radakov 1973), or can ind~lge in a variety of tactics as noted above. Despitethis fluidity of form, the theoretical expectation is borne out in a statistical tendancy towards 4It an oblate spheroid elongated in the direction of trave1. Dimensions of this oblate spheroid tend to be in the ratio of 3:2:1 (L:B:D), but this figure is an average over many swimming speeds and species (Pitcher andPartridge, 1979a). This average shape cou1d be considered when building fishing gear, provided that the great variability in shape was also allowed fore Direct observation of school shnpes adopted by fish encountering fishing genr mny reveal more predictable relationships.

This paper will only briefly touch upon the problems nssociated with detection of schools by sonar. At a within-school level, recent improvements ta resolution menn that individual flsh cnn be seen on sonar ,traces. Despite thls improvement, published details from flsh schoal targets do not bear much relation to' the findings of experimental work, even for such a relatively simple aspect 9

as the dcnsity of fish in sehools, as noted above. The distanees. . found :between sChools· 'are on the other !land of obvious importanee in planning efficient searehing strategics·' for~b·oats~'--; ·.Üthourih:'in'a river there was an indieation that the distaneesbetwecn small minnow sehools within their home range were randomly distributcd (Piteher 1973b), in the sea sehools . arepatehy and usually fit a negative binomialdistribution (Craig, 1969, Smith, 1970~1978). This means that finding a pateh of schools in one area deereases the likelihood of finding a similar pateh in the same locality. Smith (1978) diseusses the scale of patehiness in anehovy schools, and the design of optimum seareh taeties for eommereially viable eoneentrations·of fish in the Californian eurrent.

Conclusions from recent experimental work on the sensory basis of sehooling and shoaling behaviour ean be briefly summarised here (Piteher, 1979a, • a~. Piteher et 1976, Partridge and Piteher, 1979b and in prep. Partridge, 1978). The lateral line appears .to be used to maintain minimum neighbour distanees in the school and ean probably give bearing information as weIl. Vision is used in spaeing, in the deteetion of large turns, and in the approach to preferred distance. Olfaetion and vision are important in keeping members of a shoal in eontaet in the same general area. The eentral proeessing of the sensory inputs is quite eomplex and therefore probably at quite a high level in the CNS. Processing involves the eonflation of information from several senses at once, as with most· true soeial behaviour. For examplej both lateral line and vision are involved when bream learn to ignore some aspeets of the abnormal feedback provided by a mirror: they ean do this only when the mirror ean be approached eloser than the normal minimum interfish distanee. As seeond example, saithe fitted with opaque eyeeaps eould sehool, as eould saithe with cut lateral lines, but both types of sensorily deprived fish reveal quantitative differenees infue way in whieh they followed neighbours in the school. The general eonelusion was that both senses had partially overlapping roles in normal sehooling. The optomotor response, (that of keeping station with a moving external visual stimulUS) has usually been eonsidered as the most important in reaetions of sehools to.gear but the situation now appears more eomplieated. Unfortunately, it is not elear how this new knowledge ean be exploited directly in fish eapture, espeeially as the noise produced by aetual moving gear (trawls ete.) eould weIl provide the main sensory input to fish prior to the gear eoming into visual range. As with the work on sehool taetics and shapes, I feel that in this ease the knowledge gained in the sensory field has so far been more help in devising the methodology whieh might now be employed in studies 10 of schooling and fishing gear, rather than being of direct help to design.

To conclude this paper, I will mention three pieces of work whieh are still in progress and under analysis. First, the reactions of bream schools to a burst of water from asubmersible pump should provide more reliable information on the strueture of schools following compaction (Pitcher in prep.). Similar experiments using stimuli other than water jets are planned. Seeondly, data from a further series of experiments in the gantry tank at Aberdeen are under analysis, and should provide information of direet relevanee to fixed fishing gear. In these experiments, schools of cruising saithe, herring and eod were· split by a wedge-shaped barrier, and the subsequent reformation of the sehool followed. Schools of cruising saithe and herring were also eompressed with a funnel-shaped barrier and the . . re-organisation of the sehool followed as the fish emerged from the opening • of the funnel. In both types ofexperiment, barriers of different width were compared. Further work of this type is envisaged. Thirdly, the exactly positions of perch caught ·in c~ypti~·nylongillnets·in large lakes are being studied to see if they are-eorrelated·withsehool structure

Finally, I suggest that the most profitable way forward is to study direetly the reaetion~ of sehools and shoals to different types of gear in thewild, in paralell with experiments on certain eontrolled aspects in large tanks. 11

LITERATURR CITED

Breder, C.U. 1959. "Stud!es on social groupings- in fish". Bu11. Amer. Hus. Nat. 'His-t. '1'17:397-481 Breder, C.M. ' 1965 "Vortices and fish schoo1s" Zoologica '50:97-114 Breder, C.M. 1976 "Fish schools as operational structures" Fishery Bulletin '74:471-502 Bu1yakas, V.R., A.A. Darkov, D.V. Radakov and V.V. Cekolayav 1978 "Mathematical model of the movement of a flsh school" Problems in Ichthyology !:924-936 (unofficial tran~lation available to the author) Clark, C.W. 1974 "Possible ef:fects of schooling on the dynamics of exp10itedfish populations"

J. du Const!l Int., Expl. Uer~ '~:7-14 Craig, R.E. 1969 "Designs- of echo' surveys" MS 1ecture for' r.c~E"~S-~ Sonar course (mimeo) Cushing, D.ff. 1977 "Observations on fish schoo1s with the ARL scanner" Rapp. P.V. Reun. Cons. Int. Exp10r. Uar~ '170:15-20. Cushing, D.H. and F.R. Harden-Jones 1968 "Why do fish school?" Nature'218:91S-920 Edmunds, E. '1974 "Defence in Anima1s" Longmans, London Hun~er, J.R. 1969 "Communication of velocity change in jack makera1 ' schoo1s" Anim. Behaviour'17:507-514 • Hur1ey, A. 1978 "School structure of the squid~ 'Lo1igo'opalescans" Fishery Bulletin '76:433-442 Jacobson, S. andT. Jarvi 1977 "Anti-predator behaviour of 2-year-01d hatchery reared At1antic Sa1mon and a description of the predation behaviour of'the'burbot" Zool. Revy.38:57-70 Leach, C. 1975 "Energy and food production" International lnst. for Environment and Deve10pment, London 151p. Nei11, S.R. St. J., and J.M. Cu11en 1974 "Experiments on whether schooling by their prey affects the hunting behaviour of cephalopod and fish predators" J. Zoo!. .!E., 549-569 Nurse11,J.R.1973 "Some behavioura1 interactions of spottai1 shiner, yellow pereh, and northern pike" J.'Fish. Res. Bd. Canada 30:1161-1178 12

Major, P.F. 1976 tlpredator prey interactions in sehoo1ing fishes during periods of tWi1ight: a study of the silverside:PranesuS'iIisu1arium in Hawaiitl Fishery Bulletin 75:415-426 Mi1insky, M. 1977 tlExperiments on the se1eetion of predators against spatia1 oddity of their preytl Z. fur. Tierpsyeho1. ~:311-325 Partridge, B.L. 1978 tlSensory aspeets of sehoo1ing" D. Phi!. thesis University of Oxford, 262p. Partridge, B.L. 1979 tlMinnow sehoo1 dynamies" Anim. Beh. (in press). Partridge, B.L. and T.J. Piteher 1979a tlEvidenee against a hydrodynamie funetion of fish sehoo1stl Nature 279:418-419 Partridge, B.L. and T.J. Piteher 1979b tlExperiments on the sensory basis of fish sehoo1s: ro1es of lateral 1ine and visiontl Partridge, B.L., T.J. Piteher, J.n. Cu11en and J. Wilson 1979 tlThe three dimensional strueture of fish sehoo1stl Beh. Ecol. and Soeiobiol. (in press). Piteher, T.J. 1973a tlThe three dimensional strueture of sehoo1s in the minnow Phoxinus phoxinus (L).tI Anima1 Behaviour 21:673:686 Piteher, T.J. 1973b tlSome fie1d measurements on minnow sehoo1stl Trans. Amer. Fish. Soe. 102:840-843 Piteher, T.J. 1977 tlAn energy budget for a rainbow trout farmtl Environmental Conservation 4:59-65 Piteher, T.J. 1978 tlFirst conference of the l.A.F.E.: Session IV: • Sehoo1ing Behaviourtl Newsletter lnt. Assoe. Fish Etho1. 6:80-81 Pitcher, T.J. 1979a tlSensory information and the organisation of behaviour in a shoa1ing- cyprinid fishtl Anima1 Behaviour 27:126-149 Pitcher, T.J. 1979b tlSome ecologica1 eonsequences of fish schoo1 vo1umes tl Freshwater Bio1ogy Pitcher, T.J. and B.L. Partridge 1079a "The density and volume of fish in schoo1s" Marine Bio10gy (in press). Pitcher, T.J. and B.L. Partridge 1979b "The structure, sensory basis and soeiobio10gy of fish schoo1s" New Seientist (in press) Pitcher, T.J. B.L. Partridge and C.S. WardIe 1976 "A blind fish can schoo1" Seience 194:963-965 13

Pitcher, T.J., G.J.A. Kennedy and S. Wirjcatmodjo 1979 "Links between the behaviour and eco1ogy of fish" Proc. 1st Freshwater Fisheries.Conf. 1:162-175 Poo1e, T.B. and N. Dunstone 1976 "Underwater predatory behaviour of the American mink, Mustela vison" 'J. Zool. 178:395-412 Potts, G.V.N. 1970 "The schoo1ing etho1ogy of Lutianasmonostiomata in the sha110w reef environment of A1dabra" J. Zool. 101:223-235 Radakov, D•V•. 1973 "Schoo1ing in the eco1ogy .C?f fish" Israel Prog. for Scientific Trans1., Wi1ey, New York, 173p. Robinson, M. 1969 "Defences against visua11y hunting predators" Evo1utionary Bio1ogy ~:225-259 Seghers, B. 1974 "Schoo1ing behaviour in the guppy: an evo1utionary response to predation" Evolution 28:488-489 Squire, J.L. 1977 "Northern anchovy schoo1 shapes as re1ated to problems in schoo1 size estimation" Fishery Bulletin 76:443-448 Smith, P.E. 1978 "Precision of sonar mapping for pe1agic flsh assessment in the Ca1ifornian current" J. Cdns. Int. Exp1or. Mer. 38:33-40 Smith,. P.E. "The horizontal dimensions and abundance of fish schoo1s in the upper mixed 1ayer as measured by sonar" pages 563-591 !n G. Farquhar (ed) proc. Int. Symp. on • Bio1ogica1 Sound Scattering in the Ocean. Dept. of the Na~~1 Washington, D.C. Triesman, M. 1975 ."Predation and the evolution of gregariousness" Anima1 Behaviour 23:779-800 Ward1e, C.S. 1977 "Effects of size on the swimming speeds of fish" pages 229-313 in Ped1ey (ed) "Scale Effects in Anima1 Locomotion" Academic Press, Lond. Weihs, D. 1975 "Some hyd.rodynamica~ aspects of fish schoo1ing" 1n. T. Wu. et a1. (eds) Symposium on Swimming and F1ying in Nature" Pasadena, Ca1ifornia, 1974.

ACKNOWLEDGEMENTS I am gratefu1 to Anne Magurran for criticising the manuscript and to Andre Duva1 for the French abstract.

(' j rI GlIRE 1 Tactics producod by minnow schoole whon under Dttück by pike•

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20 .

10

o I 2.8 3.0 3.5 4.0 ' 4.5 trawling speed v fE'iJ