This Paper Not to Be Cited Without Prior Reference to the Author R.C.E.S
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." 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.).