The impact of pelagic fish behaviour on fisheries

8CIENTIA MARINA

SCI. MAR .. 59(3-4): 295-306

1995

IN TE RNATIO NA L SYMPOSIUM ON MIDDLE-S IZED PELAGIC FISH. C. BAS . .!..!. C1\ STRO and .f.M". LORFNZO I eds J.

The impact of pelagic fish behaviour on fisheries* TONY J. PITCHER Fi she ries Ce ntre. University of Briti sh Columbia. Vancouver V6T IZ-+. Canada.

SUMMARY: This paper details the mismatch between human fisheri es and the beha viour that has evo lved to fit pelagic fishe s to th eir niche. Beha vioural adaptation s of pelagic fi sh in fe eding. spawning. migration and sc hoolin g arc drive n by the opportunity to ex ploit high leve ls of plankton ic production: the highes t le vels are intrinsically patchy. On an annual scale . the mismatch generates vo latility. range reduction and catchability - led stoc k collapse (CA LSC ). On a tim e sca le of dec ades . the human re spon se to unce rtaint y in pelagic fi sheri es ha s been to deve lop eve r more effective levels of fish catc hing technology. Three quantitativ e model s or catchabi lity collapse are ex plored. Th e nonequilibrium Schaefer surplu s production model is used for baseline comparisons: catch-per-unit-effort is direct ly proportional to stock abundance and th e ca tchability of fi sh is constant. Thi s mode l is r~ markabl y re s ilie nt against rapid stock co llap se. In the Csirke-MacCall mode l. catchability increases with dec rea s ing -abundance a nd the stock is led into a CALSC at catch rates only sli ghtly above the Schaefer MSY. Thi s paper introduces a ne w model in whi c h catc h-pe r-unit-effort is constant whi le catchability is directl y proportional to stock size. th e opposite of standard fi shery theory but congruent with the effects of high-technology fi sheri es . In thi s model CALSC ca n occ ur very rapidly. The socia l behaviour that makes CALSC pos s ible is a behavioural response that can act. in concert w ith adverse environmental change. to exacerbate stock collapse. Management of pelagic fish e ries mu st recogni se that th e be ha v iour or pelagic fishes . tun ed by evo lution ror persistence in thi s vo latil e ni che . can make human exploitation of thi s re source a fragil e e nte rpri se.

Kn 1rnrrls. Pe la gic fi shes. sc hooling behaviour. stock collapse. fisheries managem e nt . RESUME N : EL 1\11'..\CTO DEL CO .\JPOR TA~llE'iTO DE LOS PECES PELAGICOS E'-: LAS PESQUERIAS. - Este trabajo detalla el acoplamiento entre la actividad pe sque ra y c l co111porta111iento dcsa rrollado por lo s peces pelagicos en sus nichos. La s adaptacione s de! comportamiento de los peces pelagico s e n la al imentaci6n. freza. 111 igraci6n y forrnaci6n de card C1111 cnes es tan influe nciadas por la oportuniclad de ex plotar altos ni ve les de producc i6n planct6ni ca: los ni veles mas al tos son intrin secamente irregulare s. En una esca la temporal anual. e l acoplarniento genera fuga c idad. lirnit a la reducci6n y cl co lapso de! stock por capturabilidad inducida (CA LSC ). En una esca la de ti empo de decadas. la resp ues ta hurn ana a la incertidumbre en las pesq ue ria s pelag icas ha generado un dcsa rrollo m as cfcc ti vo de la tecno logia de captu ra. Se exp loran trc s rnodclos cuantitativos de colapso por capturab ilidad. El mode lo de producci6n exede nte e n s ituaci 6 n de no equi librio de Schaefer se usa como 1·efe rc nc ia: la captura por unidad de cs fu erzo es directa rn en te proporcional a la abundancia de! stocky la ca pturabilidad de peces es constante. Este modelo es rem arca bl em e nte e last ico frente a un rap ido co lapso de! stock. En c l rnodc lo de Csirkc- MacCa ll. la capturabilidad aumenta co n el descenso en la abundancia y el stock es conduci do de ntro de una CALSC a ni ve les de c aptura so lo li gerarne nte supe ri ores a la MSY de Schaefer. Este trabajo introdu ce un nuevo modelo en e l c ua l la captura por unidad de esfue rzo es constante mientras la captu rabilidad es directam e nte proporc ional al tarn a ilo de! stock . opuesto a la teoria pesquera cst

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Figure 1a illustrates the eq uilibrium yield upon effort curves for the three mode ls while figure I b shows the catchability curves: the model parameters a re given in the legend to thi s figure. Next, I consider the annual effec ts of these models in a simul ated fi shery whose pattern of fi shing effort over 25 years is shown in Figure 2: effort inc reases in steps up to

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FIG. 3. - Annual yield and stoc k biomass over 25 years in the simulated fi shery. (A ) Schaefer model fi shery; (B) Csirke-MacCa ll model fishery: (C) Constant CPUE mode l fi shery. Broken lines indica te biomass, soli d lines yield.

tes the course of catches and fish stock biomass in the three models. Each year, the simulated fish stock is ass umed to respond according to a simpl e difference equation using logist ic stock growth diminished by a catch deriving from effort according to the three eq uation s for q ' . Figure 4 shows the catch and effort history of the three simulated fish erie s. In all three cases , the fishe ry is heading for extinc tion ; in fact , in an attempt at pessimistic realism , the effort values were deliberately chosen to be well above sustainable level s in the lon g term . Overfi shin g is shown in gene ral on these plots by points above the sustainable curves . But the re is a critical diffe rence in resilience in the three models. The Schaefe r stock is clearl y overex-

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Fi e . 4. - Catc h and effort history of th e three mode l fi she ri es plot ted (open circles) upon the sustainable yield curves (broken lines). (A ) Schaefer model fishery : (B ) Csirk e-MacCall rnodel fi she ry: (C ) Constant CPUE model fishery.

ploited, and is heading for extinction. But the collapse is in slow motion, and we still see catches one quarter the peak in year 25. The Csirke stock is also overfished and has collapsed by year 25 . However, it has taken 15 years at the highest effort to do this. (An economic analysis at high discount rates might make this look an attractive scenario). However, the equal CPUE model stock in figure 3C suffers a disastrous collapse, and is wiped out in year 13 , only 4 years after the highest effort is reached. Although the model parameters have been chosen as an extreme example for illustrative purposes , many fisheries in the real world parallel the Csirke or equal CPUE analyses compared to what might have happened to a wel-behaved "Schaefer stock" . For example, the Moroccan sardine collapsed only just after the Schaefer MSY level of effort was passed (TURNER and BEl\CHERIFI 1984). The problem is that conventional CPUE from commercial fleet s is a very poor indicator of stock size in the Csirke model , and moreover in the equal

CPUE model it cannot be used at all to estimate stock size . In conve ntional surplu s production models, and in Delmy depletion model s commonl y used in fishery assessment, CPUE is assumed to track fi sh stock den s ity in such a way that stock size can be well estimated. But in pelagic fishes the situation is more commonly closer to our models 2 and 3 and the disconcerting implication is that we will be unable to use commercial CPUE in the fishery assessments. For example, using conventional CPUE data for the Southwest African pilchard stock failed to predict the collapse (BUTTERWORTH 1989). As a consequence of this type of problem it has been suggested that catch/effort plots cannot be used to help assess pelagic fisheries . So we may have rather few tools with which to ward off the dangers of CALSC in our pelagic fisheries. If we were able to measure or estimate catchability directly, we could use models like the second or third above to assess pelagic fisheries . Unfortunately, catchability is difficult to measure independently of CPUE and with low variance, and so changes in catchability are very difficult to detect. Behavioural collapse can be confused with reduction of area in which stock is found: in the Northwest Atlantic herring collapse (WI NTERS and WHEELER 1985) CALSC was probably the real culprit. One simple technique that may still work is the approximate catch/effort method of WALTER (1985) that confirmed other indicators of an incipient stock collapse in Ecuadorean chub mackerel (PATTERSON et al. 1993). It is interesting that a collapse of capelin stocks off Iceland may have been brought about by greatly increased cod predation (BOGSTADT 1990): this may be a natural analogue of the collapses caused by human predation. Finally, although we understand schooling on a spatial scale of metres, we have very little insight of the exact processes that drive aggregation on a large scale of kilometres. In general , the same behavioural decisions to be in a social grouping must be operating, but experimental verification of the exact costs and benefits that are being evaluated are much harder to perform. It is behavious , at this size/time scale,that needs investigating as a priority.

Spawning Behaviour In reproduction , pelagic fish buffer inte r-annu al variation with high fecundity achieved through serial spawning over protracted pe riods and w ith a high number of spaw nings per yea r (Mc Evoy and IMPACT OF PEL AG IC FIS! I BEH J\ V IOUR 301

McEvoY 1992 ; ALHEIT 1989: serial spawning is also known as batch. partial o r heterochronous spawning). Californian anchovy may spawn 20 tim es in a season (Ht.;'lTER and L EOt\G 198 1); Chi lean sard ine spawn every 6-7 days (OLIVA et al. 1989); C hesapeake Bay anchovy spaw ned 54 times in 1988 (Leo and MUSICK 199 1): Atlantic mackerel may s pawn at any time of the day o r night over a period of weeks (N ICHOLS and WAR:\ES 1993 ). Furthem10re. from an intra-annual perspecti ve. the re is good evidence that batch spawning is likel y to be a mechani sm to buffer short-term food parch volatility and at the same time take advantage of high food densities. Batch spawning frequencies are ex treme ly variabl e (ALHE IT 1987). Experimental work demonstrates that batch spawn ing frequency is directly determined by ration size (Ts URATA and HIROSE 1989: W OOTT0:-.1 1990). Thi s is confirmed by a nal yses in the w ild: for example WRIGHT and BLABER (1990) showed batch spawning frequency in South Java sea anchov y was energet ically dependent on preva iling food availability. So we see here a direct reflection of the patch- seek in g behaviour of pelagic spawning fish seek in g to fuel gonad products and maximi se the ir li fe tim e fecundity. Batch spawning causes pelagic fish to spend longer in an aggregated phase when they are more attractive to fishing . So these re productive behaviours may impact fisheries by producing vu lnerable aggregati ons. In shore pelagic spawners like hetTing may be es pecially easy to over ex ploit. and need extreme ly careful exami nation of appropriate management regulation (e.g. Pacific heITing. HALL er al. 1988).

Miqration Behaviour Life hi story migration patte rns of pelagic fishe s are well -understood at a theo retica l level (K AWASAKI 1992): the trian gular movements of feed in g. spawning and recru itm en t mi g ration s ( HARDE'\ Jo'\1ES 1968) are we ll establi shed by observati on. Fastmoving pelagics tend to show an exagge rated spatial triangle as they cover vast distances. On a diel time scale. mi gration as a refuge from predators impacts fisher ies for pelag ics. For example , Mexican flat-i ron herring hug the shore by day to minimi se predation from large pelagics (PARRISH er al. 1988). Many clupeids are not catchable by hum ans and predators when refuging on the bottom by day. only becoming vu lnerab le when they migrate verti ca ll y towards the surface at night. On migration. individual fish choose to be social. and since they migrate to the same spawning or fee302 T.J. PITCHER

ding areas on a sca le of tens or hundred s of kilometres. they travel together in sc hoo ls. feeding as th ey go. Fisheries for migrating pelagics are therefore affected by the same behav io ural problems of forag ing and sc hoo ling that we re d isc ussed above . An additional impact of migration behaviour on fisheries for pelag ics is to allow ex ploitation of dense stocks as they pas s on their reg ular and predictable migration ro utes. Since the fi she ry is only for a sho rt period. op po rtuni stic la rge-capac ity vesse ls are enco uraged; for example in th e UK west coast mackerel fi she ry w here large purse se ine vessels may catch their weekly quota in o ne night (LOCKWOOD 1988). ln fact, migrati o n routes are not always complete ly predictable, and shifts in these routes have adversely affected the economics of fishing ports that re ly principally on migratory stocks. Furthermore. it is possible that heav y ex pl o itation may be partly responsible for such changes and actively select against fi s h that use regular mi g ration path s. The hi story of capture method s for large bluefin tuna in the Medite1nnean is another example of the indu strial archaeology of pelagic fisheries providing insight to a modern probl em. thi s time of the impact of mi gration behaviour. Traditional tonnare traps for migrating bluefin tuna date bac k to c lassical Gree k times, were perfected by the Arabs, and were used throughout the Mediterranean (SARA 1990). Using deflecting nets 5 km away and g uiding wall s of netting up to 4 km long. a tonnare trap comprises a stunningl y c lever way of catc hing huge fast-moving fi sh with simple rope and net technology, using the concerted manpower availabl e to a small coastal settlement. There were different variants of the trap system to catch tuna on autumn and spawning migrations. The tonnare gear did not deplete the stocks over two millenia. The introduct io n of canning after the apoleonic wars in the early 19th Century broug ht a bout an indu strial expan sion, and many tonnara had to close on account of depleted stocks by the turn of the cen tury (CcsHJNG 1988) . When bluefin fisheri es began to use mechanised purse seines in the 1960s, remaini ng tuna stocks were decimated. Reg ular m igrati on routes allowed the deve lopment of human ftshery: the technological race first depleted the stocks and then wiped them ou t as new ways of catchi ng pelagic fish were developed.

CONCLUSIONS ln summary. the behavioural adaptations of pelagic fish are driven by food and by predation on a

range of time scales. In sight of these behaviours comes from consideration of individual costs and benefits. The impact of these behaviours on fisheries is to engender, and exacerbate other influences on volatility of the resource. The hypothe sis is conceptualised in Figure 5.

OCEANOGRAPHIC DRIVERS

human fishing. So the year-to-year environmental capacity of pelagic systems is evidently driven by geop hys ical processes. We are just beginning to unders tand how to use this information on a scale local e nough to set guide-lines for fishery yield expectation (e.g H ARR IS et al . 1992 ).

(geophysical factors

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(geochemical factors

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biotic factors

FI G. 5. - Concept diagram of the hypothes is put forward in this paper.

On a global scale, there is a major ocean-atmosphere influence on the general abundance of fish stocks through effects on the two main pelagic food chains: diatoms and bacteria/flagellates (MA>:N 1993). These physical factors operate on time/space scales from microns to hundred s of kilometres (MANN 1992). Some individuals advocate such environmental factors as the principal cause of a pelagic fisheries collapse (e.g M ULL ER 1984; YANEZRoDRIG CEZ 1989; AVA RI A 1985 ). Indeed the recent discovery of oscillations over the past 2000 years between anchovy and sardine in the Californian current gives credence to thi s view. This observation was based on the abundance of fish scales preserved in anoxic sediments: the very large fluctuations of sardines and anchovy cannot have been driven by

But it ha s long been recognised that we need to look at the particular adaptations of pelagic fishes in order to gain insight of what is rea lly happening (e.g. ILES 1980), and it has often been suspected that more factors than environme ntal change or the replacement of one species of clupeid by another are at work in collapses of pelagic fishes (e.g. ARMSTRO"IG et al. 1985) . The fish social behaviour that makes CALSC possible is one exa mple of a behavioural res pon se that may act in concert with adverse environmenta l change to speed stock collapse. Mobilit y, schooling and s pawning behaviours of pe lagic fi s he s . tuned by evo lution for persistence in thi s volatil e niche, can m a ke human exploitation of thi s resource a fragile ente rprise. JJ\ IP..\ CT OF PEL\ GIC FISH BEHA \ ' !OUR 303

Collapsed pelagic srocks are unlikel y to be completel y wiped out (BEVERTO N 1990). but there is a danger that the species compos iti on of pelagic systems may alter such that recovery of the original fishery may be only partial. As a consequence fish comm uniti es and ecosystems may be altered wi th no prospect of reversal. During the collapse, the stock distribution may change in a way that precipitates the decline. As the population declines, the fish , exercising their social behaviour dec isions, aggregate into a smaller region. Not onl y do they continue to sc hool , but they aggregate on larger scale of ten s of kil ometres. Recall that thi s is th e scale of pattern th at we understand least about. Something like this happened in the Newfoundl and cod fi shery. Anoth er example is the Ecuadorean chub mackerel. which was heav il y fi shed following a period of off-shore tran sport which dimini shed recruitment in the midI 980s. The stock reduced its area at the lower density, probabl y beca use of soc ial behaviour. From that date , a combination of heavy fishing on schools combined with periods of cooler water increasing catchability brought about a di sastrous CALSC (PATTERSON et al. 1993). I have necessarily focussed on pelagic fishes in thi s paper, but, to the horror of fishery scientists. it is becoming evident that the effects discussed here may not be confined to pelagic fisheries . Modern fishing technology and the stability of commercial catch rates in the face of stock depletion may have unwittingl y allowed over-fishing to bring about a catastrophic CALSC in the rece nt dramatic collapse of the Newfoundland cod stocks (Walters, pers. comm .). The final take-hom e message is that fishin g method s and fisheries assessment procedures are not congruent w ith fish foraging, mi gration. social and spawning behav iours that have evolved to enable th e persistence of pelagic fi sh spec ies over geological time. If we had greater understanding of the responses of individual fi sh through sc hooling. foraging , sc hooling, spawning and mi grati on, we might be better able to evaluate changes in catchability and hence use more powerful assessment models in management. TYLER and ROSE ( 1994) have recentl y reviewed individuall y based models (IBMs) that include the spatial heterogenei ty conseq uent on shoaling. Different spatial patterns themselves derive from the opportunity for indi vidua l variation among the fish in a stock. Such heteroge neity over small areas on a scale of kilom etres is to be expected if fish took the kinds of individual behavioural decision s outlined here . From a different perspective. 304 T.J. PITCHE R

H uTCHT:-.JGS (1992) looks to fishery management models that can incorporate the resu lts of frequent local scale habitat monitoring. The future of pelagic fish eri es may depend upon the next generation of such models th at can confl ate local production. changes in location. density and catchability in a fashion predicted by our understanding of pelagic fish behav iour.

ACKNOWLEDGEMENTS I am most grateful to Dr Paul Hart, Professo r Dani el Paul y and Alida Bundy for comments on thi s manuscript, and thank the organisers of the Las Palmas symposium for mak ing it poss ible. REFERENCES ALHEIT. J. - 1989. Comparative spawning biology of anchovies. sardines. and sprats. In J.H .S. BLAXTER. G .~~l llLE. J.C. and von WESTER1\ HAGE\;. H. (eels). The Early Life History of Fish. Rapp . P-\" Re1111 . ICES. 191: 7-14. ALll EIT. J. - 1987. Variation or barch fecundity of sprat. Spra!//IS sprarrus during spawning season. ICES CM 1987/H: 1-44. ALLA>!. J.R. and T.J. PITCHER. - 1986. Species segregation during predator evas ion in cyprinicl fi sh shoal s. Fresl11rnrer Biology_ 16: 653-659. AR:-ISTRO'.'G. M.J .. P.A. SHELTO\; and R.M. PROSCH. - 1985. Catchbasecl assessments of population size variability of pelagic fish spec ies exploited in TCSEAF Di vision 1.6. Coll . Sci. Pap. !CSEAF .. 12: 17-29. A\".-\Rl.-1. S. - 1985. Efectos de El Niiio en las pesquerias de! Pac ifi co Sureste. lmesr. Pesq. Sa111iago . 32: I 01-116. B.\ILEY. R.S. - 1992. The global pe lag ic fi sh resource and its biologica l potential. pages 1-20 in J.R . BURT. R.H·IRDY and K.J.WHrTTLE (eds) Pelagic Fi sh: rhe Resource and irs E.rploirario11 . Fishin g News Books. Oxford. 352pp. BAKU>!. A. - 1993. The Cali fornia cu rrent. Benf!uela current. and south western Atlantic shelf ecosystems: a comparative approach to identifying factors regulating biomass yields. Pages 19922 1 in K.Sherman. L.M.Alexancler and B.D.Golcl (eds) Large Marine Eosnrerns: Srress. Miri gario11 and S11swi11ahililL

Ame rican Associat ion for the Advancement of Science Press. Washington. 376p. BrnDl\IGTO'