Pau1y, D. 1982 Dynamics of mu1tispecies stocks: ICLARM/CSIRO Workshop on theTheory and Management of Tropica1 Mu1tispecies Fish Stocks, Cronul1a,Austra1ia, 12-23 January 1981. Marine P01icy 6(1) :72-74.

DynamicsofmultispeciesstocksICLAAM-CSIAO Wor1<shop on theTheory and Management of Tropical Multi-species Fish Stocks, Cronul/a, Australia. 12-23 January 1981

There has been a flurryof symposiaandworkshops devoted to multispecies fishstocks in the last few years, along withcopious papers, ranging from descrip-tive accounts to highly mathematicalapproaches, on the dynamics of suchstocks.

Almost all of these activities, how-ever, were concerned with temperatewaters, especially on the demersalstocks and fisheries of the North Sea,the North-east Atlantic and the NorthPacific.

These stocks. and fisheries havemainly been (mis)managed under theimplicit or explicit assumption that theinteractions such as predation or foodcompetition between the various popu-lations could be neglected. Only in thelast few years has this assumption beenchallenged and gradually shown to beuntenable.

In the tropics, for demersal fisheriesat least, the multispecies nature of theresource could never be 'assumed

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away', since there are simply too manyspecies in the catch and the fishery isnever selective, ie based on any singlespecies or group of species. Also, in theearly 1950s and 1960s, when large-scaledemersal fisheries started in South-eastAsia (eg in Manila Bay, Philippines andin the Gulf of Thailand), scientistscalled on to advise the governments onpotential yields, etc could not build onprevious knowledge of the biology andexploitability of the major species inthe catch.

Massive changes in species compo-sition of South-east Asian stocks fol-lowing exploitation were first docu-mented by Tiews et al. I These authorswisely abstained from interpretingthese changes. Later, Gullandzsuggested treating the whole speciesassemblage as if it werea singlespecies,thus turning attention away fromchanges in catch composition, whilesetting the basis for a useful rule ofthumb - the stock size should not be

MARINE POUCY January 1982

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reduced to less than '12the virgin stocksize, or the even better known 'poten-tial yield = '12M.Bo'.

Yet these rules, born out of expe-diency and still widely used for lack ofsuitable alternatives, cannot mask thefact that the dynamics of multispeciesstocks, especially in the tropics, are atpresent not understood.

Recently, PopeJ and this writer\working with essentially the same datafrom the Gulf of Thailand, attemptedto derive further 'rules' and general-izations which could be used in tropicalmultispecies stock management. How-ever, these attempts were severelylimited by the scanty database (eg theunavailability of detailed catch data)and by the existence of contradictoryconcepts in the literature pertaining tothe behaviour of these complexsystems.

Aims

John Marr, then ICLARM (Inter-national Center for Living AquaticResources Management) DirectorGeneral, approached the AustralianGovernment in 1978 to sponsor aWorkshop on this subject. In 1980,agrant was awarded to ICLARM by theAustralian Development AssistanceBureau (ADAB) to hold, in conjunc-tion with the CommonwealthScientificand Industrial Research Organization(CSIRO), a 'Workshop on the Theoryand Management of Tropical Multi-species Stocks'.

The workshop, which was held on12-21 January 1981 at the CSIRODivision of Fisheries and Ocean-ography premises in Cronulla, nearSydney, included participants from sixcountries (Australia, the UK, Canada,the USA, Papua New Guinea, Philip-pines). It was convened by Dr G. I.Murphy (CSIRO) and chaired byProfessor P. A. Larkin (UniversityofBritish Columbia, Canada), with thewriter acting as technical secretary.The alms of the workshop were:

. to reviewmodelsthat are presentlyin use, or that have been proposedfor multispecies stock assessmentin the tropics; . .

. to define the data requirementsofvarious assessment and manage-ment methods;

MARINE POUCY January 1982

. to review current research onmultispecies stock assessment intropical waters;

. to identifythe majorconstraintsfor stock assessment in tropicalwaters and to formulate a feasibleplan of action to overcome theseconstraints.

Fourteen papers were presented anddiscussed, after which workinggroupswere formed to discussspecialtopics indetail. The contributions (all invited)covered the following broad areas:Fisheries Management and Research(J. Gulland, FAO, Rome; J. Marr',formerly ICLARM; A. Simpson, for-merly South China Sea Programme);Biologyof Single-speciesStocksand ofCoral Reef Fishes (P. Sale, Universityof Sydney; J. Munro, University ofPapua New Guinea; G. Murphy,CSIRO; D. Pauly, ICLARM);Ecology6 (G. Marten, East WestCenter, Hawaii; J. T. Polovina,'Southwest Fishery Center, Hawaii; R.Jones, Marine Laboratory, Aberdeen;K. Sainsbury, CSIRO); and Modellingof Stocks and Fisheries (G. P.Kirkwood, CSIRO; P. Larkin and W.Gazey, Universityof BritishColumbia;J. Majkowski, CSIRO; M. James andK. P. Stark,' James Cook University,Townsville, Australia). These fourgroups of papers, while not providinginstant solutions to any of theinvestigated problems, have helpedfocus attention on a number of specificpoints.

Concerning management and re-search, it was felt that although ourunderstanding of the dynamics ofmultispecies stocks may be deficient inbiologicalterms, it is often sufficienttoprovide reasonable managementadvice (eg on the need for effortreduction or redirection). Manage-ment advice. generally directed athard-pressed governments, shouldalsobe given to regional banks and similarbodies capable of channelling moneyinto infrastructure development (egroad construction) when these couldresult in preventing distortions in therelationships between the fishery sec-tor and the overall economy.There isadefinite need to strengthen the abilityof South-east Asian countries to con-duct management-oriented fisheriesresearch and to publish the results of

Conferencereports

such research. Generally, decline intotal catch with increasing effort is notapparent from multispecies stocks(although tremendous changes inspecies composition can and do occur)as long as habitat degradation can beprevented. This suggests that total andsudden collapse, such as has occurredin several single-species fisheries (egthe Peruvian anchovy) may not occurin multispecies stocks, which wouldallow for strategies aimed at employinga relatively large number of persons inthe fisheries.

Scanty data

With regard to singlespeciesstocksandcoral reef fish, the papers and dis-cussions suggested that special topicsdeserving attention in single-speciesstudies, particularly in reefs. aregrowth studies (which should concen-trate on daily otolith rings) and stock-recruitment relationships, the latterbeing a highly controversial topicamong reef ecologists.A reviewof thescanty data available on the stock-recruitment relationshps of tropicalnon-reef fish suggestedthe existenceofRicker-type relationships in severalstocks, as well as a strong dependenceof recruitment on abiotic factors in atleast one stock. These findings aresurprising in that strong biotic inter-actions have generallybeen expectedintropical stocks. Further studies on thisare urgently needed.

The papers on modelling rangedfrom 'simple' models, such as POpe'S7extension of the parabolic surplus-yield model. to more complex simula-tion models, one of which, presentedby P. A. Larkin and W. Gazey, wasdevised especially to be run at theWorkshop to simulate the Gulf ofThailand demersal stocks and trawlfishery. Questions of models' useful-ness and sensitivityto input parameterswere discussed. No particular consen-suswasachieved, butsomeparticipantsfelt that models can and shouldbe builtof tropical communities and fisheries,without necessarily waiting for moreand more reliable data to becomeavailable. Such models could be par-ticularly useful to identify the majorgaps in our understanding of how thesystems work. It was also suggested

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that. possibly, simulation modelscouldbe used in South-east Asia and other.areas as a training tool to help fisherymanagers obtain a 'feel' for the possibleimpact of their various policies.

The papers concerning general eco-logical principles as applied to tropicalmultispecies stocks were invited in theexpectation that general ecologicalprinciples could help identify andinterpret patterns which fishery biolo-gists have hitherto failed to notice.However. the papers and ensuing dis-cussion on this topic suggested thatecological theory. as it presentlystands.has generated too many untestablehypotheses and us.::s too many un-quantifiable parameters to be of greatuse to fishery biologists. Many of theconcepts used in theoretical ecologylack applicability to real lifesituations;also, too many of these concepts andparameters contradict each other. Onthe other hand, the empiricaldata athand allow for the derivation of anumber of useful rules, some of themquantifiable, whichallow, for instance,preliminary estimates of yield per sur-face area for different systems, giventhe structure of the food-web, thewater depth and/or the primaryproductivity. Correlational studies,such as those above, may yield, in thelong run, an understa,ndingof systemsbehaviour leading to theoretical,advances, and may be the most usefulapproaches to rationalizing tropicalfishery management.

Although some emphasis was givento South-east Asia, the contributionsand discussions effectively covered avariety of tropical systems, from theGreat Lakes of Africa to Caribbeanreefs. Similarly, the methods andapproaches reviewed ranged from thesimplest methods (eg for assessingmortality rates from fishlengthdata) tobuilding and validation of complexcomputer-based models.

Daniel PaulyICLARM

Metro Manila

Philippines

The papers and discussions of the workinggroups will be published, along with theChairman's summary of the workshop, inICLARM's Conference Proceedings series,as a volume edited by Dr G. I. Murphy andDaniel Pauly.

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'K. Tiews, P. Sucondhamam and A.lsarankura, On the changes In the abun-dance of demersal fishstocks Inthe GulfofThailand from 1963/1964 as a con-sequence of the trawl fisheries develop-ment, Contr. Mar. Ash. Lab., Bangkok,1967, in Thai and English.lJ. Gulland, The fish resources of theoceans, FAD/Fishing News (Books),Surrey, UK,1971.JJ. G. Pope, Stock assessment In multi-species fisheries, withspecial reference to

the trawl in the Gulf of Thailand, SCS/DEV/79/19, South China Sea FisheriesDevelopment Programme, Manila, 1979.40. Pauly, Theory and management oftropical multispecles stocks: a revIew withemphasis on the Southest AsIan demersalfIsheries,lCLARM Studies and Reviews 1,Intemational Center for living AquaticResources Management, Manila, 1979.'Not present at the workshop."Ecologicallheory, food chain studies, yieldper area studies, etc.