I. Overviews

ICES mar. Sei. Symp., 192: 2-5. 1991

Extensive aquaculture: a future opportunity for increasing fish production and a new field for fishery investigations

Jean-Paul Troadec

IFREM ER155, rue Jean-Jacques Rousseau 92138 Issy-les-Moulineaux Cédex, France

Preliminary definitions

The developm ent and m anagem ent of extensive aqua ­

culture raises a num ber of challenging questions o f both

an intellectual and econom ic nature . Before presenting some of them , how ever, the concept of extensive aqua ­

culture needs to be clarified.As in agriculture, the concept of intensification can

be am biguous, its meaning changing in accordance with

the variables to which it refers. A first definition cor­responds to the stepwise extension of m an ’s control

over variables o f the cultivated stocks, habitats, and

organisms. D espite im portan t overlaps, such an exten­sion is not continuous, but follows certain patterns.

Only certain farming systems characterized by specific

sets of controlled variables are viable. F rom an eco­logical poin t of view, extensive systems correspond to modes of production in which m an’s interventions are concentrated , through the manipulation of population recruitment, on the enhancem ent of wild populations, or changes in the composition (species, strains) of na t­ural ecosystems. It is essentially through such systems that fish production from large, open aquatic ecosystems (freshw ater and m arine) can be intensified. Semi-inten- sive systems correspond to the additional stimulation of food production through the fertilization of ponds that are small enough to enable the ir natural productivity

to be enhanced. Intensive systems correspond to the progressive extension of m an ’s control over m ajor

physiological functions of the cultivated organisms

(reproduction, feeding, gene pool, diseases). In those

systems, the environm ent is used essentially as a physical

support. In contrast to extensive and semi-intensive

systems, intensive systems can be established in large as well as in small m arine and freshwater bodies, although

systems w here the environm ent is controlled are necess­arily restricted to small volumes.

A second definition of intensification refers to the stock density within a given farming system. Within

extensive, semi-intensive, and intensive systems, the effects of density-dependent processes becom e stronger

as the density of the enhanced or cultivated popula tion

increases.Finally, the concept of intensification is com monly

used by economists to express the am ount of capital

an d /o r labour tha t is applied to a unit of biological

resource.

In this introduction, only the first definition is used.

“Extensive aquacu ltu re” refers to production systems in which m an’s intervention is restricted to, or con­centrates on, the introduction of additional organisms

in an aquatic popula tion at a particular stage of the

lifespan. This is generally achieved through the seeding or planting of open aquatic ecosystems with spat, fin-

gerlings, or o ther early stages of selected species for the purpose of sustaining or enhancing the recruitm ent of wild populations, o r creating new populations and new

productions in new areas. M ajo r examples of such sys­tems include the cultivation of bivalve molluscs or sea ­

weeds, sea ranching of finfishes, or culture fisheries in land-locked open waters.

Economic significance of existing systems

Often, the yield from extensive rearing is not separated

clearly from tha t o f wild stocks in national statistics.

As a consequence, the ir contribution to aquaculture production is frequently underestim ated. According to

the statistics published by the F A O in 1987, finfishes contributed 51% to world aquaculture production, sea­

weeds 24% , molluscs 20% , and crustaceans 4% . Since molluscs and seaweeds come almost exclusively from

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extensive systems, and because, in large freshwater

bodies, stocking program m es contribute generally to a

substantial extent to fishery yields, it is likely tha t ex ten ­sive cultivation dom inates aquaculture production in absolute terms. B ecause the difficulty in enhancing n a t­ural productivity increases with the size of w ater bodies, the im portance of extensive systems is often greater in

large ecosystems, bo th m arine and freshwater.As examples of the economic significance of extensive

aquaculture in IC ES m em ber countries, blue mussel comes am ong the top species p roduced by the fishery sector (250 000 t in Spain, and 104 000 t in the N ether­

lands in 1988). In F rance, total oyster production (130 0 0 0 1 per year) comes first in both weight and value

of all fish com modities, exceeding th a t of the tropical

tuna industry. Similarly, despite the rem arkable d e ­velopment of intensive culture in Norway, Scotland, Canada, . . . during the past decade (220 000 t in 1989),

world production of salmon from ranching is still greater (400 000 t - essentially in the Pacific - in 1989), and enjoys particularly interesting perspectives in Iceland, or the Pacific coasts of C anada and the U nited States.

G enerally , the developm ent of extensive systems

owes relatively little to fishery science. In research agendas they do not always enjoy a priority on a par with

their econom ic im portance. Shellfish culture developed largely through empirical trial and e rro r conducted by

small-scale farmers. In the history of attem pts to de ­velop salmon ranching (H arache, 1988), failures have

dom inated successes for a long time, an overall per­formance which Larkin (1988) sum m arized in the fol­lowing terms for the Pacific coast of N orth America:

Attempts to enhance salmon production by various “fish-cultural” activities were initiated almost a century ago in a wave of enthusiasm for hatcheries that was gen­erated by the discovery that it was easy to collect eggs, fertilize them, and rear the progeny. Since then, hatchery activity has been carried on throughout the range of Pacific salmon and at one time or another for all of the species. The results of this activity have not been uniform and, from time to time, doubts have been raised about their economic value. In part, this can be attributed to the enthusiasm for indiscriminant trial and error planting ( p. 173).

This apparen t contradiction betw een the economic im portance of extensive systems and the modest con­

tribution of fishery science to the ir developm ent raises

three im portan t questions:

(1) W hat are the ir prospects, in particular for the de ­

velopm ent of fish production in open w ater bodies,

and relative to m ore intensive forms of aquaculture?

(2) W hy have extensive systems apparently received

less attention from fishery research than capture fisheries and more intensive modes of aquaculture

production?

(3) Is this balance justified on bo th economic and scien­

tific grounds?

Opportunities for developing aquatic living resources in open water bodies

In fishery and aquaculture research agendas developed after the Second W orld W ar, research disciplines and

approaches w ere com monly segregated by m ajor modes

of fish production or uses of aquatic ecosystems. Stock assessment dom inated capture fishery developm ent and m anagem ent, while aquaculture developm ent was sought primarily through the domestication of cultivated species; environm ent studies developed largely aside

from fisheries and aquaculture investigations.In relation to the concern generated by the con ­

sequences of overinvestm ent in fisheries and the need for controlling fishing effort, stock assessment inves­tigations focused on the effects of fishing on the exploited phase o f fish stocks (on a yield-per-recruit

as well as on a stock-recruitm ent perspective). The tendency was to emphasize the role of parental biomass and density-dependent mechanisms in fish population regulation processes. T he instability of fish stocks, the role of env ironm ent - including m an-m ade alterations -

and the dynamics of early stages in the success of recruit­m ent, as well as the need for effective resource m an ­

agem ent to separate and quantify the different factors determining stock fluctuations, were underestim ated

(Sinclair, 1988; T roadec , 1988).The opportunities a ttached to the enhancem ent of

recruitm ent to channel natural productivity tow ards few selected species received relatively less a ttention . Even in investigations a im ed at developing extensive systems, the ecological aspects of aquatic population m anipu ­

lation and the need to regulate access to the exploitation of open ecosystems were frequently som ewhat over­looked, com pared to zoological investigations on cul­tivated organisms. R esearch agendas did not cover the full range of relevant disciplines systematically.

The production of certain m ature extensive aqua­culture systems, in particular the considerable b io­masses achieved in shellfish cultures (H éral et al. , 1989), shows the shortcomings of conventional investigations in the developm ent and m anagem ent of fishery eco­systems. R ecent theoretical developm ents in marine ecology (see Sinclair, 1988) indicate tha t spatial p ro ­cesses (e.g. dispersion through advection) taking place

during the early stages of fish popula tions play a greater role in the regulation of marine fish populations than

do density-dependent processes in adult stocks (D aan

et al., 1990). The concept of the natural limit of fish

population production and the role of trophic limitations

in the regulation of exploited fish populations were to

some extent overem phasized.Since different mechanisms and processes are in place

at the successive stages of ontogenesis, there is no

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particular reason for the ir respective effects resulting in com parable levels of abundance. W ith regulation strategies characterized in fish popula tions by high fec­undities, high mortality ra tes during the stages of high

dispersion, high dependence on environm ent conditions (e.g. gam etogenesis in molluscs; B oucher and D ao, 1989), and relative stability of cohorts after recruitm ent, even after exceptionally high or low recruitm ents, regu­latory mechanisms are obviously concentra ted on the early stages o f th e lifespan (and gam etogenesis for cer­

tain species groups such as molluscs). Consequently ,

density-dependent processes cannot be expected to have strong effects on initial increases of stock abundance, especially when m ost commercial species high in the

food chain are overfished.The considerable progress achieved in the p roduction

of fingerlings o r spat opens new opportunities for off­setting, even marginally, the enorm ous losses tha t are taking place in the n u m b er of eggs, larvae, and juvenile fishes. H ow ever, the production of juveniles is only one, and not necessarily the m ost critical, o f the con­straints which have to be overcom e simultaneously

for successive developm ent of stock enhancem ent p ro ­gram mes. T hey include in addition to the production,

o r collection, of juveniles o f good quality:

(1 ) the understanding of the popula tion regulation stra t­

egies and mechanism s, to determ ine the ecological conditions for effective recruitm ent enhancem ent (fo r the release of artificially raised juveniles at the p rope r time and space in the life cycle);

(2) the selection o f species easy to recap ture at an adequa te age (e.g. sessile species such as seaweeds or shell fish, o r am phibiotic species such as salmon); and

(3) the allocation o f adequa te use rights tha t condition investm ents.

F o r a n um ber of species of potentia l commercial in terest, no ne o f these aspects raises insuperable scien­tific, technical, o r legal difficulties. T he m a jo r problem lies in the m ounting o f research program m es covering adequately those four different aspects and in the im plem enta tion , on the basis of the research findings, of viable farming systems. T h e apparen t contradiction

betw een the im portan t developm ent of certain tra ­

ditional systems (e.g . shellfish culture) and the failure

of several stock enhancem ent p rogram m es lies, in many

cases, in the lack of a com prehensive multi-disciplinary scientific approach in stock enhancem ent programm es.

The sam e shortcom ings may account for the modest contribution o f fishery science in general to the de ­

velopm ent of extensive aquaculture.Extensive aquaculture has the potentia l to utilize

the prim ary and secondary productivity of open waters which are not used by m ore intensive forms of produc­tion. It may perm it the production of large quantities

of fish tha t intensive forms cannot p roduce , because they invariably require the supply of animal p roteins to species high in the food chain. T hus, extensive m e th ­odologies may offer the only opportun ity for reducing the growing gap in world fish production which makes

fish scarcer for low-income consum ers in developing countries (R obinson, 1984). C ontrary to m ore intensive systems, the assimilation of extensive systems does not imply m ajor changes in the econom y and social o rgan ­ization of fisher groups in rural developing regions.

They may provide the only effective approach towards

initiating aquacultu re in rural areas of A frica and Latin A m erica where two decades of effort to develop semi- intensive systems has no t resulted in any significant b reak through (T roadec and Christy, in press).

Still, these opportunities rem ain partly speculative. Investigations are needed to determ ine the conditions of the ir developm ent. N evertheless, the prospects are sufficiently great to justify the launching, on a few carefully selected species, of the com prehensive

research program m es tha t are required.

Scientific interest

Research program m es aim ed at determ ining the con ­ditions for recruitm ent and stock enhancem ent are par­

ticularly well suited for investigating m any scientific issues tha t have becom e crucial with the full exploitation and diversification of stresses (po llu tion) affecting exploited fish populations. T he experim ental m e thod ,

first through in vitro experim ents on organisms, second through in v ivo m anipulations of recruitm ent and p a r ­ental stock levels, could be used over ranges extending much beyond the range of fluctuations o f wild po p u ­lations. This implies the selection of popula tions of

small size for the investigations. In this respect, bivalve molluscs presen t several advantages. The existence of several small and discrete popula tions within the same region would perm it com parative studies. In som e cases, certain habita t conditions could also be modified. E xperim ental ecology at the population level would becom e feasible.

Owing to the high biomasses and densities reached

in established aquaculture systems, density-dependent

factors affecting successive stages of the lifespan could be investigated much m ore readily than in fisheries

where fish popula tions are depressed by fishing, change

relatively less, and cannot be m anipula ted for experi­

m ental purposes.

Potentially, investigations on the conditions for stock enhancem ent have a considerably b roader applicability than the m ere developm ent and m anagem ent of ex ten ­sive aquaculture . W ith diversification of the factors affecting the recru itm ent and abundance of fish po p u ­

lations (fishing - including the harvesting of larvae and juveniles from wild stocks for aquaculture purposes, pollution, stock enhancem ent program m es, un in tended

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releases from aquaculture farms, climatic fluctuations and long-term changes), the developm ent of stock assessment m ethods for distinguishing the various man-

made (and controllable) from natural (and uncon ­trollable) causes has becom e indispensable for the m an ­

agem ent of exploited aquatic ecosystems. This requires

research program m es focusing specifically on the popu ­lation regulation processes. Because stock assessment

program m es for fisheries purposes focus on adult popu ­lations, they are insufficient for tha t purpose.

The developm ent of m a jo r systems of production and

the m anagem ent of aquatic ecosystem uses now depend on the input of an array of scientific disciplines con­siderably broader than before. A t the same time, research program m es of an innovative nature cannot be

restricted to particular fields of application (fisheries, aquaculture, environm ent). T o a large extent the

m ethods and the basic knowledge required to inves­tigate the conditions for extensive aquaculture de ­

velopment and the dynamics of early stages of fish populations are available. They need to be mobilized

along new approaches and program m es defined in response to the new questions to which fishery science is now confronted.

In tha t context, the planning of scientific inves­tigations for the developm ent and m anagem ent of extensive aquaculture systems should start from a re ­examination of:

(1) the research problem s tha t the m anagem ent of

established systems are raising, notably the density- dependen t effects and the need to regulate cul­tivated biomasses;

(2) the investigations tha t are required for assessing the

prospects of new systems of extensive aquaculture and the conditions of the ir effective development;

(3) the state-of-the-art in the various relevant disci­

plines;

(4) the com prehensive research approaches involving oceanographers, ecologists, biologists, aquacultu- rists, and resource economists.

ReferencesBoucher, J., et Dao, J.-C. 1989. Repeuplement et forçage du

recrutement de la coquille Saint-Jacques (Pecten maximus). In L 'homme et les ressources halieutiques. Essai sur l’usage d’une ressource commune renouvelable, pp. 313-357. Ed. by J.-P. Troadec. IFR EM ER , Paris.

Daan, N., Bromley, P. J., Hislop, J. R. G., and Nielsen, N.- A. 1990. Ecology of North Sea fish. Neth. J. Sea Res. 26(2- 4): 343-386.

Harache, Y. 1988. Pacific salmon in Atlantic waters. ICES CM 1988: Mini 6.

Héral, M., Bacher, C., et Deslous-Paoli, J.-M. 1989. La capa­cité biotique des bassins ostréicoles. In L’homme et les ressources halieutiques. Essai sur l’usage d’une ressource commune renouvalable, pp. 225-259. Ed. by J.-P. Troadec. IFREM ER, Paris.

Larkin, P. A. 1988. Pacific salmon. In Fish population dynamics, pp. 153-183, 2nd ed. Ed. by J. A. Gulland. Wiley Interscience.

Robinson, M. A. 1984. Trendsand prospects of world fisheries. FAO Fish. Cire., 772.

Sinclair, M. H. 1988. Marine populations. An essay on popu­lation regulation and spéciation. Washington Sea Grant Pro­gram, University of Washington Press, Seattle. 252 pp.

Troadec, J.-P. 1988. Why study fish population recruitment? In Toward a theory on biological-physical relationship in the world ocean, pp. 477-500. Ed. by B. J. Rothschild. Kluwer Academic Publications.

Troadec, J.-P., and Christy, F. T ., Jr. (in press). Temporarily out of stock. A diagnosis and a strategy for international cooperation in fisheries research. World Bank, Washington, DC, USA (not released).

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