The preceding information on management of lagoon and estuarine capture fisheries has dealt with fisheries and fishery resources almost solely within the context of lagoons or estuaries as isolated biological and economic systems. Emphasis has been on management of the resources themseleves. Only passing attention has been given to existing or potential management conflicts between different kinds of fisheries or fishing interests within these waters, and to interactions between lagoon and estuarine fisheries and nearshore and offshore marine fisheries. Thus, in this section examples of several different kinds of conflicts within lagoons are examined -- competition between different economic and ethnic groups utilizing different kinds of fishing gears, conflicts between commercial and sport fishermen as another case, and as a third example, potential conflicts between capture fisheries and aquaculture. Finally, interactions between lagoon and estuarine capture fisheries and offshore and nearshore marine fisheries are considered.
Fig. 13. Artificial fish-attracting structures, from the simple to the complex (from Honma, 1980)
One obvious factor which may cause heavy fishing pressure to be exerted on lagoon and estuarine fishery resources and which can also bring about conflicts among the various groups of fishermen exploiting these resources is, of course, a general increase in coastal populations and a consequent increase in demand for fish for local consumption and for transport to nearby urban centres. The general increase in population is accompanied by an increase in fisherfolk. For example, in Mexico, Acosta Ruíz and Alvarez Borrego (1974) point out that at a time when the coastal fishery resources of Baja California are gradually diminishing under commercial exploitation, the numbers of fishermen in cooperatives are increasing due to demographic increases on the peninsula. Castro Ortiz and Sánchez Rojas (1976) provide graphical data showing an approximate 10 percent annual rate of increase in numbers of fishermen in three large lagoon systems along the Sinaloa (Pacific) coast over the period from about 1940 to 1976, along with trends in yields per fisherman for the most recent years of data. In two of the three lagoon systems trends of yields per fisherman have been decreasing. Barrera Huerta (1976) mentions that one lagoon fishing cooperatives in Oaxaça (Pacific Coast) increased from 270 members in 1973 to 355 members between 1974 to 1975. Information provided by Hernandez Carvallo (1976) on shrimp fisheries in Sinaloa suggests that population pressures have led to resource management problems such that cooperatives might have to be reorganized and membership held at actual numbers or otherwise limited to those individuals with land in physical contact with brackish waters. Discussing the socio-economic history of the evolution of shrimp fisheries in the same area, McGoodwin (1979) mentions that with rapid population growth along the South Sinaloa coast (an increase from about 8 000 in 1935 to 35 000 in 1973) there was increased competition for the area's marine resources. The existing fishing cooperatives became entrenched economic entities while the subsistence fishermen, “pescadores libres”, were encumbered by various regulations which prohibited their fishing in areas exclusively reserved for cooperatives. Meanwhile, the cooperatives themseleves have suffered the effects of populations growth from within to the point that they are overloaded and incomes per fisherman are dwindling.
McGoodwin (1979) states that the introduction of the outboard motor and nylon nets aggravated what was already a system under strain. Shrimp and oyster resources were already heavily exploited for export and attention was then turned to finfishes for domestic consumption. But the technological innovations and heavy fishing pressure caused the stocks of the most desirable fishes to be depleted rapidly. Fishermen have now turned to the exploitation of species of secondary economic importance.
The evoluation of the Lagune Ebrié fishery (Ivory Coast) documented by Verdeaux (1979) and Gerlotto, Verdeaux and Stequert (1980) provides a good example of the interactions of complex socioeconomic factors which must be taken into account in order to develop national or local fishery management strategies for traditional lagoon fisheries.
Ebrié is a lagoon complex of 56 6000 hm² with two natural sea connections and one artificial one. Verdeaux (1979) distinguishes three periods in the evolution of the fishery, the pre-colonial period, the colonial period, and the present period, or, the period of rapid economic expansion. In the pre-colonial period, up to the end of the 19th Century two basic kinds of fishing coexisted, one of which was “community” fishing using large fixed traps constructed of wood and palm fronds and seines of natural fibres. Each of these gears required a number of persons to construct and operate, a family or a village, depending on size. The other kind of fishing utilized gears which were individually operated such as small traps, castnets, and bottom longlines. The catch from individual gears was for daily subsistence and the catch from the blocking traps for preservation and sale. No real fish markets existed, at this time and sale of fish was undertaken by responsible family members.
During the colonial period, from about 1910 to 1958–60, traditional fish commercialization methods by family members were usurped by outsiders. With the marketing structure out of the fishermen's hands, the seines were rapidly abandoned and the large wood traps progressively disappeared from the fishery. The change to individual fishing was also stimulated by the availability of cotton fibres introduced by the Europeans which permitted the construction of enmeshing nets of various kinds which could be fished by a single person. Another factor in the change-over to individuals fishing was deforestation which made wood for the wooden traps less available (or more costly), and the opening of an artificial channel, the Canal de Vidri, from the sea to the lagoon to make a port at Abidjan. This latter event changed salinity conditions in the lagoon and opened the wooden fish traps to destruction by teredos.
The third period, one of rapid economic expansion of the fishery, began in the early 1960's and was characterized by the reappearance of large fishing gears -- beach seines and purse seines -- requiring a number of people to operate them; however, with the innovation that these gears were now owned by non-indigenous people and that the labour for the operation of the gears was salaried.
During the period from 1964 to 1975 the number of fishing gears operated by non-Ivorians increased by a factor of 3.6 while the mesh sizes used in them decreased appreciably. In contrast, practically all of the native Ivorian fishermen use small-scale individuals fishing methods which are fixed in location and selective for certain species. The large, active, and non-selective fishing gears are owned and operated by non-indigenous people, mainly Ghanaians but also Sahelian peoples.
The social and economic conflict between the two groups, sometimes violent, has been aggravated by the fact that non-indigenous fishing gears catch about 45 times more than the average catch of the individually operated Ivorian gear, equivalent to 4.5 times as much fish on a catch per fisherman basis. Although the native Ivorian fishermen outnumber the non-indigenous fishermen by a factor of more than three to one, the great bulk of the lagoon yield goes to the latter group. The situation has been further inflamed by the encroachment of the non-indigenous beach and purse seine fishermen on areas of the lagoon reserved by tradition for the individual Ivorian fishermen.
The individual Ivorian fishermen are thus caught in an ecomonic trap in that, their livelihood from fishing is not sufficient to provide them with enough funds to be able to invest in competitive gear. At the same time the non-indigenous fishermen are able to increase their wealth by investing their fishing profits in diversified holdings such as farms, or by reinvesting in fishing gears.
Management of the Ebrié lagoon fishery has to be seen in the context of sociological and economic terms. Both groups view the fishery resources of the lagoon as a means to acquire sufficient funds for investment elsewhere. For the individual fisherman, lagoon fishing is an obligatory step on the way to obtaining sufficient funds for purchasing a small farm. For the owner of a beach or purse seine the resources provide a means of rapid accumulation of money for other profitable investments. Thus, for both groups, the lagoon fishery represents an opportunity for the temporary accumulation of money to be used elsewhere without long-term commitment to conservation of fishery resources. This has led to what Verdeaux (1979) calls a “dynamique de pillage” leading to overexploitation of fishery resources as evidenced by recent catch trends. Berron (1977) provides additional socio-economic detail on the Ivorian situation but with emphasis on marine artisanal fisheries and ethnic conflicts therein.
Another example which points up the need for close socio-economic investigation before fishery management or fishery development programmes can be successfully implemented is that of the near collapse of the traditional canoe fishery of the Valenca delta in the Bahia State of Brazil documented by Cordell (1974; 1978a and 1978b). As mentioned previously (Section 2.3.3) traditional exploitation of the resource was controlled by a social organization which confined the detailed knowledge necessary for prediction of tides and currents and hence fishing success to a relatively small number of fishing captains, the evolution of proprietary fishing rights to certain fishing grounds, and community social pressure.
Disruption of the traditional fishery took place with the purposeful introduction of nylon nets as a means to increase fishery output to supply urban areas. It was also intended that the introduction of nylon nets would benefit traditional fishermen. However, because of the high cost of these new gears, traditional fishermen were unable to afford them and they were purchased by businessmen who employed fishermen on a salary basis for their operation. As a consequences of the entry of the new gears there was a struggle for the control of the existing fishing grounds which resulted in damage to nets and boats. As an additional result, territorial fishing claims were lost and the fishery resource became one of common property. Still other consequences were that while fishery output briefly increased, thereafter catches declined and both the nylon-net fishermen and the traditional fishermen were thus affected. Traditional fishermen then began to use individually operated gears and to extend their fishing to include shellfish while expanding their fishing operations to adjacent mangrove swamp land, much to their economic and social detriment.
The point here is that the introduction of a new, more efficient fishing technology was not inappropriate per se, but that the potential effects of the introduction were not properly accounted for beforehand. This led to social conflict and economic disruption of the traditional fishing community as well as to a likely overexploitation of the resource.
At present, serious conflicts between commercial and sport fisheries in coastal lagoons and estuaries of developing countries are probably nearly non-existent. However, limited information suggests that recreational fishing is becoming increasingly important in some areas. For example, it is believed that about 10 percent of the yield from the 274 000 hm² of the Brazilian waters of Lagoa Mirim is taken by recreational fishermen (Machado, 1976). Therefore, the subject is brought up as a possible fututre management problem based on the course of events in more developed countries where ample leisure time and other economic factors have combined to make sport fishing a significant factor in the exploitation of inland, brackish, and nearshore marine waters. For example, recreational fishing catches for finfishes tend to exceed commercial harvests in five of the seven principal bays along the Texas (U.S.A.) coastline (Hefferman et al., 1979). The management objective in the Texas bay situation is to provide an equitable and sustainable harvest for the two user groups, and a variety of controls are utilized which include quotas on commercial and recreational catch, prohibiting the use of certain gears for certain periods (e.g., week-ends), as well as licensing for both groups (Hefferman and Kemp, 1980). The management programme is based on applied biological research, but major inputs for management controls are determined by direct or indirect monitoring of the activities of both commercial and sport fishermen.
Recreational fisheries are also an important consideration for the management of Australia's estuarine and lagoon fishery resources. The management objectives of the State of Victoria are to ensure accessibility to the resources by the community and to ensure that the resources are conserved for continued community benefit. In this context, the importance of commercial fisheries as the provider of fresh fish to the community is recognized and a goal of management in the face of increasing recreational fishing pressure is to try to maintain the commercial fishery harvest and effort near present levels. Commercial fishing is regulated along two lines. One line serves to conserve resources by regulating commercial fishing where combined sport and commercial harvest is deemed to be excessive or potentially too heavy through closed areas, limitation of entry, and restrictions on gear types. Another line sseks to minimize conflicts or confrontations between commercial and sport fishermen by such means as restricting commercial net fishing on week-ends in estuaries, and bans on all commercial fishing in certain areas for longer periods (Winstanley, 1981).
Many of the authors cited in the previous sections on management have recommended aquaculture as one of the means by which lagoon and estuarine fishery yields could be increased through a more efficient use of the water surface available, through increasing water surface area in suitable fringe areas, and as a panacea for avoiding the necessity and attendant difficulties of managing lagoon and estuarine capture fisheries.
Several Kinds of interactions of aquaculture with capture fisheries are foreseen. Those of a biological/chemical/environmental nature, expanded on below, could include changes in water quality and loss of natural productive substrate to aquaculture installations either in the system itself or in the adjacent shoreline areas. Other effects are perhaps more readily evident such as interference of aquaculture installations with capture fisheries, and still others, of a socio-economic nature, are more stuble. In this latter category might be the displacement of labour and livelihood of capture fishermen by aquaculture if the fishermen cannot be accommodated for employment in the aquaculture industry or if they cannot otherwise benefit as owner/operators of the type of culture undertaken. However, apparently little attention has been given by fishery managers, proponents of aquaculture, and socioeconomists as to how capture and culture fisheries might beneficially interact, compete, or conflict with each other in coastal lagoons and estuaries, other than from the aspect of environment.
Odum (1974) treats the impacts of aquaculture in a number of broad categories including aquaculture as a pollution source, special problems of raft culture, physical alterations of the environment, and the introduction of exotic organisms.
Among the adverse consequences of pollution from aquaculture are the changes in composition of natural plant and animal populations brought about by increased or decreased aquatic production, changes in pH, and decreased dissolved oxygen concentration. Where exotic chemicals are used for disease or predator control these may also adversely affect adjacent natural communities.
Considering raft culture as a special case of possible adverse effect because of the high density of shellfishes under culture, Odum (1974) calls attention to lowering of down-current dissolved oxygen concentrations and phytoplankton densities as a consequence of raft culture and thus raft-cultured organisms could be in competition with naturally occurring shellfishes as well as providing adverse conditions for other organisms. Another adverse effect caused by the rafts may be increased sedimentation rates associated with reduction in circulation.
Chesney and Iglesias (1979) have investigated the effects of raft culture of mussels on the distribution and abundance of demersal fishes in the upper part of the Rio de Arosa estuary (Spain) by trawling in raft and non-raft areas of the estuary. Although no surface area is given for the inner part of the estuary by the authors, it seems that the 80 hm² of rafts which produce about 2 000 t hm-2 annually, cover less than 10 percent of the semi-enclosed upper Rio.
Chesney and Iglesias (1979) found no remarkable differences in demersal fish biomasses in raft and non-raft areas, although diversity, species richness, and eveness indices were generally higher in the raft area. Reasons given for these results were that demersal fishes were unable to utilize the vast epifaunal resources associated with the raft because many of the kinds of epifaunal organisms were unsuitable as food. Likewise, the benthic infauna below the rafts was adversely affected by mussel feces and pseudofeces and thus presumably less infauna per unit area were available to demersal fishes than in non-raft areas.
Other forms of aquaculture not specifically mentioned by Odum (1974) which under some circumstances could be harmful to aquatic systems in which they are practised are cage and pen culture. Leopold and Bninska (1981) and Korycka and Zdawowsky (1981) call attention to the negative effects of cage culture in fresh waters in Poland. These include mineral and organic loading leading to eutrophication and attendant proliferation of algae, and permanent changes in the distribution of dissolved oxygen.
In contrast, it seems that by carefully matching the cage or pen culture “load” to the capacity of the system in which the cages or pens were installed, oligotrophic lagoons and estuaries might be made to produce more fish for capture purposes to the mutual benefit of both capture and culture fishery interests. There must be much information of use in the voluminous literature on the effects of pollution on aquatic systems which could be used as a frame of reference and a starting point for developing criteria to aid such matching.
The physical alteration of the environment to form fish enclosures, by constructing ponds, and by diking and closing off sections of lagoons may cause problems by limiting or changing circulation patterns, increasing sedimentation from dredging and filling, interfering with freshwater input, and by destruction of productive systems (marsh grass, mangroves, eel grass) adjacent to or within the estuary or lagoon. This latter problem could be particularly serious in the case of organisms which are estuarine dependent and for the fisheries existing for them, both within and outside the systems affected. The apparent dependence of shrimp yields on intertidal areas has been demonstrated for Indonesia by Martosubroto and Naamin (1977) and on a worldwide basis by Turner (1977) who points out the seriousness of loss of these areas through any means.
Odum (1974) suggests that one management solution would be to have the aquaculture facility release the equivalent amount of juveniles which would have been produced naturally if the estuary had not been closed. However, he points out that the equivalent release may not imply the equivalent survival as achieved by natural stocks. This solution begs the issue, and not only that, is also impractical because aquaculture is likely to be based on only one or a few species of high economic value. Even if adequate numbers of these species were released they might not be the same species on which a lagoon, estuarine, or offshore fishery is based. Furthermore, the loss of space to other non-commercial fishes, invertebrates, and flora, which are important to maintaining the productive capacity of these waters should be considered.
Odum (1974) goes on to discuss the possible effects of artificial upwelling used for aquaculture (probably negligible, if for small projects) and the danger of accidental escapement into natural waters of exotic organisms used in fish culture. The latter problem can be quite serious, not only because of the adverse effects of the introduced organisms itself, but also due to any disease organisms which might be inadvertedly introduced with them. Finally, Odum (1974) draws attention to the possible proliferation of disease organisms and their introduction into natural waters due to the high density of the cultured animals which favours a greater incidence of disease than is usually normal in natural systems.
One possible adverse interaction of aquaculture with capture fisheries is the use and dependency of extensive brackishwater aquaculture on the collection of natural seed. In India, concerted research efforts have been made to determine the temporal and spatial distribution of shrimp larvae and of juvenile fishes in natural waters for use as seed to support culture operations (Shetty et al., 1971; Bhanot, 1971; Thakur, 1975, for example). In the Philippines extensive fisheries are based on the capture of Chanos fry for culture. There the by-catch, which may include the larvae of the valuable Penaeus monodon, is discarded. The extent to which seed gathering operations effect natural productivity and capture fisheries is unknown, but may require consideration and eventual management where seed collection is intensive until artificial reproductive techniques are perfected and the technology disseminated.
Another possible source of conflict is for space. This does not necessarily mean that the number of aquaculture installations would be so great as to physically crowd out capture fisheries altogether. Rather, if for example, extensive pen culture installations were placed on fishing grounds traditionally used by one family or a small cooperative, then the community's capture fishing allocational process could be upset. Direct interference with some capture fishing operations is another possibility, for example, the raft culture of mussels. Similarly, Owen (1981) remarks that the activities of net fishermen are severely restricted by oyster cultivation in many estuaries in South Wales(Australia). There, the problem is, in part, that some of the oyter leases are poorly managed and inadequately marked.
On the positive side there is the possibility that certain types of culture operations could beneficially interact with capture fisheries, much in the same way as has been suggested for traditional brush park “aquaculture” in Section 3.5. For example, pen culture installations could serve to attract wild fishes to their peripheries by providing shelter, for feeding on food items lost from inside the pen, and for feeding on fish food organisms associated with the pen itself. Ingestion by wild fishes of food items lost from the cage could reduce water quality problems engendered by the decomposition of unused food. Concentration of wild fishes in the vicinity of aquaculture installations could benefit capture fishermen by locally improving catch for effort expended. However, there is frequently mutual suspicion and antipathy between fishermen and fish culturists. A social and/or economic integration of these two activities is thus a prerequisite to the realization of any combined benefits.
Generally, in Australia decisions on the desirability of expansion of aquaculture in bays and estuaries are taken through consultations with many organizations and agencies to ensure a rational development of coastal resources. For example, in Tasmania leases are issued by the Lands Department but also participating in the issuance of leases are the Tasmanian Fisheries Development Authority, the Professional Fishermen's Association, and a number of other government agencies involved with health, navigation, environment, national parks and wildlife, as well as adjoing landowners, recreational boating organizations, and the public at large (Dix, 1981).
Competition between lagoon and estuarine fisheries and marine nearshore and offshore fisheries where the fishing is on the same or different life stages of the same species already exist for some shrimp species. For example, García (1978) has called attention to this problem with regard to Penaeus duorarum notalis lagoon/offshore fisheries in Ivory Coast, and García, Boely and Domain (1980) also treat the problem within the context of West African fisheries. Marcille (1978) mentions a similar shrimp fishery problem in Madagascar with P. indicus, and McGoodwin (1979) provides an overview of traditional versus marine shrimp fishery conflicts for the Pacific Coast of Mexico. Doubtless, there are instances of similar conflicts for finfishes. As lagoon and estuarine fisheries intensify, the incidence of such conflicts is bound to increase. Going the other way, one can imagine that with intensive effort exerted on nearshore and offshore stocks, estuarine and lagoon fisheries could be increasingly adversely affected in proportion to the extent to which they depend on the influx of species fished offshore or nearshore. For example, Jhingran and Natarajan ( 1973 ) have expressed concern that rapidly developing nearshore fisheries in the Bay of Bengal will adversely affect Chilka Lake (lagoon) finfishes.
The biological basis for such conflicts is the “estuarine dependence” of some organisms. That is, for some organisms passing a portion of their life cycles in estuarine or lagoon environments is obligatory for the completion of these life cycles. Most evidence exists for shrimps and prawns ( Figs. 14 and 15). Other potential conflicts may have their biological basis in anadromous species, such as the Indian shad Hilsa ilisha, which pass through lagoons and estuaries on upstream spawning runs. Gopalakrishnan (1971) has drawn attention to this problem in that many of the most important species in the Hooghly-Matlah estuarine fishery are anadromous and are also fished to some extent in the Bay of Bengal marine fishery. Conflicts also may be based on marine species which are seasonal visitors to estuaries and lagoons purely for trophic purposes. One can also envisage potential adverse interactions between lagoon/estuarine and inland water fisheries for catadromous species such as Macrobrachium ( Fig. 14).
Cases of lagoon/estuarine fishery interactions with marine fisheries which are quantifiable to the extent that management alternatives can be evaluated appear to be rare in the literature for tropical and sub-tropical regions. Lhomme (1979) has looked at the effect of artisanal estuarine Penaeus duorarum notalis fisheries on offshore fisheries for the same species in four West Africa estuaries, the Senegal, Sine Saloum, Gambia, and Casamance, but has found little evidence for negative interactions between estuarine and high-seas fisheries.
Garcia (1977 and 1978), however has studied the problem in greater detail in connection with interactions between industrial offshore fisheries and lagoon fisheries for Penaeus duorarum notalis in Ivory Coast.
For practical purposes industrial offshore shrimp fishing for Penaeus duorarum notalis began in Ivory Coast in 1969. The main Ivorian fishing grounds cover about 1 000 km.
Migration of Penaeus duorarum notalis from lagoons and estuaries takes place at an age of 3 to 4 months after a period of 2.5 to 3 months in brackish waters. Migration peaks occur when rivers are in flood and lagoon/estuarine salinities are lowered. Size at migration can vary from one part of a lagoon to another, and among lagoons in the same year and among years in relation to salinity (García, 1977; García and Lhomme, 1980).
Ivory Coast has some 1 800 km² of coastal lagoons and mangroves which can serve as nursery areas for post-larval P. duorarum notalis. Lagoon shrimp fisheries are executed with closely grouped set nets fished on the ebbtide which fish the emigrating juveniles, and by a kind of scissor net which is pushed and effectively acts as a trawl. This latter gear fishes shrimp which have not as yet completed their growth in brackish waters. Artisanal shrimp fishing in lagoons generally gives higher income to fishermen than other kinds of fishing (García and Lhomme, 1980). In 1973, 55 percent by number of potential recruits to the marine fishery were captured by the lagoon artisanal fishery.
Fig. 14. Distribution in different environments of commercial prawns in India (from Mohamed and Rao, 1971)
Fig. 15. Expected degree to which commercial penaeid shrimps inhabit estuarine environments during ontogenetic development (from Kutkuhn, 1966)
García (1978), using the available marine and lagoon yield data and other fishery parameters combined with biological data on P. duorarum notalis, has looked at artisanal lagoon/industrial fishery interactions through simulation of variations of exploitation rates in terms of offshore and lagoon yield, and relative value (Fig. 16).
Generalized results of this study indicate that variations in exploitation rates in lagoons (0–50 percent) would have only a sight effect on total yield (lagoon plus offshore) where the exploitation of the adults at sea is close to a MSY level but that the overall value of the catch could be increased from 12 to 40 percent by increases in industrial fishing at sea on the larger, higher-value individuals.
García (1978) noted that neither one nor the other of extreme resource management measures would be viable -- elimination of the lagoon artisanal fishery, or elimination of industrial fishery. Elimination of the lagoon fishery would, of course, cause socio-economic, not to mention, political havoc. Suppression of the marine industrial fishery with all future effort concentrated in lagoons could lead, at least, to the economic, if not biological extinction of the species through lagoon overfishing, if a minimum abundance of spawning stock was not maintained. García (1978) proposes four general lagoon fishery management measures which would help to ensure increased recruitment of shrimp to the offshore industrial fishery: (1) control of mesh sizes of fixed fishing gears (set nets); (2) strict limitation or prohibition of the gears which fish juvenile shrimp before their migration; (3) regulation on the locations of nets set on shrimp emigration routes in channels; and (4) a closed season both at sea and in lagoons at the time of peak migration towards lagoons and during the period of maximum post-larval growth.
Artisanal/industrial shrimp fishery interactions have been looked at by Marcille (1978) on the northwest coast of Madagascar. Artisanal fisheries there are executed with barrage traps of natural woods set in a V-shape in tidal waters in, or near river mouths which are adjacent to the nursery areas of the shrimp, mostly P. indicus. As a result of their location and because of the characteristics of the trap itself, many shrimps of less than commercial size are caught. The use of such traps has increased in recent years in the Baie d'Ambaro area due to the stimulus of the formation of shrimp export companies which send refrigerated trucks to the coastal village to collect the catch. Beach seines are also used to some extent.
Using data and analytical techniques much like those employed by García (1977) (above), Marcille (1978) was able to show that suppression of artisanal fishing in the Baie d'Ambaro could increase the catch of the industrial fishery from 8 to 30 percent, based on differing assumptions of natural mortality rates, with consequent shrimp size-related increases in profits of 15–45 percent.
Among the artisanal fishery management options which were entertained by Marcille (1978) were: (1) closed areas; (2) a closed season with closure to begin after a certain yield had been obtained; (3) closed season by fixed time period; and (4) protection of young shrimps. The first three management options were rejected as unworkable because of the inability to identify various nursery areas for various species (three species are included in the offshore fishery) with grounds where they are fished and because of differing timing in life history stages among stocks and therefore differing temporal availability.
Control of the selectivity of the barrage traps was believed to be a viable management option. Data indicated that the mean distance between the “meshes” of the trap was only 7.5 mm and should be increased to about 11 mm, which could be easily accomplished by modifying the materials used. Adoption of such a regulation would benefit the barrage fishermen in that the average size of shrimp in the catch would increase as would their value per unit while at the same time increasing recruitment to the industrial fishery. Limitation of the number of barrages in use was not considered advisable because the barrage traps also catch finfish which are important to the villages. However, a gradual change-over to beach seines was advocated on the basis that their mesh sizes could be more easily controlled than those of the barrage traps.
Fig. 16. The effect of rate of exploitation by lagoon artisanal fisheries and effort exerted by marine industrial shrimp fisheries on total yield and total value of the shrimp fishery in Ivory Coast. Actual situation in 1973 and other simulated situations (from García, 1978)
