326 Aspects of the Reproductive Strategy of Sdrotherodon melenotberom Comparison between a Natural Population (Ebrie Lagoon, Cote d'lvolre) and Different Cultured Populations
M. LEGENDRP Institut Irenceis de recherche scientifique pour le developpement en cooperation (ORSTOM), 2 13, rue Lafayette 754BO Paris Cedex 10, France Centre de recherches oceenologiques (CRO) BP VIB, Abidjan, COte d'Ivolre
J.M. ECOUTIN ORSTOM, BP 5045, 34032 Montpel/ier Cedex I, France
LEGENDRE, M. and J.M. ECOUTlN. 1996. Aspects of the reproductive strategy of Sarotherodon melanotheron: comparison between a natural population (Ebrie Lagoon, C6te d'lvoire) and different cultured populations, p. 326-338. In R.S.V. Pullin, J. l.azard , M. Legendre, J.B. Amon Kothias and D. Pauly (eds.) The Third International Symposium on Tilapia in Aquaculture. ICLARM Conf. Proc. 41, 575 p.
Abstract
The size at first sexual maturity, absolute fecundity and egg and spawn weight were compared in females of Sarotherodon melanotheron (RGppel, 1852) collected in the wild (Ebrie Lagoon) and reared in different lagoon culture conditions: intensive farming in pens and extensive farming in "acadja-enclos" (pens fitted with branches or bamboo poles). In pens, females reach maturity at a smaller size and produce smaller eggs and in larger quantities than in the wild. In "acadja-enclos" where conditions are intermediate between intensive culture and natural conditions, an intermediate situation is also observed in fecundity and in the size of eggs produced. However, the size at first maturity is similar to that observed in the wild. In contrast, the relationships between spawn weight and female weight hardly vary with the different growth environments. These results suggest that the quantity of matter produced during a breeding cycle (based on the spawn weight) is a specific constant that may be genetically determined. Environmental conditions have an effect on the gonadal material division process and on the breeding strategy: many small eggs or large ones but few.
Introduction and breeds in a wide range of salinity conditions (0-90 ppt; Albaret 1987). Its potential for aquaculture has been Sarotherodon melanotheron (Ruppel, noted by several authors (Pauly 1976; 1852) is a typical estuarine species found Sivalingam 1976; Legendre 1983). in large quantities in most West Afri However, the different tests done in can lagoons and estuaries where it lives intensive culture conditions using ar tificial feeds yielded disappointing re sults (slow growth and poor feed con version), making this species at present
'Current address: ORSTOM. Kemang Indah Kav. L2, a poor candidate for this type of cul JI. Kemang Selatan 1, 12730 Jakarta, Indonesia. ture (Legendre et al. 1989). 327
What seems to be giving more prom in culture conditions, sexually mature ising results is the extensive farming females of S. melanotheron display suc of S. melanotheron in "acadja-enclos" cessive breeding cycles throughout the (Hem 1992), a farming system derived year without interruption. from the highly productive traditional fishery in acadjas developed in the la goons of Benin (Welcomme 1972). Materials and Methods Acadjas are organized stacks of branches submerged in shallow lagoon waters All culture experiments were con where some species of the natural stock, ducted at the Layo Aquaculture Station, including S. melanotheron, converge and 40 km west of Abidjan, in an oltgo multiply. The power of these structures mesohaline area of the Ebrie Lagoon to attract fish seems to result, on the (Cote d'lvoire). one hand, from a significant increase in the surfaces on which microfauna and Fish Origins ,and Culture Conditions epiphytes (source of food for fish) de velop and, on the other hand, from their Four populations of S. melanotheron function as shelter. Based on the same were studied: one natural population, principle, the objective of the "acadja one population reared in pen using ar enclos" is to promote the growth of tificial feed and two populations reared natural food in the culture environment, in two different "acadja-enclos" with no hence considerably reducing the need complementary feed. for artificial feed and, consequently, the Wild specimens were collected in costs of production (Hem 1992). The oligohaline areas of the western part first extensive culture experiments us of Ebrle Lagoon. They were bought di ing the "acadja-enclos" technique have rectly from lagoon fishers soon after given encouraging results: annual fish capture and dissected the same day in yields of over 8 t-ha', of which 60-90% the laboratory. composed of S. melanotheron, are pos The intensive culture of S. melanotheron sible without adding any artificial feed of different sizes was conducted in asso (Legendre et al. 1989; Hem 1992; Hem ciation with Tilapia guineensis in a 625 and Avit, this vol.). m- pen without acadja, at an initial stocking Considering this new orientation given density of 5 lndlvlduals-rn". Fish were given to the culture of S. m elan0 th eron, it may pelleted feed with 3 t % crude proteins be useful to specify the major charac (Legendre 1986), distributed twice daily teristics of the reproduction of this spe six days a week at a daily ration of 5% of cies in this particular environment. In total fish biomass. Initially, these fish were the present work, the size at first ma captured in the station ponds which they turity, the absolute fecundity, the egg had spontaneously colonized from the and spawn weight have been estimated lagoon (AIbaret and Legendre 1983). Test for two different "acadja-enclos" designs fish were both individuals that penetrated (stacks of branches or bamboo poles). the ponds while at the fry stage and first Results are compared to those of a pre generation fish hatched in the ponds. vious analysis of the reproduction of S. In the first "acadja-enclos," an artificial melanotheron based on samples col reef made of 200 stacks of branches was lected in the wild (Ebrle Lagoon) and submerged in an area covering 200 m l in in intensive pen culture (Legendre and a 625-ml pen. This structure was stocked Ecoutin 1989). Whether in the wild or with 1,000 individuals of cultured S. 328 melanotheron (initial mean body weight = females or presence of intratesticular 40 g) at a density of 1.6 fish-m". sperm for the males). Moreover, in or The second "acadja-enclos" was made der to determine the size range in which of 4.000 bamboo poles planted verti first maturity is likely to occur, it is useful cally in the sediments in an area cov to specify, in addition to L5O' the length ering 800 m Z in a 1,250-mz pen. This of the smallest mature individual as well structure was stocked with 4,000 ju as the size at which nearly all (95%) the veniles of cultured S. melanotheron with observed fish are at an advanced stage initial mean body weight of 5 g at a of maturity. stocking density of 3.2 flsh-rn". Fecundity, determined from the ovaries The juveniles used to stock the "acadja of females at the end of the matura enclos" were produced from broodfish tion process, represents, in this study, initially captured in the Ebrie Lagoon the number of oocytes belonging to the and frequently renewed with wild in modal group with the largest diameter. dividuals. This group of oocytes is clearly distin guished from the rest of the egg popu Sdmpllng dnd Study lation and corresponds approximately of the FIsh ReproductIon to the ova that will be spawned. The mean weight of the eggs was determined Monthly samplings of over 30 indi by weighing (to the nearest 1 mg) 50 viduals were taken simultaneously in the oocytes carefully cleared of all traces wild and in the pen over a period of of superficial moisture using absorbent 16 months, between t 982 and 1983. paper. The spawn weight (i.e., the to In the two "acadja-enclos," fish were all tal weight of oocytes ready to be harvested in one harvest, 12 months after spawned) was estimated by the prod stocking. In both cases, observations uct: fecundity x mean weight of one were made on a sample size of approxi oocyte. The estimation of the spawn mately 250 individuals. The first "acadja weight being relevant only in individuals enclos" was harvested in November 1986 with completed oocyte development, and the second in March 1988. oocyte mean weight was determined In all cases, fork length (FL± 1 mm) only for females with a GSI>5. No re and weight (W± 1 g) were estimated lationship existed between the oocyte for each individual. Sexual maturity was mean weight and GSI as defined here determined by macroscopic examina in S. melenotberon studied here (unpubl. tion of the gonads using the scale de data). fined by Legendre and Ecoutin (1989). The mean oocyte weight of S. After dissection, gonads were weighed melanotheron in the four environments to the nearest 0.1 g to estimate the was compared using one-way ANOVA gonadosomatic index (GSI = [gonadal and the Duncan multiple range test. The weight x 1001/ fish weight). relationships between fecundity and The size at first sexual maturity (L so) female weight, on the one hand, and is defined here as the fork length at which between spawn weight and female 50% of the fish are sexually mature, l.e., weight, on the other, were compared their first sexual cycle is at an advanced using covariance analysis applied to the stage (ongoing vitellogenesis for the slopes of the regressions. 329
Results meJanotheron. For an equal weight of female, the eggs produced in culture SIze at FIrst Sexual MaturIty conditions are systematically smaller and (FIg. 1, Table 1) produced in larger quantities than in natural conditions. In contrast, relation In pen culture conditions with artifi ships between spawn weight and female cial feed, females of S. meJanotheron body weight are equivalent in both reach maturity at a much smaller size environments (Fig. 2 and Table 2).
(L so= 140 mm) than that observed in the wild (176 mm). A higher proportion COMPARISON WITH THE "ACADJA-ENCLOS" POPULATIONS of small, sexually active individuals is also observed in the pen population (Fig. In the "acadja-enclos" system, con 1). In contrast, the size at first sexual trasting with the previous description, maturity of the populations of the two the correlation coefficients between "acadja-enclos" is high (166 and spawn weight and female body weight 189 mm), bracketing the values for are not higher than between fecundity females in the natural environment. and female body weight (Table 2). How Results indicate, furthermore, that first ever, this may be due to the smaller sexual maturity occurs at a similar size number of females considered in the es in males and in females (Table 1). timation of spawn weighttfernales with GSI>5 only). FecundIty, Egg and Spawn WeIght In both "acadja-enclos" (Fig. 3), the re (FIgs. 2 and 3, Table 2) lationships between fecundity and weight of female are similar. In contrast, these NATURAL AND PEN-CULTURED POPULATlONS relationships are clearly intermediate with those observed in pens and in the wild, In the wild as in pen culture condi differing significantly by the slope or by tions, the absolute fecundity and the the position (P<0.001). Additional obser spawn weight are positively correlated vations (unpublished) have shown that the with the body weight of the females (Fig. fecundity of S. meJanotheron varies with 2). However, in both cases, the esti season and is slightly higher in the dry mated correlation coefficients between than in the rainy season. Individuals (la spawn weight and body weight are goon and pen populations) considered for higher than between fecundity and this study were sampled throughout the weight of female (Table 2). In each en annual cycle-and therefore reflect this vironment, individual fecundity varia seasonal variability-while sampling for tions are compensated, in terms ofspawn the "acadja-enclos" experiments was done weight by chances in opposite direc at specific intervals. It is clear, however, tion of egg weights. Egg weight and that the difference in fecundity observed fecundity are thus inversely related such between the females from the "acadja that for equal weight, females show enclos" and the females from the other ing the highest fecundity produce smaller two environments exceeds that which eggs (Fig. 3, based on the natural could be due to seasonal variability, there populations of females). fore reflecting a real difference between There are significant differences in the populations. reproductive characteristics of the natural The mean egg weight of the females and pen-cultured populations of S. reared in "acadja-enclos" no. 1 (19 mg) 330
Table 1. Size at first maturity (fork length) observed in Sarotherodon melanotheron collected in the wild (Ebrle Lagoon) and in two different culture systems {pens and "acadja-enclos"}.
b L c No. of fish SMI' Lso 9 5 Environment Sex observed {mm} {mm} (mm)
Lagoon F 365 146 176 2Z3 M 96 148
['ens F 783 100 140 180 M 211 105 138 200
"Acadja-enclos" 1 F 170 173 189 no M 62 153 170 199
"Acadja-enclos" 2 F 158 161 166 205 M 91 152 170 203
'SMI: smallest mature individual.
bLso: size at which 50% of the fish are sexually mature. cL.s: size at which 95% of the fish are sexually mature.
100 ~.... / /.
Pens I" 50 ;
/~ La<;pal / / ~ / III .~/ Q> "5 0 " • iI E ....Q>
e:::J 100 '0 ;¥--e :2: 0/r J / "Acodjo-erclos' No.2 ~ 50 I Aaxljo-enclos I No.I I le / J / o L L._-._-._-...... q~---'-_---'-_---'-_----' 85 105 125 145 165 185 205 225 245 Length (Ft.irrm) Fig. 1. Size at first sexual maturity in fe males of Sarotherodon melanotheron in different environments. 331
Table 2. Relationships between fecundity (F) and weight (W) of female; between spawn weight (S) and weight of female; and mean egg weight in Sarotherodon melanotheron collected in the wild (Ebrle Lagoon) and in two different culture systems (pens and "acadja-enclos").
Relationship Mean egg 95% Conf. Environment N (regression) weight (mg) Int. (±)
Lagoon 31 F=-15.0+1.72 W 0.871 31 S= 1.60+0.041 W 0.963 28.03 1.90
Pens 46 F=203.9+2.61 W 0.777 46 S=0.32+0.045 W 0.939 12.06 1.28
"Acadja-enclos" I 31 F= 132.6+ 1.81 W 0.881 12 S=2.29+0.034 W 0.832 18.94 2.28
"Acadja-enclos" Z 24 F~267.3+1.25 W 0.874 18 S~0.38+0.043 W 0.870 15.07 1.74
16 .". ". ". e /C A ". B ". ". 12 B ". A 12 ". 'l ". ~ DO 0 ". ... .". . /' ...'"0 01> » ~ o ... ~ oc8 8 0...... (I)."" · .. o~o~.,
I I I I I 0 200 400 600 800 1,000 200 400 600 800 1,000 I Body weight (g) Body wolght (gl Fig. 2. Compari- 40 40 son of the rela- .. .. tionships fecun- 0. 30 +.__~..! __A 30 A .§ ... dity-weight of fe- ~ . male, egg . ~ . 0 20 _L ____..... _c ";; 20 J . 0 weight-GSI and 0 !o ... D • . 0 0 environments (natural environment, pen 1,000 l" I culture with or without complementary ~ .. 1,<00 / feed) either in the slope or in the posi 1,>00 " tion. In contrast to fecundity and egg size, spawn weight appears here as a 1,000 variable that is independent of the en vironment of the fish. :t, -i 5 Discussion >00LLI _--'--_"------'-1------l-~~-.l i 0 200 300 400 500 600 700 800 900 Body weight (g) In this study, all fish have the same origin-the Ebrle Lagoon, i.e., do not Fig. 3. Relationship between fecundity and egg originate, for the cultured individuals, weight in female Sarotherodon melanotheron in from a strain of broodfish long isolated the natural environment (Ebrle lagoon). Each data from the natural population. In addi pair (fecundity. egg weight) placed on the same vertical line corresponds to one female. Individual tion, it seems that even when the "acadja variations in fecundity and egg weight show a enclos" is artificially stocked, part of the mirror effect. population is composed of wild indi viduals that entered into the structure also shows an intermediate position as juveniles and grew there until the (P<0.05) in comparison with that of nets could no longer let them through females cultured in pens (12 mg) and (Hem, pers. comm.). It is considered, females in the natural environment therefore, that all fish studied under this (28 mg; Fig. 2). For females reared in experiment are genetically similar, and "acadja-enctos" no. 2, mean egg weight the differences observed in their breeding does not differ significantly from that characteristics seem to be adaptive observed for females cultured in pens (phenotypic) responses to the different or in "acadja-ericlos" no. 1. Other ob environments. servations have shown that egg size tends to increase with the weight of the Size at First Sexual Maturity fish, up to about 100 g, then reaches a plateau. Thus, for the pen population In pen culture conditions, the size at where the sample included few small first sexual maturity (L so) is much smaller individuals (Fig. 2), the mean egg weight than that observed in Ebrte Lagoon. In (12.06 mg±4.20) was slightly under contrast, first maturity in the "acadja estimated and became 13.35 mg±3.70 enclos" occurs at a size similar to that when only individuals weighing over of the natural population of females, 100 g were considered. This, however, slightly smaller in one instance and does not invalidate our previous con slightly bigger in the other. clusions concerning the comparison The fact that the surface areas and between different populations. the sites for pens and "acadja-enclos" The relationships between spawn are similar indicates that neither cap weight and body weight are considered tivity in the Layo site nor available vi equivalent regardless of the origin of tal space (in terms of enclosed lagoon the females (Table 2; Fig. 2) as there is volume) seems to be responsible for the no significant difference between the significant decrease in L so observed in regression lines estimated for the four intensive culture. 333 Lowe-McConneJl (1982) reported that, males of equal body weight, variations in Oreochromis niloticus, natural in fecundity are coupled with a varia populations composed of individuals tion of egg weight in the opposite di with a low weight for their length tend rection, thus confirming the observa to reproduce at a size smaller than that tions of Peters (1963) on the same spe observed for populations composed of cies. This also applies to interspecific individuals in better condition. The comparisons: females from populations present study yielded similar results: with the highest fecundity levels also fish with a low weight for length had a produce the smallest eggs. This balance smaller size at first maturity (Fig. 4). between fecundity and egg size results Although these results suggest a trophic in a relationship between individual effect in the determinism of sexual spawn weight and weight of female that maturity, they do not imply that food remains unchanged regardless of the supply is the only important factor. population considered. The quantity of AI though authors are not unanimous matter produced during a breeding cycle on the subject (see Noakes and Balon (based on spawn weight) is, therefore, 1982), it is generally agreed that in a specific constant that may be deter tilapias, the reduction of the size and mined genetically, the environment in age at first sexual maturity is an adap fluencing the division of the gonadal tation to adverse life conditions (Fryer material and the breeding strategy: many and lies 1972; Ruwet et al. 1976; Lowe small eggs or few large ones. McConnel1 1982). This tends to be sup At the interspecific level, the exist ported by the poor condition observed ence of an opposite relationship between for fish reared in pens which also dis fecundity and egg size is well-known played the lowest Lso. in fish (Bagenal 1978; Mann and Mills In intensive culture conditions,S. 1979; Albaret 1982; Elgar 1990). This melanotheron's first sexual maturity balance between the number and size occurs at the age of six to eight months. of eggs produced was also shown or But in the absence of data on the growth suggested for different groups or popu of this species in Ebrle Lagoon or in lations of a same species (Mann and Mills "acadja-enclos," it is not clear whether 1979; Springate et al. 1985; de Silva the differences in Lso result from a dif 1986). However, this balance does not ference in growth or are also coupled seem to exist in all species studied (Mann with a difference in age at first matu and Mills 1979). To our knowledge, there rity. Eyeson (1983) reported that when is no other clearly demonstrated example fish are kept in captivity,S. melanotheron where the compensation number/size can be sexually active as early as four of eggs observed for various populations to six months at a size as small as 4 of the same species implies a constant 5 cm (standard length). spawn weight. Variations in fecundity have sometimes been analyzed in con Fecundity dnd Egg Size nection with variations in GSI this gives only an approximate measurement of This study shows two distinct fecun the reproductive effort and can change dity levels with variations observed with the number or the stage of devel between individuals of a same popula opment of the groups of young oocytes tion and strong variations observed which, apart from those that will be between populations. At the lntraspe spawned, are present in the gonads cific level, results showed that for fe- (Scott 1979; Mann et al. 1984). 334 .'Acodjo- enclos' I • Ebrie Lagoon ."ACadja-enclosll 2 • Pens ! I I I~ 160 170 Weight (g) at length of 200 mm Fig. 4. Relationship between the size at first sexual maturity (L so) and the condition (weight at length) of females of Sarotherodon melanotheron in different environments: natural (Eurle Lagoon) or culture (pens. "acadja-enclos" nos. t and 2) environments. The condition is here estimated by the weight of a fish at FL=200 mm. This weight is estimated by the length-weight relationship estimated for each environment. Three physiological processes are likely Townshend and Wootton (t 984) re to cause variations in fecundity: the rate ported that in laboratory conditions, the of oogonia multiplication, the recruit egg size of Cichlasoma nigrofasciatum ment of oocytes beginning vitel increased with increasing spawning in logenesis and the atresia of part of the tervals. Thus, in the present study, the developing oocytes (Springate et al. higher egg weight and lower fecundity l 985). Observations of the ovaries of levels observed in wild females could prespawning females of 5. melanotheron be explained by the combined effect of from the four environments have gen longer spawning intervals and higher erally shown atresia. The exact proportion levels of atresia (or the same level over of atresia has not been determined but a prolonged period) compared to what it is always low, which compares well has been observed in cultured females. with the condition reported by Peters While the mean spawning interval of (t 963) in 5arotherodon galilaeus. These 5. melanotheron cultured in tanks is observations, however, do not consider approximately two weeks (Legendre and the potential extent of atresia at dif Trebaol , this vol.). there are no avai ferent stages of oogenesis and the sig lable data on the spawning frequency nificant effect that it can finally have of this species in the wild. The rate of on fecundity. oocyte recruitment and the atresia level 335 can, however, vary simultaneously with fish reared in pens. Since high levels certain environmental factors. of feed (5% of fish biomass) were dis Townshend and Wootton (1984) attrib tributed daily to the fish reared in pens, uted the low fecundity of C. nigrofas this raises questions about feed qual ciatum reared with restricted feeding ity rather than quantity. In addition, al regimes both to a decrease in recruit though the feed is natural and appar ment and to an increase in atresia. At ently found in sufficient quantity in the present, the relative importance of these "acadja-enclos" and in the lagoon, this processes in the control of fecundity is does not exclude possible differences not very well-understood in fish and can in the nature and nutritional quality of vary with species (Springate et al. 1985). the organisms (animals and plants) avail Several scenarios are therefore possi able for fish in both environments. ble to explain the responses observed The effect of the dietary proteins on and may involve different rates in oocyte the production of eggs and fry was stud growth. An in-depth, histological com ied recently in various species of tilapias. parative study of the ovarian develop In S. melanotheron, Cisse (1988) did ment of S. melanotheron maintained in not observe any significant difference different environments would be nec either in spawning frequency or in essary to clarify this problem. number of eggs produced per spawn Among the external factors likely to ing with varying protein levels, which, have an effect on the production of eggs, however, could be explained by the small feeding has been the subject of the larg number of fish used in this study. San est number of experiments in fish. A tiago et al. (1985) and Chang et al. decrease in absolute fecundity is gen (1988), studying 0. niloticus and the erally observed with a reduction of feed red hybrid (0. mossambicus x O. ing levels (Bagenal 1969a; Wootton niloticus), respectively, showed a sig 1979; Billard and de Frernont 1980; nificant increase in the production of Springate et al. 1985). Wootton (1982) fry by broodfish fed with a protein-rich indicated, however, that considering the diet. Although there was no direct evi positive relationship between fecundity dence, these authors attributed this and size of females, the effect of feed response to an increase in spawning on fecundity can be difficult to sepa frequency and fecundity due, in turn, rate from that resulting from a simple to a higher weight of female. In a de difference in growth and therefore in tailed study, Wee and Tuan (1988) fish size. For Cichlidae, Mironova (1977) analyzed the reproductive characteris reported that, in Oreochromis tics of 0. niloticus fed ad libitum with mossambicus, a decrease in feeding five isocaloric feed containing 20-50% levels had a limiting effect on growth proteins. They showed that fish fed with and reduced the number of eggs pro a lower or intermediate protein diet (20 duced per spawning, but increased 35%) had a higher fecundity and pro spawning frequency and total number duced smaller eggs than fish fed with of eggs produced. In C. nigrofasciatum, a higher protein diet (42-50%). In ad Townshend and Wootton (1984) also dition, fish receiving a feed relatively observed a decrease in fecundity at the poorer in proteins had a higher spawning lowest feeding levels. In our study, it frequency. These results contrast with is therefore difficult to suggest inad those of studies previously cited, but equate feeding to explain both the low tend to confirm the notion of a com condition and the-high fecundity of the pensation between fecundity and egg 336 size, and conform to the general trend mentioned above (and perhaps others) of our observations. These results could rather than to a single factor. This is also also suggest that natural feed consumed illustrated by the comparison of the re by S. melanotheron in the lagoon or in productive characteristics observed in this the "acadja-enclos" has, in fact, a higher species in the natural environment and protein content than the artificial feed in "acadja-enclos" no. t. In this acadja, (31 %) distributed in the pens. fecundity (for an equal female weight) is Other environmental factors such as significantly higher than in the lagoon (Fig. reduced vital space, increased density 2) while size at first maturity and fish or repeated harvests (in pens) may con condition (weight for length) are similar stitute factors of stress and directly or (Fig. 4). This suggests, on the one hand, indirectly affect the production of eggs. that reduced size at first maturity and high For example, the varying fecundity levels fecundity do not necessarily go together observed between different Sri Lankan and, on the other hand, that these char reservoir populations of 0. mossambicus acteristics may be influenced by differ do not seem to be related to feeding ent proximate factors. levels, but are positively correlated with In aquaculture, early sexual maturity the fishing pressure applied to the and high fecundity are two negative waterbodies (de Silva 1986). characteristics for fish in the grow-out In addition to feeding levels, the or phase because of the resulting prolif ganization of the available vital space eration of fry and the likely stunting constitute the major difference between effects. In contrast, these characteris pens and "acadja-enclos." While pens are tics are positive in tilapia broodfish stock limited areas of open waters, the stacks management as the production of fry of branches or bamboo poles placed in can be optimized. However, as increased "acadja-enclos" multiply the hide-outs and fecundity is negatively related to egg shelters which can have an effect on the size, a negative effect on the fry sur behavior of this territorial species and on vival rate is to be expected. It is well the social interactions among individu known that in the wild, larvae from small als. In this study, since the surface areas eggs are smaller and their chances of covered by "acadja-enclos" and pens are survival are reduced (Bagenal 1969b, similar, the perception that the fish have 1978; Mann and Mills 1979). However, of the available vital space seems to be in culture conditions where fish are more more important than the actual space avail protected, there is no indication of dif able. In Ebrle Lagoon, mangroves or bay ferences in survival rates between lar zones strewn with decomposing branches vae or juveniles produced from eggs constitute the preferred biotopes for S. of different sizes (Billard and de Frernont melanotheron (AI baret, pers. comm.). Since 1980; Springate et al. 1985). In S. the "acadja-enclos" can be viewed as an melanotheron, a high rate of survival intermediate environment between the has been observed in the larvae from natural environment and the pen, it is not the smallest eggs produced by females surprising that the fecundity of this spe cultured in pen. Mortality is low dur cies is also intermediate in this particu ing the mouth-brooding phase (Legendre lar environment. and Trebaol, this vol.) and there is still This discussion implies that the varia about 95% fry survival after four weeks tions observed in the reproductive strat of culture in tanks (Legendre 1983). egy of S. melanotheron are most certainly To conclude, the present results il due to a combination of the different factors lustrate the remarkable flexibility of the 337 reproduction of S. melanotheron under Albaret, J.I. and M. Legendre. 1983. Les especes varying environmental conditions. Im colonisatrices des etangs d 'une station de pisciculture lagunaire en Co te d'lvoire. portant variations are observed for size Description et incidence sur I'elevage. at first maturity, fecundity and egg size Doe. Sci. Cent. Rech. Oceanogr. Abidjan except for spawn weight which remains ORSTOM 14:57-67. constant. Bagenal, LB. 1969a. The relationship between food supply and fecundity in brown trout In intensive farming (using artificial Sa/mo trutta L. I. Fish BioI. 1:167-182. feed), S. melanotheron reaches matu Bagenal, LB. 1969b. Relationship between egg rity at a smaller size and produces more size and fry survival in brown trout Sa/mo eggs and smaller ones than in the wild. trutta L. I. Fish BioI. 1:349-353. Bagenal, LB. 1978. Aspects of fish fecundity, In "acadja-enclos" where conditions are p. 75-101. /n S.D. Gerking (ed.) Ecology intermediate between intensive farm of freshwater fish production. Blackwell ing and the natural environment, an Scientific Publications, Oxford. intermediate situation is also observed Billard, R. and M. de Frernont. 1980. Taux d ' all mentation pendant la g arnetogenese et for fecundity and egg size, the size at performance de reproduction chez la truite first maturity remaining similar to that fario. Bull. Fr. Peche Piscic. 279:49-56. observed in the wild. Chang, S.L., CM. Huang and I.C Liao. 1988. However, taken together, the results The effect of various feeds on seed pro duction by Taiwanese red tilapia, p. 319 of this study are not sufficiently explicit 322. /n R.S.V. Pullin, L Bhukaswan, K. to imply any causal effect between the Tonguthai and I.L. Maclean (eds.) The Second different environmental factors involved International Symposium on Tilapia in and the reproductive characteristics. In Aquaculture. ICLARM Conf. Proc. 15, 623 p. tilapias, knowledge on the nature, role Clsse , A. 1988. Effects of varying protein lev and possible interactions between the els on spawning frequency and growth of external factors (biotic and abiotic) in Sarotherodon me/anotheron, p. 329-333. volved in the control of the different /n R.S.V. Pullin, T. Bhukaswan, K. Tonguthai and J.L. Maclean (eds.) The Second Inter stages of gametogenesis is still gener national Symposium on Tilapia in Aqua ally insufficient. An experimental ap culture. ICLARM Conf. Proc. 15,623 p. proach remains necessary and could de Silva, 5.5. 1986. Reproductive biology of Oreochromis mossambicus populations of greatly contribute to the understand man-made lakes in Sri Lanka: a com para ing of the adaptative strategies devel tive study. Aq uacu It. Fish. Manage. oped by these species. Furthermore, this 17:31-48. approach could also, in time, contrib Elgar, M.A. 1990. Evolutionary compromise be ute to an improvement in breeding prac tween a few large and many small eggs: comparative evidence in teleost fish. Oikos tices through a better control of the 59:283-287. influence these external factors have on Eyeson. K.N. 1983. Stunting and reproduction fecundity, spawning frequency and first in pond-reared Sarotherodon me/anotheron. sexual maturity. Aquaculture 31 :257-267. Fryer. G. and T.D. lies. 1972. The cichlid fishes of the great lakes of Africa. Oliver and Boyd, Edinburgh. 641 p. References Hem, S. 1992. Acadja-enclos : de la p eche de cueillette a. la peche de culture, p. 101-113. /n G.M. Bernacsek and H. Powles (eds.) Albaret, 1.1. 1982. Reproduction et fecondite des Recherches sur les systern es aquacoles en poissons d'eau douce de Cote dIvoire, Rev. Afrique. Workshop. 14-17 November 1988, Hydrobiol. Trop. 15:347-371. Bouake. Cote d'lvoire. IDRC-MR308 ef. Albaret, J.J. 1987. Les peuplements de poissons International Development Research Cen de la Casamance (Senegal) en p erlode de tre, Ottawa, Canada. se ch e r e s se . Rev. Hydrobiol. Trop. Legendre, M. 1983. Observations pretlmlnalres 20:291-310. sur la croissance et le comportement en 338 e levage de Sarotherodon melanotheron Teleostei). Translated from German by D. (Rup pel , 1852) et de Tilapia guineensis Pauly. ICLARM Tran sl , 2, 28 p. (Bf e e k e r , 1862) en lagune Eb r i e (Cote Ruwett , J.C, J. Voss, L. Hanon and J.C Micha. d'lvoire). Doe. Sci. Cent. Rech. Oceanogr, 1976. Biologie et e levage des tilapias, p. Abidjan ORSTOM 14: 1-36. 332-364. In Proceedings of the FAO/CIFA Legendre, M. 1986. Influence de la d en si t e , de Symposium on Aquaculture in Africa, Accra, letevage monosexe et de I'alimentation Ghana. CIFA Tech. Pap. 4. sur la croissance de Tilapia guineensis et Santiago, CB., M.B. Aldaba, LF. Abuan and M.A. de Sarotherodon melanotheron eleves en Laron. 1985. The effects of artificial diets cage-enclos en lagune Ebrle (Cote d'lvoire). on fry production and growth of Oreochromis Rev. Hydrobiol. Trop , 19: 19-29. niloticusbreeders. Aquaculture 47:193-203. Legendre, M. and J.M. Ecoutin. 1989. Suitabil Scott, CB.C 1979. Environmental timing and ity of brackishwater tilapia species from the the control of reproduction in teleost fish. Ivory Coast for lagoon aquaculture. 1 -Re Symp. Zool. Soc. Lond. 44:105-132. production. Aquat. Living Resour Sivalingam, S. 1976. The biology of cultivable 2:71-79. brackishwater and marine finfish in Africa, Legendre, M., S. Hem and A. Cl ss e , 1989. Suit p. 283-291. In Proceedings of the FAO/ ability of brackishwater tilapia species from CIFA Symposium on Aquaculture in Africa, the Ivory Coast for lagoon aquaculture. 2 Accra, Ghana. CIFA Tech. Pap. 4. Growth and rearing methods. Aquat. Liv Springate, J.R.e., N.R. Bromage and P.R.T. ing Resour. 2:81-89. Cumaranatunga. 1985. The effects of dif Lowe-McConnell, R.H. 1982. Tilapias in fish com ferent ration on fecundity and egg quality munities, p. 83-113. In R.S.V. Pullin and in the rainbow trout (Salmo gairdnen) , p. R.H. Lowe-McConnell (eds.) The biology 371-391. In CB. Cowey, A.M. Mackie and and culture of tilapias. ICLARM Conf. Proc. J.G. Bell (eds.) Nutrition and feeding in fish. 7.432p. Proceedings of the Fisheries Society of the Mann, R.H.K. and CA. Mills. 1979. Demographic British Isles. Academic Press, London. aspects of fish fecundity. Symp. Zool. Soc. Townshend, T.J. and R.J. Wootton. 1984. Effects Lond.44:161-177. of food supply on the reproduction of the Mann, R.H.K., CA. Mills and D.T. Crisp. 1984. convict cichlid, Cichlasoma nigrofasciatum. Geographical variation in the life-history J. Fish BioI. 24:91-104. tactics of some species of freshwater fish, Wee, K.L. and N.A. Tuan. 1988. Effects of di p. 171-186.lnG.W. Pottsand R.J. Wootton etary protein level on growth and repro (eds.) Fish reproduction - strategies and duction in Nile tilapia (Oreochromis tactics. Academic Press, London. niloticus), p. 401-410. In R.S.V. Pullin, T. Mironova, N.V. 1977. Energy expenditure on egg Bhukaswan, K. Tonguthai and j.L. Maclean production in young Tilapia mossambica and (eds.) The Second International Symposium the influence of maintenance conditions on on Tilapia in Aquaculture. ICLARM Conf. their reproductive intensity. J. Ichthyol. Proc. 15, 623 p. 17:627-633. Welcomme, R.L. 1972. An evaluation of the Noakes, D.L.G. and LK. Balon. 1982. Life his acadjas method of fishing as practiced in tories of tilapias: an evolutionary perspective, the coastal lagoons of Dahomey (West Af p. 61-82. In R.S.V. Pullin and R.H. Lowe rica). j. Fish BioI. 4:39-55. McConnell (eds.) The biology and culture Wootton, R.j. 1979. Energy costs of egg pro of tilapias. ICLARM Conf. Proe. 7, 432 p. duction and environmental determinants of Pauly, D. 1976. The biology. fishery and poten fecundity in teleost fishes. Symp. ZooI. Soc. tial for aquaculture of Tilapia melanotheron Lond.44:133-159. in a small West African lagoon. Aquaculture Wootton, R.j. 1982. Environmental factors in fish 7:33-49. reproduction, p. 210-219. In CJ.J. Richter Peters, H .M. 1963. Fecundity, egg weight and and J. Th. Goos (eds.) Reproductive physi oocyte development in tilapias (Cichlidae, ology of fish. PUDOC, Wageningen. Legendre Marc, Ecoutin Jean-Marc. (1996). Aspects of the reproductive strategy of Sarotherodon melanotheron : comparison between a natural population (Ebrié Lagoon, Côte d'Ivoire) and different cultured populations. In : Pullin R.S.V. (ed.), Lazard J. (ed.), Legendre Marc (ed.), Amon Kothias J.B. (ed.), Pauly D. (ed.), Lhomme-Binudin C. (trad.) The third international symposium on tilapia in aquaculture. Paris (FRA) ; Makati City : ORSTOM ; ICLARM, (41;1325), 326-338. (ICLARM Conference Proceedings (PHL): ICLARM Contribution ; 41;1325). ISTA III : International Symposium on Tilapia in Aquaculture, 3., Abidjan (CIV), 1996. ISBN 971-8709-42-8