G rowth and ontogeny The inland water fishes of Africa G rowth is one of the most complex processes for an organism . On the metabolic level, part of the energy consumed will be devoted to increasing its weight, but the proportion of energy used to generate living matter depends on the age of the individuals, their physiological state, their environmental conditions, etc. Firs t stages of deve lopme nt Little is know n about the first stages of development in Af rican fishes. A review of literature shows th at data is only available for 18 of 74 ident ifie d fam ilies (Carnbrav & Teugels. 1988). ONTOGENY AND MAIN STAGES OF DEVELOPMENT Ontogeny is the process of differentiation • the juvenile period begins w hen the fins of the diff erent stages of development are we ll-diffe rentiated and w hen all temporary in the life of an organism. We usually distinguish organs are replaced by final organs. several periods in the life of a fish. This stage ends w ith the first maturation (BaIon, 1981, 1984 and 1986): of gametes. This is usually a period of rapid • the embryonic period wh ich begins w ith growth sometimes characterized by a specific fertilization and is characterized by exclusively colouration; endogenous nutrition from the egg yolk; • the adult period begins w ith the first • the larval period w hich begins with maturation of gametes. the progressive but rapid transition from It is characterized by a decrease in somatic an endogenous food supply to exogenous grow th rate; feeding. This period is characterized by • finally, there is sometimes a period the presence of tem porary larval organisms; of senescence. Growt h and ontogeny CHRISTIA ~ LEVEOUE Salon (1 985; 1990) distinguishes two broad types of ontog enic trajectories. In the indirect development or "altricial" model, eggs are generally small and produced in large numbers. They yield small, underd eveloped young larvae with only a small volume of yolk that is not suff icient for producing the final phenotype. These young larvae mu st feed rapidly on sma ll partic les to complete their develop ment and are highly vulne rable durin g this period. The Alestidae Alestes baremoze is a good example of t his type of fis h (Durand & Loubens, 1971) (figure 11.1), as are Clarias gariepinus (Bruton, 1979a) and Heterobranchus longifilis. 25 .,--------------------------- -------- -, 20 !APparition of the anal fin raysl < '» Apparition of fi ns caudal--->dorsal--->anal vent ral·->adipose o 10 15 20 25 30 35 Days f lCiU Rf.11.1.__ Early stages of development of Alestes baremoze in the Chad basin Development of the terminal part of the spine (12-24 days) (from Durand & Loubens, 19711. In the direct developm ent ("precocial") model, fishes produce a limite d number of large eggs w ith a large amount of yolk, w hich allows the embryo to develop to an advanced stage. This shortens or eliminates the larval period, and juveniles are already w ell-formed and thus less vulnerable w hen they start seeking an external diet. Labeotropheus, a Cichlidae mouth-brooder from Lake M alawi, is a good examp le of this type of fish that releases a large juvenile (14% of adult size) only 31 days after fertilization (Salon, 1977) (see box " First stages of development in Labeotropheus") . Cyphotilapia frontosa illustrates an even mo re advanced style, as embr yos already start consuming an external diet even while in the buccal cavity w here they still have a yolk reserve (Salon, 1985). The inland water fishes of Africa FIRST STAGES OF DEVELOPMENT IN LASEOTROPHEUS (FROM BA LON, 1977) Lebeotroptieus is an endemi c The eggs are incubated for 6 days before Cichlidae in Lake Malawi. hatching. At birth, juveniles have a large yolk It is a mouth-brooder for eggs sac and stay in the oral cavity w here they and juveniles. develop and acquire their main morphological Just after spaw ning, 30 to 50 oval-shaped eggs structures (skeleton, fins, etc.). The yolk sac (3 mm in diameter and 4.4 mm long) is absorbed 21-22 days afte r fertilization, are immediately collected in the mouth but embryos stay in the oral cavity of the female by females. up to the 31st day. At this age, they are very They are fertilized in the oral cavity we ll-formed, autonomous, large (approximately of the female w hich is almost completely filled 15 mm , i.e., 14% of the adult size) and begin by eggs. their exogenous foraging. There are, of course, interme diate modes between th e tw o broad styles discuss ed above. Pollim yrus isidoti (table 11.1) is an example . IABL£.1t.l Compa rative characteristics in the development of some African fish species. References. 1 Durand & Loubens, 1971; 2: Cambray, 1985; 3: Bruton, 1979; 4: Legendre & Teugels, 1991; 5 Kirs hbaum , 1987; 6: Baton, 1977; 7: Balon, 1985. Gonado-somatic index (GSI) Altricial de velopment Intermediate Pr ecocial developm ent de velopment Species Alestes Barbu s Clarias Hetrobranchus Pollimyrus Labeotropheus Cyphotilap ia baremote tre velyani gariepinus longifilis isidori lrewavasat' [ron tosa References 2 4 6 Egg size (mm) 1-1.3 1.5 1.6- 1.9 1.5- 1.8 2 3 x 4.3 5.6x4.0 Egg numbe r x 10,000 x 10,000 x 100,000 120 30-50 GS[ 9% 7- 10 % 20-23 % Parental care no no no nest protected mouth-brooder mouth-brooder (male) (female) (female) Embryonic development < J day 2.8 days 1 day 1 day 3 to 4 days 6 days 5 days Exogenous diet 5 days 3-4 days 3 days 15 days 25-30 days 14 days Size (mm) 6 0101 7 0101 6.20101 10mmTL 8 mm [5 mm 15mm Disappearance of the yolk sac 12 days 11.5 days 3 days 3-4 days 14 da ys 2 1 days Size (mm) 7 108 7. 1 62 8 13-14 End of larval lime 30 days 50-60 days 14 day s ? 40 days feedi ng Size 17.5 mm 20 mm 12.1 mm 15mm oral cavil)' Juvenile freedom 31 days 54 days Size 15 mm 23 nun LT Growth and ontog eny CHRISTIAN L F. VEQUE Growth estimation Growth can be estimated using changes in size or biomass in a given time interval. It corresponds to the organism 's energy intake that is not used for maintaining metabolism . There is a great deal of literature on growth estimation me thods, and researchers agree that it is difficult to determine the age of trop ical fish. Marks on bony structures are harder to read and interpret than on temperate fishes (Meron a et et.. 1988). Nonetheless, cert ain investigations have shown that growth marks generally coincide with drops in temperature, which often occur during low water periods. In some cases, growth marks can be associated with gonad maturation which generally follows a long period of fasting. Formation of marks on bony structures is thus the result of various physiological disturbances, and it is necessary to determine which ones are at play for a given species. Determination of growth and age were long considered key elements for models of fish stock dynamics. As a result. a large numb er of growth curves were calculated for African fishes (Me rona et et., 1988). but most did not atte mpt to account for the relationships between grow th and othe r biological parameters. Consequently, many of these results are of limited use to unders tand species biology. Growth models Annual growth can be described as an asymptot ic curve, and the most w idely used is von Bertalanffy's model (1938). It is based on bioenerget ic principle s, with the hypothesis that growth rate is equal to the difference between the anabolic rate and the catabolic rate. The von Bertalanffy model is expres sed as the equation : Lt = Loo ll- e-K(Holl w here Loo is the asymp totic length, Ltfish lengt h at age t. K is a constant that describes the rate of growt h, and to is the hypothetical time at w hich size is zero. Similarly, fish weight at time t is given by the equation : Wt = W oo ll-e-K(Holj Wh ile von Bertalanffy 's mode l provides us with a smooth annual growth curve, growt h is not a cont inuous proce ss. At times there are long periods in a year during wh ich grow t h stops or is sharply reduced .In other words, growth can take place during rather short periods. For instance, 75% of the growth of young Oreochromis andersoni and 0. macrochir in the Kafue River occurs during the six weeks of peak water levels (Dudley, 1972). During the dry season, growth is very slow but does not stop (Kapetsky, 1974). Welcomme & Hagborg (1 977) proposed a growth model for fishes in flood zones, w hich takes into account a rapid init ial increase in length fo llowed by a slower ~ growth period. In the formula L1+1' = l., + G (et'l. the values of for successive years are the same as the ones provided by the von Bertalanffy model, but growth during the year is calculated for each w eek. This gives a growth curve that is a more accurate representation of what happens on the field . The inland water fishes of Africa Grow th in length and w eight gain are not alwa ys correlated. W eight gain includes th e creation of fat reserves, for instance, or gonad developme nt.