Marc Legendre

Seasonal changes in sexual maturity and fecundity, and HCG-induced breeding of the catfish, Heterobranchus longifilis Val. (Clariidae), reared in Ebrie Lagoon (Ivory Coast)

Abstract Oocyte maturation and ovulation were induced with human chorionic gonadotropin (HCG) in the African catfish, Heterobranchus longifilis . Females with a mean oocyte diameter of at least 1.1 mm were used. 100% ovulation was obtained after a single intramuscular HCG injection of between 1.0 and 2.5 I.U./g body weight; a lower dose led to high variability in individual response. Latency time depended more on temperature than on the hormone dose. Eggs could be stripped within 14 h of a dose of 1.5 I.U./g of HCG, at a temperature of 27 to 29°C. After stripping, most of the eggs were fertilized, and high percentages of normal larvae (76% ± 8) were obtained after hatching. Fish hatched in captivity and becoming sexually mature at 1 year of age (mean weight 1.5 kg) were, in turn, artificially reproduced. Seasonal changes in oocyte diameter (determined by periodic intraovarian biopsy of brooders reared in lagoon enclosures) showed a clear decrease in sexual activity at the beginning of the dry season (December) which seemed to be related to higher water temperature. However, even at that time, good quality eggs were obtained after HCG injection. Thus, eggs and fry could be produced all year round, although the mean quantity of eggs collected per female kg proved to be much less at the beginning of the dry season (26 000 ± 14 000) than during the rainy season (68 000 ± 13 000).

Induced ovulation of Pangasius bocourti (Sauvage, 1880) with a progressive hCG treatment

Abstract The Mekong catfish Pangasius bocourti (Pangasiidae, Siluriforme) has been widely cultured in southern Vietnam in floating cages since 1989 (15,000 tonnes annually). However, the supply of fingerlings has been dependent on catches from the wild, which has led to a reduction in the natural resources. Reproduction of P. bocourti was studied with brooders reared in earthen ponds or in floating cages on the Mekong River. Brooders did not show any sexual dimorphism but females were more developed than males in terms of body weight (+26%) and fork length (+7%). Induction of oocytes maturation and ovulation required a progressive hormonal treatment in two steps. Several daily injections of hCG at a low dose (500 IU kg −1 ) were applied first. These injections induced the development of ovarian follicles, indicated by an increase in their diameter. The second step consisted of two successive hCG injections applied at higher doses (1500 and 2500 IU kg −1 ) at an 8–10-h interval, which induced oocytes maturation, followed by ovulation at 19±3 h after the first injection. Mean ovulation and hatching rates were 66% and 55%, respectively. Ovulation was generally induced once a year for each female although a second ovulation could be obtained for some individuals in the same reproductive season. Fecundity was highly variable, from 400 to 16,700 ova kg −1 , and average fecundity in ponds was twice as high as in cages.

Induced spermiation and milt management in Pangasius bocourti (Sauvage, 1880)

Abstract Males of Pangasius bocourti (3.3–6.9 kg) raised in floating cages on the Mekong River were used to study the production of milt and its use for fertilization. Although spontaneous spermiation occurred in mature males, the average volume of collected milt was low (26.5 μl kg −1 ). A single human chorionic gonadotropin (hCG) injection (2000 IU kg −1 ) induced a 13-fold increase in sperm volume, 12 h after injection (344.5 μl kg −1 ). The associated sperm concentration and production were 26.9×10 9 spz ml −1 and 8.4×10 9 spz kg −1 , respectively. Injection of GnRHa (30 μg kg −1 ) with domperidone (3 mg kg −1 ) induced a lower increase of sperm volume and sperm production. The motility of spermatozoa (spz) in well water lasted only 57 s, but was doubled in low saline solution (NaCl 34 mM). Crude milt could not be stored but five-fold dilution in immobilization solution (NaCl 207 mM) allowed fertilization to be postponed for 36 h. Fertilization was triggered by well water with milt diluted 1000 times.

Evidence of environmental effects on reproductive characteristics of Nile tilapia (Oreochromis niloticus) populations from man-made lakes of Ivory Coast

Abstract Large differences in reproductive traits were observed between populations of Oreochromis niloticus sampled in 9 manmade lakes of Ivory Coast. In order to assess whether these variations in reproductive characteristics resulted from short term adaptation or from a longer evolutionary process, living specimens were caught in the two most differentiated populations in term of life history traits, and placed in a common environment in culture conditions. Genetic analysis of fish from these two populations were performed using four microsatellite markers and revealed that both descended originally from the same strain (Bouake station) which was constituted from broodfish initially caught in the Nile and Volta basins. Fish from the two populations were subjected to a common environment (pond and aquariums) for five months. Then, their reproductive characteristics were analysed and no significant differences were found in fecundity, egg size and spawning frequency. These results indicate that reproductive differences between the two populations, originally observed in the two reservoirs, mostly reflect the phenotypic plasticity of the species in facing different environmental conditions.

Maximum observed length as an indicator of growth rate in tropical fishes

Abstract A positive linear relationship between maximum observed length (MOL) and growth rate is shown for 69 marine, fresh- or brackishwater African fish species in the natural environment and for six cultured species. When accurate data on growth are missing, the MOL appears to be one of the most useful criteria for the preselection of candidate fish species for aquaculture.

Rapid phenotypic changes of reproductive traits in response to experimental modifications of spatial structure in Nile tilapia, Oreochromis niloticus

Abstract The reproductive characteristics of Oreochromis niloticus, a multiple spawner, have been compared between populations submitted experimentally to three types of spatial structure. Fish from a same original stock were divided and reared for 2 months in: 1) concrete tanks with a bare bottom, 2) concrete tanks with a sandy bottom, or 3) concrete tanks equipped with an artificial reef. The females were then killed and their gonads examined for sexual stage and fecundity determination. Fecundity was inversely correlated to the spatial complexity of the environment whereas oocyte size tented to increase together with spatial complexity. Females placed in tanks with artificial reefs presented a lower fecundity and reproductive investment and bigger oocytes than those maintained in the bare bottom tanks. Fish reared in tanks with a sandy bottom had intermediate reproductive traits. The response of females to changes in the spatial structure of their environment was observed after only one or two reproductive cycles, providing the first experimental evidence of such rapid phenotypic adaptation to environmental complexity. The results also indicate that the perception of living space, arising from the availability of refuges and spawning sites, is more likely to influence the reproduction of tilapias than substrate quality.

Peuplements et biomasse zooplanctonique dans des etangs de pisciculture lagunaire (Layo, Côte d'Ivoire)

Abstract Variation of physico-chemical parameters, phytoplankton biomass, and the composition and abundance of zooplankton were studied in brackish water pisciculture ponds after liming and filling. Two successive peaks of abundance were observed for both phyto- and zooplankton, followed by a decrease in biomass which then became more or less stable at lower values. The first zooplankton peak (2–2.5 g DW/m3) was almost exclusively made up of rotifers and it occurred 8–10 days after liming. Crustacea developed later and dominated the second peak of abundance (1.2–1.4 g) occurring 13–20 days after liming. Total biomass then fell and fluctuated around 0.3 g. The distribution in size classes was estimated in terms of numbers and biomasses. It shows that the population of the ponds studied is composed mainly of small organisms in spite of the lack of predators. Results are analysed with regard to possible application in rearing fish larvae.

Assessing aquaculture sustainability: a comparative methodology

Little work dealing with the evaluation of aquaculture system sustainability has so far been undertaken on a global and comparative basis. Moreover, such work is mostly based on very unbalanced approaches in terms of the dimensions of sustainable development that are taken into account. The approach adopted in this article is designed to encompass all the dimensions of sustainability including the institutional one (governance). The taking into account of this latter, in particular, together with the role played by aquaculture in sustainability at the territorial level gives the approach its original and innovative nature. The process of establishing the checklist of sustainability indicators in aquaculture relies on a hierarchical nesting approach which makes it possible to link indicators with general sustainability criteria and principles. At once multidisciplinary and participatory, the approach compares several countries with highly differentiated types of aquaculture system. An original finding from this work is that the technically most intensive farming model scores better than more extensive systems, which might have been thought to be closer to natural systems in their environmental dimension and therefore intuitively more ‘sustainable’. This result suggests relating sustainability outcomes to the level of control and of devolved responsibilities.

Rapid phenotypic changes of reproductive traits in response to experimental modifications of spatial structure in Nile tilapia, Oreochromis niloticusAdaptation phénotypique rapide des caractéristiques de reproduction des tilapias, Oreochromis niloticus, en réponse à des modifications expérimentales de lˈenvironnement.

Abstract The reproductive characteristics of Oreochromis niloticus, a multiple spawner, have been compared between populations submitted experimentally to three types of spatial structure. Fish from a same original stock were divided and reared for 2 months in: 1) concrete tanks with a bare bottom, 2) concrete tanks with a sandy bottom, or 3) concrete tanks equipped with an artificial reef. The females were then killed and their gonads examined for sexual stage and fecundity determination. Fecundity was inversely correlated to the spatial complexity of the environment whereas oocyte size tented to increase together with spatial complexity. Females placed in tanks with artificial reefs presented a lower fecundity and reproductive investment and bigger oocytes than those maintained in the bare bottom tanks. Fish reared in tanks with a sandy bottom had intermediate reproductive traits. The response of females to changes in the spatial structure of their environment was observed after only one or two reproductive cycles, providing the first experimental evidence of such rapid phenotypic adaptation to environmental complexity. The results also indicate that the perception of living space, arising from the availability of refuges and spawning sites, is more likely to influence the reproduction of tilapias than substrate quality.

Evaluation of Aquaculture System Sustainability: A Methodology and Comparative Approaches

Over the last 30 years, aquaculture has experienced an unprecedented development in global animal production with an average yearly growth rate of over 10% between 1980 and 2000 (FAO, 2009). During the same period, capture fisheries saw their progression gradually grind to a standstill and growth stopped from 1995 (total catch fluctuating between 90 and 95 Mt/year according to the year). The growth of aquaculture, despite its benefits and the fact that it is the only way to meet the increase in demand for sea products, evaluated at 270Mt in 2050 (Chevassus au Louis et Lazard, 2009; Wijkstrom, 2003), raises a certain number of issues directly related to its sustainable development. Amongst these are issues related to feed for the farmed organisms, to their biological diversity, to the farms’ economic sustainability, to the impact of aquaculture development on social equity and to the set of arrangements constituting the sector’s governance. Feed, for example, is currently the subject of significant controversy as shown by the emblematic article of Naylor et al. (2000) that exposes the impact on catches of the massive use of fish meal and fish oil in fish and prawn aquaculture and advocates the return to sparser aquaculture systems, directly inspired by traditional Asian systems which use more extensive techniques based on polyculture and fertilisation and where artificial feed is only seen as a potential supplement. This diagnosis, although interesting as it generated much debate, was, however, incomplete and, in fact, inaccurate: by focusing on a single criterion and a single dimension (environmental) of sustainability, the authors were led to make proposals that had no chance of being adopted by the actors. De facto, farming systems have continued to intensify and this has led to a sustained increase in the use of