Les espaces de l'halieutique

Le poulpe commun (#Octopus vulgaris) est très abondant au large des côtes NO-africaines où il est exploité à grande échelle. En Mauritanie, il représente la principale ressource halieutique grâce à la production de la zone de pêche du Cap-Blanc. Les caractéristiques globales de répartition spatio-temporelle de la population d'#Octopus vulgaris présent dans cette zone (20°30 N, nord de la Mauritanie) sont décrites à partir de données de chalutages scientifiques réalisés au cours de 24 campagnes mensuelles effectuées de mai 1993 à avril 1995. On constate que des poulpes de différents sexes, tailles (âges) et stades de maturité sont présents, quel que soit le mois et quelle que soit la profondeur (entre 10 et 100 m). Des phénomènes cycliques existent néanmoins à l'échelle de cette population. Ils se manifestent autant par des variations saisonnières et bathymétriques dans les proportions des différentes composantes précitées que par leurs variations d'abondances absolues. Ainsi le recrutement, qui est continuel, est structuré par deux pics principaux par an (juin et période autonmnale) de même que la ponte (septembre et période hivernale). A part cela, on ne retrouve pas en Mauritanie la trace de migrations génésiques saisonnières telles que décrites en zone tempérée pour cette même espèce. Le schéma observé ici est plutôt celui d'une stratégie d'occupation permanente du milieu, et en particulier des différentes strates bathymétriques, par les différentes composantes de la population. (Résumé d'auteur

Typologies des entomocénoses benthiques soumises à des épandages d'insecticides antisimulidiens

During the tests in gutters, five insecticides (B.t., Abate, chlorphoxin, carbosulfan and permethrin) employed by the Onchocerciasis Control Programme (OCP) have neither the same toxicity nor the same selectivity against for the principal aquatic taxa. In long run, the utilization of these insecticides led to dry season entomocenose structures characteristic for each product with differ from those observed during the pretreatment period. More the insecticides are estimed toxic in the gutter tests, more the long run typologies differ from the reference ones. The differences are slight for B.t., intermediate for Abate and of stronger magnitude for carbosulfan, chlorphoxim or permethrin. The populations of the taxa react differently to the insecticides; some stay the same or are reduced while others increase, due certainly to their low susceptibility, the shortness of their life cycle or the diminution of the spatial and trophic competition. (Résumé d'auteur

Homologie d'évolution de peuplements benthiques soumis aux épandages d'insecticides antisimulidiens

L'utilisation hebdomadaire d'insecticides pour lutter contre les Simulies modifie l`abondance des populations benthiques. Les variations ne sont ni de la même ampleur ni du même signe, selon l'insecticide (Abate, chlorphoxime, B.t.) et les taxons.Parmi les dizaines de rivières savanicoles traitées aux insecticides antisimulidiens, seules quelques unes font l'objet d'une surveillance de la faune non-cible. La problématique de ce travail est de savoir s'il est possible de généraliser à l'ensemble des rivières, les fluctuations d'abondance des populations observées sur une station de référence.Sur la station de référence, l'évolution de l'abondance des populations pour les trois insecticides étudiés est mesurée à l'aide du rapport : R/P = log APTi/log AVT, où log APTi est le logarithme de l'effectif moyen pour la période de traitement à l'insecticide i et où log AVT est le logarithme de l'effectif moyen avant traitement. Pour chaque taxon ainsi que pour la faune totale on obtient donc un rapport Abate, un rapport chlorphoxime et un rapport B.t.Les valeurs de ces rapports obtenus pour la station d'Entomokro, sont ensuite appliquées aux effectifs récoltés avant traitement sur trois autres stations. Les valeurs calculées sont comparées aux valeurs observées dans le cadre de la surveillance des rivières.Pour la faune totale, l'écart entre les valeurs calculées et les valeurs observées est très faible, r = 0,98.L'étude par taxon montre que la prédiction de ce modèle est excellente pour les Caenidae et les Hydropsychidae, bonne pour les Baetidae, les Chironomini, les Tricorythidae et les Orthocladiinae, médiocre pour les Tanypodinae et les Tanytarsini.The weekly utilization of insecticides sprayed by the Onchocerciasis Control Programme to control Simulium damnosum larvae modifies the abundance of the non target benthic populations. These insecticides (B.t., Abate and chlorphoxim) have neither the same toxicity nor the same selectivity for the principal taxa. In consequence, the long-term variations of the abundance of the populations are neither of the same importance nor of the same sign (some of them remain the same or decrease white others increase), according to the product and the taxa. In long term, these variations being mainly the consequence of direct toxicity but also take into account the duration of the life cycles as well as spatial and trophic competitions.Iin this study, only the saxicolous fauna is taken into account. If has been collected on rocks with the Surber sampler during the low water period (December to March).Among the ten Savannah rivers treated with antiblackfly insecticides, only few are the object of a non-target fauna monitoring. The abject of this work is to land out if it is possible to generalize to all the rivers, the variations in population abundance observed for an insecticide on a control station.In the reference station, the variation of population abundance for each of the insecticides is calculated by means of the ratio R/P = log APTi/log AVT, where log APTi is the logarithm of the average for the period treated with the i insecticide, and where log AVT is the logarithm of the average before treatment. For each taxa as well as for the total fauna, Abate, chlorphoxim and B.t. ratio were defined.The values of these ratio obtained for the Entomokro station on the Marahoué river in the Ivory Coast are then applied to the data collected before treatment on three rivers or stations (Amou-Oblo on the Amou river in Togo, Asubende on the Pru river in Toge and Danangoro on the Marahoué river in the Ivory coast). The calculated values are then compared to the values collected after treatment in the river monitoring programme.In the reference station the ratio of the populations collected before and after treatment are sometimes greater, sometimes lower than unit (when the ratio is greater than unit, it means that there is an increase of the abundance of populations during the processing period).The ratios are very close to the one for the B.t. which is the less toxic insecticide against the non target fauna, the greatest for the chlorphoxim which is the more toxic product and intermediate for Abate which has an intermediate toxicity. It appears that the more insecticides were estimated to be toxic in gutter tests, the more population abundance differ tram the reference ones.For the total fauna, differences between the values calculated and observed were very low (r 0,98). For the taxa, the prediction of this model is excellent for Caenidae and Hydropsychidae, good for Baetidae, Chironomini, Tricorythidae and Orthocladiinae, but mediocre for Tanypodinae and Tanylarsini. The authors submit some hypotheses to explain the bad adequation of the model for these two taxa; hypotheses based on taxonomic problems and niche releasing

Anomalies de surface et pêche thonière: SPOT et la détection de zones de pêche par l'inventaire des hauts-fonds

Par une analyse théorique et quelques exemples pratiques, il est montré que, dans une zone de pêche des thonidés, la présence de hauts-fonds est un facteur d'amélioration des rendements, et que, par l'utilisation des caractéristiques spécifiques de l'imagerie SPOT et par un traitement simple, la télédétection des hauts-fonds est réalisable. Une nouvelle stratégie d'exploitation de cette imagerie est proposée

Spatio-temporal variability of fish assemblages in the Gambia estuary (West Africa) observed by two vertical hydroacoustic methods : moored and mobile sampling

indexedccInternational audienceThe Gambia Estuary is one of the last estuaries in West Africa not to have been affected by strong environmental changes and adverse human disturbances. In order to describe the spatio-temporal change in fish biomass in this estuary, and the seasonal changes in fish size classes, five research surveys have been conducted at characteristic periods in the river's water regime and meteorological seasons. The fish assemblages were sampled by two vertical echosounding protocols: mobile surveys (zigzag transects) in the estuary and moored stations. The two sampling methods produced a similar image of the fish assemblages in terms of the distribution of total biomass and target strength distributions. Large fish aggregations were detected in the most marine area at the end of the wet season, and fish biomass increased through the dry season in the upstream zone. Fish were smaller at the beginning of the wet season and they were always larger in downstream areas

Нові підходи в керуванні процесами електронно-променевого зварювання

The Gambia Estuary, a “normal” estuary with a decreasing salinity gradient from the mouth towards the head, is moderately exploited by small-scale fisheries and does not receive any severe pollution from either agriculture or industrial activities. Neither the Gambia River nor its estuary are artificially impounded. As the last large West African estuary free of major human disturbance, it is of considerable interest for comparative studies on the effect of major environmental perturbations in West African estuarine ecosystems. The aquatic environment and fish communities of the Gambia Estuary (about 250 km long) were studied by purse seine sampling at different periods in the river cycle, covering all hydro-climatic seasons that are characteristic of West African estuaries. Emphasis was placed on the diversity, composition, structure and distribution of fish assemblages in relation to fluctuations in physico-chemical factors such as water temperature, salinity and turbidity. Results on the aquatic environment, mainly the salinity range (from freshwater to 39) and dissolved oxygen (never a limiting factor for fish in the estuary) and on the main characteristics of the fish fauna (high diversity of life cycles, all the ecological categories represented) indicated that the Gambia Estuary was free of major climatic perturbation and reinforced the choice of this system as a reference for the study of the effects of major perturbations on estuarine tropical fish communities