Exploitation des données sonar, type omnidirectionnel (type SR240)

Le SR240 est un sonar multifaisceaux de longue portée, son utilisation au niveau du traitement des données passe par divers calculs et méthodes qui lui sont spécifiques. La méthode de dépouillement des données n'est pas répétée (cf. rapport précédent), seuls les principaux calculs et méthodes à connaître sont résumés ici (correction de la taille des bancs, erreurs systématiques de taille, réglages, densité, indice de compaction, les biais...). Des résolutions sont à prendre pour les prochaines campagnes VARGET (IRD, ISRA, FLASA) au niveau des réglages acoustiques. Test de la fonction TVG, en 30 log R, validité du paramètres "along beam dimension" : taille des bancs. Des méthodes sont proposés afin de diminuer voir contrôler les différents biais apportés par le sonar. (Résumé d'auteur

International workshop AWA-PREFACE : "Socio economic impact of climate change in West Africa : insight from modeling and questionnaires" [résumés]

International audienc

Efficient multibeam sonar calibration and performance evaluation

Quantitative applications of mobile multibeam sonar in aquatic ecology and fisheries require accurate and efficient in-tank calibration methodologies. Calibration factors for a Simrad SM20 multibeam sonar are experimentally extracted thereby enabling sonar estimation of target strength and volume backscattering strength. Measured and modeled sonar characteristics are systematically compared and show good overall correlation. Due to the limited angular span of the sonar head array, well quantified sonar operation is restricted to an equatorial angular sector of only 80° (vs. rated 120°) in 'imaging' mode. In 'echo-sounder' mode, the 'high' power transmit setting appears to introduce artifacts. A routine in-tank measurement procedure is described which for given multibeam sonar minimizes the time required for quality multibeam calibration

Spatial distribution of main clupeid species in relation to acoustic assessment surveys in the continental shelves of Senegal and The Gambia

This work compiles hydroacoustic recordings and catch data over Senegambia (Senegal and The Gambia) from assessment surveys on the major clupeid species to identify sources of bias in abundance estimates caused by their horizontal distribution. The latitudinal distribution of small pelagic fish is often well known, while their “across shelf” distribution on the continental shelf is less understood. The southern part of the Senegambian shelf has a wide shallow water (<10 m) area that makes up 20% (1500 NM2) of the total shelf surface, while the northern part accounts for 3% (200 NM2). These areas are not assessed by conventional fisheries acoustics surveys and therefore increase the uncertainty of the assessment of these species. Our findings show that this likely introduces a bias in the assessment of Sardinella maderensis, while for S. aurita no major estimation-error is caused by their horizontal distribution. The data confirm that Ethmalosa fimbriata and Ilisha africana are challenging to assess by conventional surveys, due to their mostly inshore distribution. We emphasise the usefulness of assessing S. aurita through fisheries independent hydroacoustic surveys, and propose alternative methods to survey shallow water areas to reduce biases in biomass estimates and distribution mapping

Fine-scale vertical structure of sound-scattering layers over an east border upwelling system and its relationship to pelagic habitat characteristics

Understanding the relationship between sound-scattering layers (SSLs) and pelagic habitat characteristics is a substantial step to apprehend ecosystem dynamics. SSLs are detected on echo sounders representing aggregated marine pelagic organisms. In this study, SSL characteristics of zooplankton and micronekton were identified during an upwelling event in two contrasting areas of the Senegalese continental shelf. Here a cold upwelling-influenced inshore area was sharply separated by a strong thermal boundary from a deeper, warmer, stratified offshore area. Mean SSL thickness and SSL vertical depth increased with the shelf depth. The thickest and deepest SSLs were observed in the offshore part of the shelf. Hence, zooplankton and micronekton seem to occur more frequently in stratified water conditions rather than in fresh upwelled water. Diel vertical and horizontal migrations of SSLs were observed in the study area. Diel period and physicochemical water characteristics influenced SSL depth and SSL thickness. Although chlorophyll-a concentration insignificantly affected SSL characteristics, the peak of chlorophyll a was always located above or in the middle of the SSLs, regularly matching with the peak of SSL biomass. Such observations indicate trophic relationships, suggesting SSLs to be mainly composed of phytoplanktivorous zooplankton and micronekton. Despite local hypoxia, below 30 m depth, distribution patterns of SSLs indicate no vertical migration boundary. The results increase the understanding of the spatial organization of mid-trophic species and migration patterns of zooplankton and micronekton, and they will also improve dispersal models for organisms in upwelling regions.publishedVersio

Insight from PREFACE & AWA on Tropical Atlantic Tuna ecology and effects on western African fisheries economies

International audienc

Book of Abstracts submitted to the IV Congress of Marine Sciences

Sardinella aurita, or round sardinella, has been the main small pelagic fish species harvested off Senegal and Mauritania coasts over the last 4 decades and plays a central role for sub-regional food security and economic incomes. The landings of this species have strongly declined recently. Intense exploitation and climate change are acting together but population&#8217;s dynamics are too poorly understood to disantangle the different drivers and to clearly evaluate the current state of S. aurita&#8217;s population. In the present study, we developed a bio-physical, individual based model for S. aurita population off North-West Africa. The hydrodynamic environment was simulated by a regional model ("ROMS") configuration covering the area 5°-40°N and 5°-30°W, with a 8km resolution and 32 sigma-levels in our area of interest. The biogeochemical compartments were simulated using the PISCES model coupled with ROMS. Fish schools of S. aurita were represented by active lagrangian markers affected with ad hoc larval, juvenile and adult fish swimming behavior. Individual&#8217;s physiology was described following the local temperature and food availability by the Dynamic Energy Budget model "DEB". The extended kinesis algorithm ruled the horizontal fish movement and depends on food research, individual temperature preference and spawning migration, whereas fish vertical position in the water column was set for each stage according to scientific knowledge. We investigate the predicted seasonal migrations pattern of S. aurita off West Africa over the period 1980-2006. Then we also evaluate the fluctuations of fish biomass available for coastal fisheries (h<200m) in 4 distinct areas from the western Saharan bank (24°N) down to the Bijagos islands (11°N) and compare with the available fish landings data on this period. Finally, we calculate seasonal connectivity indices between the populations in the 4 areas selected as the percentage of fish present in an area that was born in another area

Importance de la zone côtière dans les évaluations des stocks de petits poissons pélagiques : analyse d'une série de campagnes acoustiques et d'une expérimentation en zone côtière

International audienc

Three dimensional fish school underwater scene (3D)

Fish schools are aggregative structures encountered in all types of aquatic environments but have as yet been little studied in freshwaters except at small spatial scales. This study represents the three dimensional description of juvenile fish schools (Perca fluviatilis and Rutilus rutilus) in a lake environment using high resolution multibeam sonar system operating at a frequency of 455 kHz, composed of 60 beams of 1.5° allowing a 90° observation plane. The in situ diurnal schooling behaviour of young-of-the-year fish of both species is confirmed. The morphological, energetic and spatial variables of these schools are described and related to one another. The structures described are of the same order of magnitude as those described in the marine environment. The school shape is elliptical, they are shallow and they display a temporal and spatial stability over the course of a day but a highly variable morphology. The number of vacuoles, a descriptor of the internal morphology of the schools, was found to be significantly (p < 0.01) correlated with the volume of the school, and showed two distinct relationships, with proportions similar to the percentage occurrence of both species sampled by a pelagic trawl. The relation may be efficient for fish species discrimination by 3-D acoustics methods in this lake with two main aggregative fish species