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

Approche acoustique de la dynamique et la distribution spatiale des ressources halieutiques de petits pélagiques dans l'upwelling sénégalo-mauritanien

Along the coast of North-West Africa, among the richest waters over the word, fish supply is important at both socio-economic and cultural levels. In this region, the small pelagic fish are the most abundant marine resources and are monitored using hydroacoustic stock assessment methods since more than three decades. These surveys have allowed providing to local authorities a sound scientific basis for deciding about the management of pelagic fisheries. A specific challenge is to know the representativity of these surveys with respect to the stocks targeted. In this work, we confirm this representativity for Sardinella aurita stock while for S. maderensis, a part of the stock could have not been sampled and for pour Ilisha africana and Ethmalosa fimbriata these surveys are not the best tool. Furthermore, Northward shifts in the distribution of sardinella in the Canary Current Large Marine Ecosystem are recorded in this work and have been attributed to the redistribution of upwelling intensity and productivity, resulting in the abundance of sardinella decreasing in the inter-tropical coastal region. Here for the first time using independent observational time series, we report a robust northward shift in S. aurita since 1995 and attribute it to the strong ocean warming in the region. The observed spatial shifts in biomass in the last 20 years are of the same order of magnitude as those recorded for surface isotherms. Such changes will impact policy considerations in the management of food security in several West African countries and should be considered with respect to “Intended Nationally Determined Contributions”.Le long des côtes nord-ouest africaines, parmi les plus poissonneuses au monde, les ressources halieutiques sont importantes tant du point de vue socio-économique que culturel. Les petits poissons pélagiques sont suivis au niveau sous régional depuis plus de trois décennies en s’appuyant sur les méthodes hydroacoustiques d’évaluation de stocks. Ces campagnes ont ainsi permis de fournir aux gouvernants une base scientifique en matière de prise de décision à des fins d’aménagement des pêcheries pélagiques. Un défi spécifique est de connaitre leur représentativité par rapport aux stocks ciblés. Nous rassurons sur cette représentativité sur la couverture de Sardinella aurita par ces campagnes. Par contre, pour S. maderensis une partie du stock ne serait pas échantillonnée et la méthode n’est pas adaptée pour Illisha africana et Ethmalosa fimbriata. Nous reportons des déplacements vers le nord dans la répartition de S. aurita et d’autres espèces de pélagique, attribués à la redistribution de l'intensité et de la productivité des upwellings [découpé en 5 sous zones du sud Maroc au sud Sénégal], diminuant ainsi l’abondance de la sardinelle dans la région côtière intertropicale. Pour la première fois en utilisant des séries chronologiques d'observation indépendantes à la méso-échelle, nous montrons un changement vers le nord dans la distribution de S. aurita depuis 1995 et l'attribuons au fort réchauffement de l'océan dans la région. De plus les changements spatiaux observés dans la biomasse de S. aurita au cours des 20 dernières années sont du même ordre de grandeur que ceux enregistrés pour les isothermes de surface. De tels changements auront une incidence sur les considérations politiques dans la gestion de la sécurité alimentaire dans plusieurs pays d'Afrique de l'Ouest

Monitoring Senegalese small pelagic stocks: current state of the resource, variability and levels of exploitation [résumé]

International audienceIn Senegal, the coastal pelagic resources occupy a leading place in the landing and are mainly constituted by both sardinella species, horse mackerels, mackerels and ethmalosa. On average, they represent about 70 to 80 % of the global annual catch. The exploitation of these resources is mainly done by the small-scale fisheries. The pelagic resources follow a migratory pattern in the South part of the Canary Current Large Marine Ecosystem area located between the South Senegal and South Morocco. It is thus a shared fish resource and their management is made at a sub-regional level within the framework of the Workgroup Fishery Committee for the Eastern Central Atlantic (CECAF) under the supervision of the FAO. The current state of these resources in the light of our last annual acoustic assessment survey led in January, 2015 on Senegalese continental shelf, on the framework of the Preface project is presented in this work as well as the historic series of the biomasses estimated by the FRV Fridtjof Nansen vessel in Senegal over the last twenty years. All the acoustics surveys carry out was completed with physical measurement with CTD probe (temperature and salinity profile) and sometimes ADCP (current). At the sub-regional level, the last management measures delivered by the Workgroup CECAF are presented and state an overexploitation of the main Senegalese stocks of small pelagics i.e. Sardinella, horse mackerels, mackerel, anchovy and ethmalosa, with the exception of the sardine, the biomasses of which are not significant in Senegal until now. This work led independently by Senegal allow to discuss the effect of the departure of the last industrial foreign fisheries in 2012 which targeted small pelagics and also the potential effect of climate change as the 2015 survey complete the Nansen series, stopped in 2006, to get an 20 years overview.Au Sénégal, les ressources pélagiques côtières occupent une place prépondérante dans les débarquements et sont principalement constituées par les deux espèces de sardinelles, les chinchards, les maquereaux et les ethmalosa. En moyenne, elles représentent environ 70 à 80 % des captures annuelles globales. L'exploitation de ces ressources est principalement le fait de la pêche artisanale. Les ressources pélagiques suivent un schéma migratoire dans la partie sud de la zone du grand écosystème marin du courant des Canaries, située entre le sud du Sénégal et le sud du Maroc. Il s'agit donc d'une ressource halieutique partagée et leur gestion se fait au niveau sous-régional dans le cadre du Groupe de Travail Comité des Pêches pour l'Atlantique Centre-Est (CECAF) sous la supervision de la FAO. L'état actuel de ces ressources à la lumière de notre dernière campagne annuelle d'évaluation acoustique menée en janvier 2015 sur le plateau continental sénégalais, dans le cadre du projet Préface, est présenté dans ce travail ainsi que la série historique des biomasses estimées par le navire FRV Fridtjof Nansen au Sénégal au cours des vingt dernières années. Toutes les études acoustiques réalisées ont été complétées par des mesures physiques avec des sondes CTD (profil de température et de salinité) et parfois ADCP (courant). Au niveau sous-régional, les dernières mesures de gestion délivrées par le Groupe de travail COPACE sont présentées et font état d'une surexploitation des principaux stocks sénégalais de petits pélagiques à savoir la sardinelle, le chinchard, le maquereau, l'anchois et l'ethmalosa, à l'exception de la sardine dont les biomasses ne sont pas significatives au Sénégal jusqu'à présent. Ce travail mené indépendamment par le Sénégal permet de discuter l'effet du départ des dernières pêcheries industrielles étrangères en 2012 qui ciblaient les petits pélagiques et aussi l'effet potentiel du changement climatique puisque l'étude de 2015 complète la série Nansen, arrêtée en 2006, pour avoir une vue d'ensemble sur 20 ans

Monitoring Senegalese small pelagic stocks: current state of the resource, variability and levels of exploitation [résumé]

International audienceIn Senegal, the coastal pelagic resources occupy a leading place in the landing and are mainly constituted by both sardinella species, horse mackerels, mackerels and ethmalosa. On average, they represent about 70 to 80 % of the global annual catch. The exploitation of these resources is mainly done by the small-scale fisheries. The pelagic resources follow a migratory pattern in the South part of the Canary Current Large Marine Ecosystem area located between the South Senegal and South Morocco. It is thus a shared fish resource and their management is made at a sub-regional level within the framework of the Workgroup Fishery Committee for the Eastern Central Atlantic (CECAF) under the supervision of the FAO. The current state of these resources in the light of our last annual acoustic assessment survey led in January, 2015 on Senegalese continental shelf, on the framework of the Preface project is presented in this work as well as the historic series of the biomasses estimated by the FRV Fridtjof Nansen vessel in Senegal over the last twenty years. All the acoustics surveys carry out was completed with physical measurement with CTD probe (temperature and salinity profile) and sometimes ADCP (current). At the sub-regional level, the last management measures delivered by the Workgroup CECAF are presented and state an overexploitation of the main Senegalese stocks of small pelagics i.e. Sardinella, horse mackerels, mackerel, anchovy and ethmalosa, with the exception of the sardine, the biomasses of which are not significant in Senegal until now. This work led independently by Senegal allow to discuss the effect of the departure of the last industrial foreign fisheries in 2012 which targeted small pelagics and also the potential effect of climate change as the 2015 survey complete the Nansen series, stopped in 2006, to get an 20 years overview.Au Sénégal, les ressources pélagiques côtières occupent une place prépondérante dans les débarquements et sont principalement constituées par les deux espèces de sardinelles, les chinchards, les maquereaux et les ethmalosa. En moyenne, elles représentent environ 70 à 80 % des captures annuelles globales. L'exploitation de ces ressources est principalement le fait de la pêche artisanale. Les ressources pélagiques suivent un schéma migratoire dans la partie sud de la zone du grand écosystème marin du courant des Canaries, située entre le sud du Sénégal et le sud du Maroc. Il s'agit donc d'une ressource halieutique partagée et leur gestion se fait au niveau sous-régional dans le cadre du Groupe de Travail Comité des Pêches pour l'Atlantique Centre-Est (CECAF) sous la supervision de la FAO. L'état actuel de ces ressources à la lumière de notre dernière campagne annuelle d'évaluation acoustique menée en janvier 2015 sur le plateau continental sénégalais, dans le cadre du projet Préface, est présenté dans ce travail ainsi que la série historique des biomasses estimées par le navire FRV Fridtjof Nansen au Sénégal au cours des vingt dernières années. Toutes les études acoustiques réalisées ont été complétées par des mesures physiques avec des sondes CTD (profil de température et de salinité) et parfois ADCP (courant). Au niveau sous-régional, les dernières mesures de gestion délivrées par le Groupe de travail COPACE sont présentées et font état d'une surexploitation des principaux stocks sénégalais de petits pélagiques à savoir la sardinelle, le chinchard, le maquereau, l'anchois et l'ethmalosa, à l'exception de la sardine dont les biomasses ne sont pas significatives au Sénégal jusqu'à présent. Ce travail mené indépendamment par le Sénégal permet de discuter l'effet du départ des dernières pêcheries industrielles étrangères en 2012 qui ciblaient les petits pélagiques et aussi l'effet potentiel du changement climatique puisque l'étude de 2015 complète la série Nansen, arrêtée en 2006, pour avoir une vue d'ensemble sur 20 ans

Vessel avoidance response: A complex tradeoff between fish multisensory integration and environmental variables

International audienceThe avoidance reaction by fish in front of an approaching vessel is a major source of bias in direct biomass assessment and ecological studies based on fisheries acoustics data. An experiment was carried out to compare echosounder data obtained using a small speedboat and a research fisheries vessel generating significant higher noise above conventional reduced-noise standard. The results show that there was no significant difference between the individual fish target strength distributions, and the numbers of schools recorded by both boats, these schools having similar areas and perimeters. However, the schools detected by the noisier vessel were significantly deeper, and unexpectedly had a significantly higher energy level. These findings suggest that noise-reduced vessels trigger a different vessel avoidance reaction. The noise-reduction standard is not sufficient to reduce avoidance behavior. It is also to take into consideration the ambient noise, which could impair perception of the platform by the fish, and the probability that the acoustic stimuli could be less important than visual perception under some local conditions. The paper introduces the concept of partial avoidance and presents a conceptual diagram of the strength of the avoidance reaction. Last, it is not recommended, because of noise reasons, that vessels routinely used for pelagic stock assessment surveys be changed. Indeed standardized time series, which could be disrupted when switching to a new vessel, are more important than the hypothetical gain from change to quieter vessels. Obviously, all long-term surveys must change vessels; best practice will be to estimate the vessel effect before any change to avoid disrupting the time series and/or perform vessel intercalibration surveys

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

Matecho: An Open-Source Tool for Processing Fisheries Acoustics Data

WOS:000442525800006International audienceMatecho is an automated processing method to extract information and perform echo-integration and fish shoal extraction from various scientific echo-sounder sources providing digital acoustic data on fisheries and aquatic ecosystem. The open-source initiative helps foster collaboration and technological transfer. Matecho supports various formats, such as the international standard format for the exchange of fisheries acoustics raw and edited data. The procedure allows the semiautomatic cleaning of echogram data and the application of automatic data filters, i.e., transient noise, attenuated signal and impulsive noise removal and background noise reduction. Echo-integration processing is executed for each depth layer and integrates their characteristics per elementary sampling unit. Sound scattered layers are automatically detected by segmentation from the echo-integrated echogram, and shoals are extracted according to an iterative process of aggregation of filtered echogram echoes that allows, in both cases, the calculation of the ad hoc parameters describing morphological, spatial location and acoustic characteristics of sound scattered layers and shoals. Matecho is open-source software for researchers and provides end users with a user-friendly, free executable program

Climate change and seasonality of small pelagics : impacts on their value chain in Senegal [résumé]

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018Factors influencing landed prices and those of processed products (fish and processed) are mainly landed quantities and product quality. The latter are in turn influenced by the seasonality of sardinella conditioned by the temperature of the water. It is usually in cooroon periods (April to June) that sardinella landings are more important and it is at this time also that the prices of fresh fish and processed products are the lowest. On the other hand sardinella is rare in lolli most often (October to December) and it is at this period also that the prices are generally higher. Nevertheless, depending on whether one is on the Petite Côte or on the Grande Côte, fishermen's appreciations of the periods of abundance of sardinellas differ. However, from the analysis of fishermen's knowledge on sardinella migration and bioecological models, it appears that sardinella are present on Senegalese coasts during periods of low temperature. As the cold water periods are later and shorter, the sardinella will be increasingly rare on the coast in Senegal, its higher price, accessibility is more difficult and consequently the animal protein deficit of the populations more accentuated

Spatial Environmental trends in the three Atlantic African Large Marine Ecosystems in a context of global warming

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018In Atlantic Africa there is a crucial need to better assess the effect of climate change on marine ecosystems, particularly over the continental shelf and inside the national exclusive economic zones. Nevertheless there is a lack of observation carried out in the African ecosystems and the times series are often short or disrupted. Space-based observations allow precise synoptic observation of marine ecosystem and is often use to monitor, e.g., Eastern boundary upwelling ecosystems, from 36 years of constant monitoring from some major parameters as Sea Surface Temperature and more than twenty years for Ocean-Colour related parameters as surface primary productivity. The spatially heterogeneous trends observed show that these systems are highly variable, at temporal scales decades) that potentially impact some of their marine resources at rates that compete with the decline of human activities, beyond over-fishing. In this work we will present the effect of global warming at regional level for the three large marine ecosystems of Atlantic Africa on the sea surface temperature, wind stress and chlorophyll concentration as a proxy of primary production. The Canary and the Benguela systems are particularly impacted by the global warming, especially in their tropical parts, while Pacific systems show a more stable trend, due to their constantly high activity that partly counteracts some effects of the global warming

Micronektonic acoustic density variations along Benguela Current Large Marine Ecosystem continental shelf from 1994 to 2001

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018The Benguela Current Large Marine Ecosystem (BCLME) is situated along the coast of south-western Africa, stretching from Tombua (Angola) in the north (16°N, 11°W) southwards to the east of the Cape of Good Hope (South Africa) (29°S, 17°E) and includes the study zone, from 17°S, 9°E to 31°S, 17°E. In this work, we focused on the Namibian continental shelf where fishing activities are mostly due to industrial fishing. Acoustic data were recorded with a 38 kHz echosounder, from 10 to 500 m depth over 8 surveys totalling 46 302 nmi from 1994 to 2001. To get homogenous data (i) only off-upwelling season surveys (October to June) were studied and (ii) only continental shelf data were considered (10-150 m). The mean volume backscattering strength (Sv in dB) was used as a micronektonic biomass proxy to assess its spatial inter-annual variability. Diel transition periods were removed from analyses to avoid micronektonic density changes bias due to diel vertical migrations. Data were echointegrated at a spatial resolution of 0,1 nmi*1 m depth using the Matecho tool. (i) On horizontal dimension, the variability in annual micronektonic densities was assessed using the mean Sv value for each 0,1 nmi Elementary Sample Unit (ESU). Then, hot and cold spots were computed from the combined analysis of the spatial correlation and the Moran's I index of these values. (ii) On vertical dimension, the change of micronektonic spatial structure between day and night was assessed using the mean Sv value for each 1 m depth step. The inter-annual variability inside the eastern boundary upwelling ecosystem of the BCLME was analysed. (i) No significant change in micronektonic density was observed over the study period. Mean micronektonic acoustic density values observed were lower than in other African Atlantic large marine ecosystems. (ii) Hot and cold spots were spatially stable over time. Further analysis of physico-chemical parameters should improve the understanding of this pattern. (iii) A different vertical structure was reported between day and night, suggesting a migration from bottom to surface at dusk, as in the well-known diel vertical migrations. In perspective, physical processes occurring in the water column from turbulence to mesoscale activities should be considered in future studies