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

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

Matecho : an open-source tool for processing fisheries acoustics data to facilitate collaborative development

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018Matecho 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

Echo level segmentation on echo-integration of fisheries acoustics data

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018In fisheries acoustics the analysis of data usually often concern biomass assessment mainly for small pelagic fish stocks using the well-known echointergration approach. Other can concern the analysis of single fish using their target strength (TS in dB) and more seldom analysis can also be done with the fish school descriptors using e.g. shoal extraction method (Movies+, Ifremer Software). In the framework of the Preface project we have focused on the micronektonic layers observed by scientific echosounder. Matecho, a friendly automatized processing method to extract information and perform echo-integration, fish shoal extraction and also performs a segmentation, on each zone of a cruise with a constant twilight, of the echointegrated echogram from an echo level threshold fixed by user to extract micronektonic layers in the water column. Here we describe this methodology which allows an accurate description of the spatial organisation and structuration of the marine ecosystem. The process is based on three main steps which consist in : (i) adjust the echo level threshold in dB, (ii) the extraction of the echoes inside each contours and the calculation of the layer descriptors, (iii) and then the correction of the extraction. Finally the echo segmentation, setup to extract micronektonic sound scattered layer, allows to get 34 layers descriptors, e.g., minimum/maximum depth (m), geographical position in 3D, maximum depth width (m), duration of the layer, surface covered by the layer, mean volume backscattering strength 'Sv' (dB re 1 m-1)': mean nautical area scattered coefficient 'Sa' (or NASC m2 nmi-2), to characterise their spatial position in the water column and acoustics properties. Moreover, a second class of descriptors, classified by elementary sampling unit (ESU), are estimated e.g. number of layer per ESU, layer depth per ESU. An innovative descriptor is also computed using this methodological approach: the water column fulling rate per layer and per ESU. Both classes of descriptors are then available for ecological studies