Relation entre l’abondance des petits poissons pélagiques et les facteurs environnementaux dans le système d’upwelling des Canaries

Relation entre l’abondance des petits poissons pélagiques et les facteurs environnementaux dans le système d’upwelling des Canaries

 Data sources

Acoustic data

The acoustic data were obtained as part of the EAF -Nansen project, a partnership between the Norwegian Agency for Development Cooperation (Norad) and Institute of Marine Research (IMR) of Bergen, Norway, and the Food and Agriculture Organization of the United Nations (FAO). The object if for these surveys was to estimate and map the distribution and biomass of small pelagic fish stocks off NW Africa (Morocco, Mauritania, Senegal and Gambia).

Acoustic surveys were carried out annually during the months October–December from 1995 to 2006 aboard RV Dr Fridtjof Nansen. The area of the continental shelf covered each year extends from latitude 10° to 35°N and depths greater than 10 m and up to 500 m and the survey always runs from the south Senegal to the north of Morocco.

Acoustic sampling was performed by means of scientific echosounders SIMRAD EK-500 with a split beam transducer of 38 kHz. The EK-500 was calibrated each year before these surveys using a copper sphere and following the methods of Foote et al. (1987). Sampling design consisted of transects oriented perpendicular to the coastline characterized by 10 nautical mile (nmi) inter-transect distance (Fig. II-1).

Each acoustic transect was divided in elementary distance sampling units (EDSU) of 1 nmi in which nautical-area-scattering strength (SA, m2.nautical mile-2; MacLennan and Simmonds, 1992) was determined. In total, about 7000 miles of transects were examined. Track lines were run at 9 knots.

During the surveys, the acoustic energy reflected by marine organisms was measured continuously and fish sampling, with two pelagic trawls and a demersal trawl, were carried out to help identify the species that are reflecting the acoustic energy.

The small pelagic trawl has 10-12 m vertical opening under normal operation, whereas the intermediate sized trawl has 15-18 m opening. The bottom trawl has a 31 m headline and a 47 m footrope fitted with a 12″ rubber bobbins gear. The vertical opening is about 5.5 m. The codend has 20 mm meshes, and has an inner net with 10 mm mesh size. The Bergen Integrator (BEI) was used for analysis and allocation of the integrated SA-values to the individual specified target groups by 5 nmi intervals.

The allocation of values to target groups was based on a combination of a visual scrutiny of the behavior pattern as deduced from echo diagrams, the BEI analysis, and the catch compositions. The relationship between fish size and acoustic backscattering from the fish is needed to transform nautical-area backscattering estimates to fish biomass (MacLennan and Simmonds, 1992).

However, within the NW Africa, fishery scientists have not yet agreed on a suitable transformation between fish length (L) and backscattering cross-section and the derived quantity target strength (TS). 85 Therefore, the present study applied nautical-area-backscattering (SA) estimates as measures of fish abundance.

The present study was focused on sardine, anchovy and sardinella without distinguishing between S. aurita and S. maderensis, whose spatial behaviors are still not completely understood (Bez and Braham, 2014) investigate the year-to-year spatial patterns of the 3 species,

we calculated the centre of gravity (CG) of their abundances (Woillez et al., 2007). The centre of gravity represents the mean location of the population and was calculated as: CG = ( ) ( ) With Z(x): density acoustic in x Annual biomasses of the studied area were estimated for each species based on acoustic surveys and catch data (Toresen et al., 1998). The total biomass estimate is obtained by summing the biomass by length group and areas within each sector of the survey. 

Environmental data

The environmental variables retained as potential useful predictors of the three species distribution and abundance were: sea surface temperature (SST in °C), chlrophyll-a concentration (Chl a in mg.m-³), upwelling index (CUI in m3.s-1.100 m-1) , wind mixing index (WMI in m3.s-3) and retention index (CRI in °C). All environmental data used were synchronized with the surveys periods and transects (dates and positions). For each parameter, we calculated the mean value for 8 days and 5 nmi.

The Sea Surface Temperature (SST, °C) is a product of the Advanced Very High Resolution Radiometer sensor (AVHRR). The temperature is measured at the skin of the ocean (Casey 2010). This is the Pathfinder Version 5.2 (PFV5.2), weekly level 3 products on a 4 km grid.

The retention index (CRI) was calculated from the SST using the method of (Demarcq and Faure, 2000). In order to take into account both the intensity and spatial extent of the upwelling plume, the retention index is constructed around an integration of the inverse SST gradient observed between the coast (Z=0) and the position of the absolute SST minimum (Zmin):