Distribution maps

The objectif of these maps is to provide a rough indication of the spatial distribution of certain fish and shelfish species as provided by scientific surveys.

Regarding bottom trawl surveys, for each area a systematic grid was defined and the simple average density across years and hauls was calculated in numbers per km2. For mapping purposes, grid cells were coloured using a percentile scale: blue: species never observed, light yellow: average density between [0- 25 %[; dark yellow: [25-50 %[, orange : [50-75 %[ and red : [75-100 %]. Thus, cells with the highest average density are in red.

A fixed 0.25° grid covering the Bay of Biscay has been defined. For each year of the PELGAS series, the mean biomass per km² has been computed 200 times in each grid cell based on yearly biomass estimates per acoustic Elementary Sampling Distance Units (ESDUs), while slightly shifting the grid origin each time. The yearly mean biomass per grid cell has been computed as the average of the 200 replicates in each cell. This smoothing methodology allows to mitigate border effects in the neighborhood of grid cells. The map presented here is the average per cells of yearly mean biomass maps.
The color scale on the left hand side represents the biomass per grid cell in metric tons (red : high biomass, blue : low biomass).

The maps should not be interpreted as biological distribution maps but as the areas where the species are caught during the scientific surveys. As the different surveys use different samplin gear and are carried out at different times of the year, there can be differences in catchability among surveys and consequently among areas.

Access atlas : http://archimer.ifremer.fr/doc/00000/6976/

Citation : Trenkel V.M., O. Berthelé, P. Lorance, J.A. Bertrand, A. Brind'Amour, M.L. Cochard, F. Coppin, J.P. Leauté, J.C. Mahé, J. Morin, M.J. Rochet, M. Salaun, A. Souplet & Y. Vérin, 2009. Grands invertébrés et poissons observés par les campagnes scientifiques. Atlas de distribution. Bilan 2008. Ifremer, Nantes. EMH : 09-003. http://archimer.ifremer.fr/doc/00000/6976/. 100 p.