(1) Institut National de Recherche Halieutique, 2 rue de Tiznit, Casablanca, MAROC.
(2) Université de la Méditerranée, Centre d’Océanologie de Marseille, UMR CNRS 6540, Campus de Luminy, Case 901, 13288 Marseille cedex 09, FRANCE.
(3) Département de Biologie, Université Mohammed V, Rabat, MAROC.

Résumé. - Les petits pélagiques constituent une ressource qui dépasse 80% de la pêcherie marocaine dans le sud de l’Atlantique marocain, entre le cap Blanc et le cap Boujdor (21°-26°30’N). Deux campagnes océanographiques ont été effectuées dans cette zone en mars (hiver) et en juillet (été) 1998, au cours desquelles ont été mesurés des paramètres hydrologiques et prélevés des échantillons ichtyoplanctoniques. L’analyse des paramètres hydrologiques a montré une séparation entre les stations côtières et les stations du large, dont la limite est l’isobathe 80 m, et a localisé des zones d’upwelling. L’analyse de la distribution spatiale des densités (individu/10 m²) des œufs et des larves de la sardine, des sardinelles, de l’anchois, du maquereau et des chinchards a montré une aire de concentration localisée entre le cap Blanc et le cap Barbas, et dans la région de Dakhla (24°N). L’analyse simultanée des données hydrologiques et biologiques a montré que ces aires sont influencées par les faibles températures et la grande richesse en sels nutritifs qui résultent des courants ascendants (upwelling), quasi-permanents dans cette zone.

Abstract. - Importance of hydrological parameters in egg and larva distribution of small pelagic fishes in the southern Moroccan Atlantic.

The small pelagic fishes constitute a resource that overgoes 80% of the Moroccan fisheries in the southern Moroccan Atlantic region between Cape Blanc and Cape Boujdor (21°-26°30’N). Two surveys were carried out in this area in March (winter) and July (summer) 1998. During cruises, the following environmental parameters were measured at 5 m depth: temperature, salinity, dissolved oxygen and phosphates. At each survey, 30 samples per season of ichthyoplankton were made using a small size Bongo net, 20 cm mouth diameter and 417 µm mesh size, and equipped with a flowmeter to measure the volume of filtered water. The sampling was carried out with oblique hauls from surface to bottom, and no more than 100 m in depth for the stations that exceeded 100 m depth. The species studied were sardina, sardinellas, horse mackerels, chad mackerel and anchovy. The statistical analysis consisted of a one way anova to test significant spatial and temporal differences among the hydrological and biological parameters, a normalised principal analysis (PCA) for the hydrological parameters justified by the heterogeneous data, and a canonical correspondence analysis (CCA). The CCA analysis permitted to explain the structure of the biological data by mixing the two tables, biologic and hydrologic. The significance of CCA was tested by the Monte-Carlo test. A grouping of stations with the same hydrological characteristics was carried out and the main result in the two cruises was the separation between coastal and offshore stations at the isobath 80 m limit. In the coastal stations, the variation of temperature and phosphates was important and permitted the localisation of the upwelling areas in the cruise periods. These areas were characterised by a low temperature and a high content of phosphates observed principally in the north, between 24°30’N and 26°30’N in July. In the south, between 21°N and 22°30’N, the upwelling was influenced by the ECSA current, which was rich in nutriments and relatively hot, and was more marked in March. In March, the spatial distribution of egg and larva densities (individual/10 m²) showed two separate high concentration areas in the north and in the south, essentially of sardina, sardinellas and anchovy. In July, the main characteristic was the occurrence of the eggs and larvae of almost all species in the area located between North Dakhla (24°30’N) and Cape Barbas (22°30’N). The choice of spawning areas was done to limit the loss by advection. This was possible in the areas where the continental shelf is wide and constitutes a retention zone, as it was the case in the region of Dakhla. In the two seasons, the samples showed a large variability of egg and larva occurrence between species and stations proved by the density of standard deviations, which was superior to the average density in all cases. This was in relation with the spawning periods of each species. Thus, at the small pelagic species, characterised by batch fecundity, the spawning was displayed throughout the year with periods of peak spawning, which coincides with favourable environmental conditions. The projection of stations concerning egg and larva densities of CCA analysis showed approximately the same groups that the one established by normalised PCA. The projection in the factorial plan of the results of CCA analysis showed a link between the distribution of eggs and larvae of sardinellas and the temperature and phosphate factors. The eggs and larvae of the other species were more dispersed. The study of the variation of egg distributions, in relation to the sea surface temperature, showed a large thermal margin of eggs occurrence of different species, which permitted a large distribution of species and an overlapping of spawning areas.

Key words. - Clupeidae - Engraulidae - ANE - Morocco - Pelagic fishes - Ichthyoplankton - Hydrological parameters.