Distribution and abundance of larval fish in the northern Aegean Sea--eastern Mediterranean--in relation to early summer oceanographic conditions

Abstract

Two ichthyoplanktonic surveys were conducted during June 1995 and 1996 along the northern Aegean Sea coast with the aim of describing species composition, abundance and distribution patterns of the assemblages of fish larvae and their relationships to oceanographic regimes. The upper water column was generally cooler, fresher and richer in zooplankton during June 1996. The mean abundance of larvae of the small-sized pelagic fish (anchovy and most mesopelagics) was higher in 1996. In contrast, larvae of the middle-sized pelagics (Sardinella aurita, Trachurus mediterraneus, Scomber jaconicus and Auxis rochei) were more abundant in 1995. The eastern part of the surveyed area (NE Aegean Sea) was colder and less saline during both years because of the influence of Black Sea and riverine waters. In the colder June 1996, larvae of many summer spawners', e.g., the middle-sized pelagics, were present infrequently and at very low numbers in the NE Aegean Sea. Cluster analysis using the Bray-Curtis similarity index revealed well-defined groups of stations and assemblages of larvae. Ordination scores derived from non-metric multidimensional scaling were compared by multiple regression with various parameters and the analysis showed that taxonomic composition and abundance was primarily explained by bathymetry. In the cooler June 1996, a significant amount of variation in ordination scores was related to longitudinal differences in temperature, salinity and the width of the continental shelf. The membership of station and species groups defined by the cluster analysis differed between 1995 and 1996 and reflected interannual differences in the distribution and abundance of component species. Spatial and temporal spawning patterns of adults played the key role in the formation of assemblages of early larvae.