Sympatric Clupeoid Fish Larvae in the Northeastern Mediterranean: Coexistence or Avoidance?

The summer (July 2002 and 2003) larval concentrations of two commercially important clupeoid species (order: Clupeiformes), the European anchovy (Engraulis encrasicolus) and round sardinella (Sardinella aurita), were measured in Kavala Gulf (northern Aegean Sea), in order to examine their distribution patterns. Anchovy was generally most abundant, reaching 13 852 larvae 10？m？2 in 2003 and occurred in more stations compared to round sardinella. Despite the spawning of both species occurring across the entire gulf, the general spatial distribution of their larvae was different, with European anchovy larvae occupying the entire gulf and round sardinella larvae mainly concentrated at offshore stations. The reason proposed for the spatial segregation of their larvae is direct competition for resources. Yet, coexistence of the two species was observed in frontal areas with abundant food resources that prevent competition. 1. Introduction Competition can be an important factor in structuring ichthyofaunal communities because it largely determines the distribution and resource partitioning of closely related species, hence affecting their population characteristics [1]. Sympatric occurrence of closely related species utilizing the same resources may result in interspecific competition [2], which in turn may lead to one species differentiating in order to survive [3]. This condition holds only when the shared resource is limited [4]. On the contrary, competing species can coexist when a resource is unlimited [5] or above the surviving level of an organism [6]. Competition among fish larval stages may be avoided through successive larval appearance in time emerging from successive adult spawning [7, 8], spatial segregation [9], or occupation of different depth ranges [10]. Temporal succession of spawning among closely related species with common life-history characteristics, as a strategy for avoiding resource overlapping, has been reported to occur within families of marine fishes (Sparidae [11]; Mugilidae [12]). Competition avoidance through the selection of distinct habitats (in terms of depth range and distance from shore) will ensure optimal growth and survival at the vulnerable larval stages [1]. Such selective adaptations to habitats with often less favourable conditions have been reported for clupeoid fishes (order: Clupeiformes) at various life stages [13]. Clupeoid fishes consist of one of the major fish groups that account for more than 30% of the global marine landings [14]. In Greek waters, three clupeoid fishes, the European anchovy (Engraulis