Vertical distribution of early developmental stages in two coexisting clupeoid species, Sardinella aurita and Engraulis encrasicolus

Abstract

In recent years a northward expansion of Sardinella aurita has been reported in the western Mediterranean. Considering the coexistence of its larvae with those of the dominant species Engraulis encrasicolus, the present study was conducted to compare their vertical distributions in 2 areas off the Catalan coast with different vertical environmental conditions. During summer, the water column was stratified with a deep chlorophyll maximum (DCM) beneath the pycnocline. However, the southern area, under the influence of the Ebro River, was characterized by a secondary surface chlorophyll maximum. Vertical distribution of larval food, nauplii and copepodites showed good agreement with the high chlorophyll layers. In the earliest stages of development, larvae of both species remained in the upper levels. From 6 mm standard length on they developed a day/night migratory behaviour to search for food during the day (feeding period). Therefore, in the south, where the abundance of potential food in the upper layers was relatively high, larvae of both species remained in the upper levels during the day. However, in the north, where food was restricted to the DCM, only E. encrasicolus larvae were able to reach these deep levels. The low temperatures (∼15°C) detected at the DCM may restrict the vertical migration of S. aurita in accordance with their thermophilic character. This limitation might represent a restriction for the northward expansion of this species in the western Mediterranean. © Inter-Research 2008.