The distribution of eggs and larvae of anchovy (Engraulis encrasicolus) in relation to hydrography and food availability in the outflow of the river Po

Abstract

      Sampling was carried out in August 1995 along a transect across the outflow of the river Po. The river plume was clearly seen as a superficial layer of turbid low salinity water with a rela­tively high concentration of chlorophyll a. Eggs of anchovy (Engraulis encrasicolus) were most abundant at the station close to the river plume front and in relatively low numbers at stations both further inshore and offshore from this position. Larvae of anchovy were also most abundant at the position of the front and remained relatively numerous further offshore from this station; at the most coastal station larvae were less abundant. The mean length of larvae increased with dis­tance from the coast. Both anchovy eggs and larvae were mostly in the top 20 m of the water col­umn, the eggs in increasing numbers towards the surface whereas the larvae exhibited a sub-sur­face peak of abundance at a depth of 6-8 m. Larvae fed during daylight hours only, their diet changing from mainly copepod nauplii at larval lengths of <6 mm, to predominately early cope­podite stages and small adult copepods at lengths >8 mm. Abundance of potential prey items for anchovy larvae, as an integrated measure through the entire water column, showed a general decline from inshore to offshore. Highest concentrations of food particles were at similar sub-sur­face depths to those at which the larvae occurred. The results suggest that for smaller larvae, feeding conditions were most favourable in inshore areas adjacent to the spawning grounds where nauplii were most abundant and occurred in maximum abundance at the same depth as the larvae; larger larvae were advected offshore in the river plume to areas where the copepodite stages, which formed their main prey items, represented a higher proportion of potential food items.