Vertical distribution and feeding patterns of the juvenile European hake, Merluccius merluccius in the NW Mediterranean

Diel vertical migration of the European hake Merluccius merluccius on the Catalan coast (north-western Mediterranean) was studied by pelagic trawls at a single location during two cycles of 24 h each. Diurnal bottom trawls were also employed to determine which part of the population performed vertical migration. The size range of the population caught by both nets did not match exactly, since very small individuals (<5 cm TL) were obtained only with the pelagic net, indicating that this part of the population had not present on the bottom. In addition, the 95.5% of the pelagic captures were obtained during the night, while the maximum catches of the bottom net were obtained at midday, indicating that juvenile hake may display nocturnal vertical migration. Pronounced quantitative and qualitative variations in the diet of M. merluccius were observed between pelagic and bottom trawl samples belonging to the same size range. Differences in the stomach fullness and in the state of digestion of the food between benthic and pelagic individuals were also noticed. These results suggest that feeding represented one of the possible constraint factors controlling the vertical migration of juvenile hake.