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Fishery Bulletin 103(2) 



size classes. Two main thresholds associated with 

 ontogenesis-related diet changes have been identi- 

 fied. The first one was observed around 16 cm TL 

 and corresponded to a significant change in depth 

 distribution. The second, around 36 cm TL, cor- 

 responded to the attainment of sexual maturity 

 (Colloca et al., 2002). 



Although hakes are demersal fishes, they feed 

 typically upon fast-moving pelagic prey that are 

 ambushed in the water column (Alheit and Pitcher, 

 1995). There is evidence that hakes feed in mid-wa- 

 ter or near the surface at night, undertaking daily 

 vertical migrations (Hickling, 1927; Papacostanti- 

 nou and Caragitsou, 1987; Orsi-Relini et al., 1989) 

 which are more frequent for juveniles. Small hakes 

 feed daily on small Euphausiacea (Nictiphanes cou- 

 chi). This school-forming planktonic crustacean 

 carries out vertical migrations at night (Casanova, 

 1970; Franqueville, 1971; Vallet and Dauvin, 2001). 

 They rise to near the surface at night to feed on 

 phytoplankton and sink during daylight between 50 

 and 800 m depth (Buchholz et al., 1995). Juveniles 

 of M. merluccius may follow such migrations, moving 

 from near the bottom, 100-200 m depth, to midwater at 

 night (Froglia 1973; Papaconstantinou and Caragitsou, 

 1987; Orsi-Relini et al., 1989). Nocturnal vertical mi- 

 gration behavior has been described for gadoids such as 

 hake and cod and is considered responsible for the re- 

 duction of trawl catches of these fish at night (Beamish, 

 1966; Bowman and Bowman, 1980). 



Considerable diet changes have been observed after 

 the first year of life (>16 cm TL) when juveniles move 

 from nursery areas on the shelf-break and upper slope 

 to the middle shelf (Andaloro et al., 1985; Ardizzone 

 and Corsi, 1997). The data indicate that such migration 

 is induced by a change in trophic requirements. In this 

 size class, diet changed to fish prey (Clupeiformes), and 

 the importance of the small epiplanktonic crustaceans 



(Euphausiacea) strongly decreased. Clupeiforms S. pil- 

 chardus and E. encrasicolus are distributed largely on 

 the continental coastal shelf forming schools usually 

 deeper than 25 m (Fisher et al., 1987). 



The size-depth distribution pattern of hake was con- 

 firmed by experimental trawl surveys carried out in 

 the Mediterranean (Relini and Piccinetti, 1996; Relini 

 et al., 1999). Juveniles (modal length of 10 cm TL) are 

 found mostly between 100 and 200 m depth. Intermedi- 

 ate hakes reach the highest abundance mainly on the 

 shelf (<100 m). Large hakes (>36 cm) are found in a 

 wide depth range but concentrate on the shelf break 

 during the spawning period (Recasens et al., 1998; Col- 

 loca et al., 2000; Alvarez et al., 2001). 



Growth induces a continuous qualitative and quanti- 

 tative change in diet that is reflected in the increasing 



