Forced settlement of metamorphosing halibut (Hippoglossus hippoglossus L.)in shallow raceways: growth pattern, survival, and behaviour

Pelagic metamorphosing halibut (Hippoglossus hippoglossus L.) larvae with w et weights between 50 and 150 mg were transferred into shallow raceways wit h a water depth of 7 mm; they were the smallest-sized 10% among 6000 mostly metamorphosed halibut. After transfer they settled to the bottom within fe w hours. Three treatment groups with two replicates were established with 1 00 halibut larvae each (initial mean wet weight of 77 mg). Each group was o ffered one of three diets: live yolk-sac larvae of cod (Gadus morhua L.), n ewly hatched Artemia salina nauplii, and a combination of the two diets. Th e water current in the raceways was maintained at 1 cm s(-1). The system wa s self-cleaning throughout the 19-day experiment; the temperature was kept at 13 degrees C throughout the study. Halibut larvae offered a mixture of t he two live organisms and those offered only A. salina had 98 and 99% survi val. Halibut larvae offered only cod larvae resulted in an overall survival of 78% at termination. The smallest-sized fraction did not ingest cod larv ae on day 7 explaining why almost all of these small halibut larvae died. P ercent coverage of the bottom by halibut increased from 5% to 8-10%. Final mean wet weight ranged from 217 to 257 mg for the three groups. The specifi c growth rate (SGR) was almost the same in all three groups, 5.8 to 6.0% af ter correcting for the biased mortality observed among larvae offered only cod larvae. The specific growth rate pattern was examined from ranking of w et weights as was the daily ingestion of cod larvae and Artemia nauplii bas ed on a gross growth efficiency, K-I, of 40%. Aggression among halibut larv ae was observed only in the group offered cod larvae; the larger larvae wer e concentrated close to the front screen through which the cod larvae enter ed. They frequently attacked smaller conspecifics as they tried to establis h themselves in the front screen area. (C) 1999 Elsevier Science B.V. All r ights reserved.