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

Abstract

Pelagic metamorphosing halibut ( Hippoglossus hippoglossus L.) larvae with wet weights between 50 and 150 mg were transferred into shallow raceways with 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 few hours. Three treatment groups with two replicates were established with 100 halibut larvae each (initial mean wet weight of 77 mg). Each group was offered one of three diets: live yolk-sac larvae of cod ( Gadus morhua L.), newly hatched Artemia salina nauplii, and a combination of the two diets. The water current in the raceways was maintained at 1 cm s −1. The system was self-cleaning throughout the 19-day experiment; the temperature was kept at 13°C throughout the study. Halibut larvae offered a mixture of the two live organisms and those offered only A. salina had 98 and 99% survival. Halibut larvae offered only cod larvae resulted in an overall survival of 78% at termination. The smallest-sized fraction did not ingest cod larvae on day 7 explaining why almost all of these small halibut larvae died. Percent 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 specific growth rate (SGR) was almost the same in all three groups, 5.8 to 6.0% after correcting for the biased mortality observed among larvae offered only cod larvae. The specific growth rate pattern was examined from ranking of wet weights as was the daily ingestion of cod larvae and Artemia nauplii based on a gross growth efficiency, K 1, of 40%. Aggression among halibut larvae was observed only in the group offered cod larvae; the larger larvae were concentrated close to the front screen through which the cod larvae entered. They frequently attacked smaller conspecifics as they tried to establish themselves in the front screen area.