Egg quality, triploidy induction and weaning of the Atlantic halibut Hippoglossus hippoglossus

The supply of juvenile Atlantic halibut, Ilippof’lossus hippoglossus, has been sporadic and until recently, has fallen short of expectations, due to difficulties associated with the hatchery phase. This thesis focuses on some specific aspects of intensive hatchery production which needed to be addressed in the areas of egg production and quality, triploidy induction and weaning. A quality assessment technique for halibut eggs, based on observations of morphological anomalies occurring during early blastomere divisions was devised. The degree of abnormality in the appearance of five features was quantified and a strong relationship between these characteristics and hatch rate of eggs incubated in microtitre plates was revealed. It was concluded that this method is of great potential use as an early predictive indicator of egg viability. The effect of temperature on egg production was studied in two broodstock groups held either at ambient or low stable temperature during spawning. High temperatures caused a delay and shortening of the spawning period as well as a reduction in egg quantity and quality. The results indicated that temperature control is a necessary feature of broodstock management at sites where ambient temperature profiles are unsuitable. The efficacy of hydrostatic pressure shocking for the induction of triploidy was tested on newly fertilised eggs. A 5 min pressure shock of 8500 psi, administered around 15 min after fertilisation resulted in high triploidy yields. This treatment had little effect on survival to hatching. However, a preliminary experiment indicated that triploid halibut may be prone to higher mortality through the hatchery cycle. Six weaning experiments were conducted to determine the influence of size, age, and developmental stage of halibut larvae on diet uptake, survival, growth and fry quality. Gradual replacement of Artemia with dry diet, co-feeding, and the use of intermediate diet types, were evaluated. In the absence of live prey, pre-metamorphic larvae (< 100 mg) would accept non-living feed particles but total replacement with a conventional dry diet was unsuccessful. However, following good growth prior to weaning, 700 “day old larvae could be successfully weaned over a nine day period. Improved growth and substantial savings in live feed were among the positive benefits resulting from early weaning.