Egg characteristics and developmentof larval digestive function of cobia (Rachycentron canadum) in responseto dietary treatments: Focus on formulated diets
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- Institutt for biologi 
Cobia Rachycentrum canadum is widely recognized as a good species for commercial fish farming. However, it is still significant limitations in the juvenile production. In this regard, understanding cobia broodstock nutritional requirements for improvement of egg quality, and larval digestive physiology for optimizing larval rearing techniques, are important research areas. In the present thesis, the effects of dietary essential fatty acid levels on egg characteristics and broodstock spawning performance of cobia have been studied. Possible egg quality indicators and ontogeny of the larval digestive function in response to different weaning strategies have also been investigated. Broodstocks of cobia were fed three formulated diets of similar proximate composition but different n-3 highly unsaturated fatty acids (HUFA) levels: 0.94 % (D1), 1.31 % (D2), or 1.72 % (D3) of the dry weight (dw), or raw fish (mean value: 1.86 % n-3 HUFA of dw). The egg fatty acid composition was directly influenced by the essential fatty acid levels in broodstock diets. The egg contents of total n-3 HUFA and docosahexaenoic acid (DHA) were significantly lower in all groups fed formulated diets than in the group fed raw fish, whereas the egg content of arachidonic acid (ARA) from groups fed diets D2 and D3 were significantly higher than the group fed diets D1 and raw fish. Although there was no significant difference in spawning quality of different dietary treatments, a tendency towards better spawning performance, such as higher fertilization success, hatching rate and lower yolk-sac larval deformity, was observed in the group fed raw fish. A total of 20 egg batches spawned naturally over two spawning seasons from broodstock fed raw fish were used to evaluate egg quality of cobia. The egg size and fertilization success decreased towards the end of the spawning season. Egg floating rate determined within an hour after spawning may be used as a predictor of batch larval production and yolk-sac larval survival. Blastomere morphology did not correlate significantly with hatching success, yolk-sac larval survival and yolk-sac larval deformity. The linear relationships between egg biochemical composition and egg quality parameters with highest correlation coefficients was firstly the decreased in abnormal blastomere rate as egg content of lysine increased (r2 = 0.82, p < 0.01) or linolenic acid declined (r2 = 0.75, p < 0.01). Secondly, yolk-sac larval deformity decreased as egg arginine content increased (r2 = 0.77, p < 0.01). The precise prediction of batch larval production was significantly increased when using combination of floating rate, batch fecundity and fertilization rate as independent variables to a multiple regression model (r2 = 0.95, p < 0.001). Similarly, using egg content of arginine and methionine as independent variables significantly increased correlation coefficient in the multiple regression model predicting yolk-sac larval deformity (r2 = 0.92, p < 0.01). The intestinal enzymes alkaline phosphatase (AP), leucine aminopeptidase, leucine-alanine peptidase, and the pancreatic enzyme trypsin were detected in cobia larvae when they were ready for first feeding (2 day post hatch, dph), and individual activities of the enzymes increased exponentially with larval standard length. Based on the development of specific activity of brush border membrane enzyme AP, cobia larvae fed cultured live feed achieved maturation of enterocytes on 26 dph, at a mean standard length (SL) of 17.0 ±} 0.6 mm and 699 degree days. However, for larvae weaned onto a commercial microdiet (Otohime) from 17 dph, the maturation of enterocytes was delayed until 30 dph (mean SL of 20.0 ±} 0.7 mm and 814 degree days), and was associated with a lower survival rate. Weaning of cobia larvae to a microdiet (Otohime) directly from rotifers was not successful, evidenced by lower growth, survival and digestive enzyme activities compared to those weaned from Artemia. Early weaning of cobia larvae from Artemia onto a microdiet (NRD) on 8 or 13 dph reduced growth, survival and intestinal maturation compared to those fed live feed. However, cobia larvae weaned from Artemia onto the NRD microdiet at 18 dph obtained comparable growth, survival and intestinal maturation as those fed live feed. This study suggest that cobia broodstock require high dietary n-3 HUFA levels (> 1.86 % dw) for improvement of egg quality. Increase in precise prediction of cobia egg quality is possible when using combination of several parameters for evaluation rather than single one. With the commercial microdiets used in this study, weaning of cobia larvae should start from Artemia at around 18 dph in order to obtain comparable growth, survival and gut maturation to those fed live feed.