Lipid metabolism of haddock (Melanogrammus aeglefinus)

Haddock, a commercially important coldwater marine fish accumulates a high amount of dietary lipid in the liver (>60% lipid, wet weight). Several experiments were conducted to study the dietary lipid utilization by haddock, their lipid transport mechanisms and the biochemical basis of liver lipid deposition and catabolism. Juvenile haddock (6.9g) were fed three isonitrogenous diets containing 14, 16, 19 and 22% lipid to examine the effects of dietary lipid on growth, feed utilization, hepatosomatic index (HSI) and tissue lipid accumulation. Growth and feed efficiency of haddock was not significantly affected by increasing the lipid content of the diet. A significant increase in HSI (9.8&ndash;12.1%) and total liver lipid content (63.2&ndash;69.0%) was observed in haddock fed 14% versus 22% lipid, however, total muscle lipid content was unaffected. The liver fatty acid (FA) composition mirrored that of the diet. The muscle lipid (1%) contained a high proportion of polar lipid (84% lipid) and polyunsaturated FA (53% total FA). A dietary lipid level of 14% or less is recommended for use in juvenile haddock grower diets based on the results of this study. A series of liver function tests were also performed on the plasma of cultured haddock grouped on the basis of their HSI. No significant differences were observed between the 11.1, 13.0 and 17.3% HSI groups for any of the liver function parameters tested.Serum lipoproteins of post-absorptive juvenile haddock showed that HDL (high density lipoprotein) was the predominant lipoprotein class (1517 mg/dL) followed by LDL (low density lipoprotein; 467 mg/dL). A low level of VLDL (very low density lipoprotein; <50 mg/dL) was observed in serum. The concentration of total lipid in the serum averaged 1297 mg/dL. The phospholipid (PL), triacylglycerol (TG), cholesterol ester (CE) and cholesterol (CL) contents of serum were 57, 16, 16 and 11%, respectively. The fatty acid (FA) composition of HDL resembled that of the serum lipid and PL and it contained a higher percentage (51%) of polyunsaturated FA than VLDL (36%). High molecular weight apo B-like proteins were observed in the VLDL and LDL fractions. An apo A-I-like protein was predominant in the HDL fraction. The low level of VLDL (