Sustaining seabass

Published September 01 2016

Farm-raised seabass can have high fat deposits that may compromise their flavor. PHOTO COURTESY OF IVAN VIEGAS

A major challenge of the aquaculture industry is meeting the worldwide demand for fish. With the rising demand, the the cost of the fishmeal is increasing. A widely used solution to cut costs is to replace some of the protein-rich components of fishmeal with plant-based carbohydrates. In a recent paper in the Journal of Lipid Research, Ivan Viegas and colleagues from the Universities of Coimbra and Barcelona investigated whether a high-carbohydrate diet increases fat deposits in seabass.

Seabass, which are carnivorous, can tolerate up to 30 percent of their diet being replaced with digestible carbohydrates without compromising their growth rate. However, the replacement causes farm-raised seabass to have more fat deposits than their wild counterparts. The fat lowers the quality and taste of meat, therefore lowering the overall value of the final product.

Various methods have been used to study lipid accumulation in different fish species fed partial carbohydrate diets, but a consistent explanation for increased adiposity has not emerged. While it is clear that farmed fish have higher fat content, scientists don’t know if the higher fat content is due to increased lipid synthesis or decreased lipid breakdown.

Farm-raised seabass have increased plasma, liver and whole-body lipid levels as a result of high-carbohydrate diets. In mammals, high-carbohydrate intake increases the activity of a pathway in liver and adipose tissue called de novo lipogenesis, which is primarily responsible for converting carbohydrates into fat. To determine if the same process occurred in seabass, Viegas and colleagues fed the fish a diet with either high-protein or a combined protein-and-starch content. To tease apart the lipid profiles, fish were placed into tanks containing deuterated water. The hydrogen isotope was incorporated into triacylglycerol, the main component of fat.

After six days, the investigators isolated liver and serum samples from the fish and measured the lipid profiles using nuclear magnetic resonance. The investigators found that even though plasma and liver triglyceride levels in the fish fed the high-carbohydrate diet were higher, the lipid profiles revealed no significant changes in de novo lipogenesis between fish fed the two different diets.

The investigators also measured enzymes important for NADPH production, as NADPH provides energy for de novo lipogenesis. Viegas and colleagues discovered that NADPH production was moderately increased in the fish on the high-carbohydrate diet, but it was not sufficient to increase de novo lipogenesis.

In the future, additional processes, such as tissue lipid retention or decreased fat breakdown, need to be evaluated to identify the mechanism of lipid accumulation in fish fed high-carbohydrate diets. Furthermore, implementing breeding selection strategies for lean farmed fish may be an effective solution to satisfy the ever-growing demand for high-quality seabass.

Alexandra Nail Alexandra Nail is a doctoral candidate at the University of Kentucky.