Production of soybeans in the US combined with an interest in exploring the use of alternative lipids in aquaculture feeds led a group of researchers from the US and Mexico to evaluate the use of different lipid sources in the diets of juvenile yellowtail, said Artur Rombenso, corresponding author and research scientist with the Center for Fisheries, Aquaculture and Aquatic Sciences at Southern Illinois University.
One interest of the examination was to find what type of soybean-based oil – partially hydrogenated, fully hydrogenated or a blend – would be best at supporting fish production, fatty acid composition in tissue by improving “fish oil sparing,” the researchers said. The sparing effect means less of the long-chain polyunsaturated fatty acids (LC-PUFA) or omega-3s are needed in the diet and that potentially fish could be raised without the need for fish oil use.
“It was not expected that the SFA [saturated fatty acid] source would outperform the other sources, in particular, MUFA-rich lipids (mainly focus of global research),” Rombenso told FeedNavigator. “Once, it was identified the potential of SFA-rich lipids in aquafeeds we further investigated this matter.”
The research team found that when generating a feed using an alternative oil in place of fish oil, they had better results using a fully-hydrogenated soybean oil, he said. “In our study fully-hydrogenated soybean oil was better for growth, fillet fatty acid profile and etc.”
Tissues from fish on the diet incorporating SFA-rich lipids were the most similar to fish raised on feeds with fish oil, the researchers said. Those fish also had higher levels of LC-PUFAs than those on any of the other diets – including the fish oil-based feed.
However, more research is needed to assess use of the oil in several areas including with larger fish or for a longer period, added Rombenso.
Why LC-PUFA sparing?
Several fish species need specific long-chain polyunsaturated fatty acids in their diets, said the researchers. It can be important that they include ARA, DHA and EPA to maintain the best growth performance.
That necessity makes it challenging to directly replace fish oil with cheaper alternatives, they said. Using other sources of ARA, DHA and EPA to supplement the alternative lipid also is expensive and can negate the saving from using a less expensive lipid.
However, when a diet includes predominately saturated fatty acids (SFA) or monounsaturated fatty acids (MUFA) it is possible to reduce the amount of LC-PUFA needed, they said. Use of those lipid types appears to improve metabolism of the LC-PUFA available and promote fish oil sparing.
It is somewhat unclear how strong the effect is, how much of the specific lipids are needed to generate it, and what the result is if both types of lipids are mixed, they said.
Methods and materials
In the feeding trial, juvenile California yellowtail was given one of six diets for a period of 10 weeks, the researchers said. Diets varied only by lipid source and fatty acid composition.
The diets included a control that used fish oil, a diet with fully hydrogenated SFA-rich soybean oil (SFA SOY), a diet with partially hydrogenated MUFA-rich soybean oil (MUFA SOY), a diet with standard soybean oil (PUFA SOY), a diet with a blend of both SFA and MUFA oils (LOW PUFA), and a diet with all three soy oils (BALANCED SOY), they said. Diets also were supplemented with LC-PUFA concentrates to ensure needed levels of the fatty acids.
Diet and lipid samples were collected for analysis along with a sample of fish on each diet, they said.
Fish metrics were tracked to establish weight gain, feed conversion ratio (FCR), specific growth rate (SGR), feed intake (FI), they said.
Overall, the implications from the feeding trial suggest that the composition of dietary fatty acids in aquaculture feeds matter, said Rombenso.
“Generally, alternative lipids do not affect growth as long as the essential fatty acid requirements are met, but they will alter the fatty acid profile of the final product (fillet),” he said. “So, depending on the composition of the lipid source the fillet composition will be modified to better or worst, in terms of nutritional value.”
Additionally, it is becoming clear that the fatty acid composition of a lipid source plays a role in areas including tissue fatty acid composition, bio-availability of LC-PUFA, fish performance and sometimes LC-PUFA requirements, he said. “Research on MUFA and SFA has highlighted all or some of these benefits,” he added.
“The key point here is that with the advancement in fish nutrition it is possible to be more efficient in terms of growth, fillet fatty acid quality, and at the same time spare the important and expensive LC-PUFAs,” he said. “Consequently, it is possible to rely on cheaper sources of lipids to maximize profitability.”
Fish survival was 100% and production performance, feed intake and FCR were not altered by lipid source or fatty acid composition, the researchers said. However, tissue fatty acid levels were different and fish on the PUFA SOY and MUFA SOY diets had lower amounts of LC-PUFA.
Producers focused on boosting the amount of omega-3 fatty acids in the fish fillet could use either a MUFA-rich or SFA-rich diet, said Rombenso. However, SFA are the “primary drivers” in LC-PUFA sparing.
“The most exciting outcome is that fish fed SFA soy diet yielded a fillet richer in DHA (%) than those fed the fish oil-based diet. On the other hand, fish fillet could have reduced levels of LC-PUFA if fed standard soybean oil-based diets (C18-PUFA diets),” he said. “A simple choice of dietary lipid source have major consequences in terms of fillet nutritional value.”
It was still necessary to ensure that there was an amount of the LC-PUFA in the diet, but that can be done in a cost-efficient manner, he said.
“With this approach, we can add a bit of fish meal or fish oil to guarantee the LC-PUFA, and it would be a feasible approach for feed companies,” he said. “The idea is not only to eliminate fish oil but provide reliable options to the industry.”
Title: Saturated lipids are more effective than others in juvenile California yellowtail feeds—Understanding and harnessing LC-PUFA sparing for fish oil replacement
Authors: Artur Rombenso, Jesse Trushenski, and Mark Drawbridge