Enhancing growth and development in olive flounder diets
by Tibiabin Benitez-Santana PhD, Aker BioMarine, Norway
Making a good diet better
In the feed production industry, fish meal is in demand due to its high-quality protein, omega-3 fatty acids, and well-balanced amino acids. While the demand may be high, the production-levels are slowing, which has sparked a search for alternatives. One natural contender has stood out among the rest: krill meal from captured raw Antarctic krill (Euphausia superba).
Based on its nutritional profile, krill meal has the right total protein and ash contents, as well as amino acid profile, to make it one of the most comparable alternatives to fish. Over the years there have been several studies on the benefits of krill meal as a part of the diets of various species of fish, all with positive effects. A new study from researchers at Jeju National University's Department of Marine Life Sciences reveals findings based on their experiment with fish meal supplementation in olive flounder diets.
Olive flounder are one of the most important fish species in South Korea, accounting for approximately 50 percent of the total finfish production. The objective of the olive flounder feed study was to determine how the fish were impacted by varying levels of krill meal in a low fish meal diet. The study specifically analysed the effect on growth performance, feed utilization, intestinal histology, digestibility, innate immunity, hematological parameters and disease resistance to Edwardsiella tarda, which is a serious bacterial pathogen in South Korean olive flounder farms.
Antarctic krill, a proven supplement in fish feeds
Krill is a marine-originated protein and phospholipid source that has shown through previous studies to be a successful supplement in fish feeds, as it lacks the unacceptable properties linked to plant proteins. Plant-derived protein sources have nutritional limitations such as low protein and high fibre content, imbalanced amino acid profiles and a wide range of anti-nutritional factors that negatively affect growth and health of the fish, as well as palatability of the feeds.
Krill meal is mainly produced from captured raw Antarctic krill, a crustacean related to shrimp. Krill aggregate in up to twenty-kilometers-long swarms and travel up and down a water column. Antarctic krill are one of 85 different species of krill around the world. They can be found in the Southern Ocean, with a nearly 500 million tonnes biomass, which means it has a huge potential among a number of under-exploited marine resources at lower tropic levels.
Krill are transparent animals with some red and green colorations; the red is due to astaxanthin pigmentation while the green is due to the algae in their digestive systems. Antarctic krill is harvested in the wild, cooked, dried and ground in order to create a powder consisting of proteins, phospholipids, omega-3 fatty acids and astaxanthin.
Krill meal diets given to olive flounder studied over 12-week period
A total of 792 olive flounder were purchased from a South Korean olive flounder farm and transported to the Institute of Marine Life Sciences at Jeju University. First, the fish were acclimatised for two weeks before beginning the experiment. Then they were randomly distributed into 24 polyvinyl circular tanks (33 fish per tank). The researchers tested diets that included varying percentages of krill meal (3%, 6%, 9% and 12%) added to a low fish meal diet (28%), versus a high fish meal diet (58%). There were four groups of olive flounder per diet, tested it over a course of 12 weeks.
At the end of the 12-week krill meal trial, all the fish were bulk weighed. Then four fish from each tank were randomly selected to provide samples for analysis. These results were compared to those from olive flounder consuming high fish meal (58% inclusion) and low fish meal (28% inclusion) diets.
Partial inclusion of krill meal improves growth performance and feed utilisation of olive flounder
The results of the olive flounder study show that inclusion of krill meal improves growth performance and feed utilisation, confirming earlier studies on dietary krill meal supplementation for other fish species. Overall there was a positive tendency of improved performance in these areas among the olive flounder fed the three percent, six percent and nine percent krill meal supplementation in low fish meal diets. Significantly higher protein efficiency ratio values were observed in the six percent and nine percent krill meal diets, as compared to the low fish meal diet.
The study revealed improved feed intake among the three percent, six percent and nine percent krill meal groups, as compared to the low fish meal group. This may be the result of the feed attractant properties of krill meal, which can lead to an increased appetite. Krill meal offers a well-balanced nutritional profile, more similar to fish meal, and contains high levels of phospholipids and astaxanthin, both of which are considered to be stimulators of fish growth. With this in mind, the increased palatability and growth stimulators could be the reason for the improved growth performance in the above-mentioned groups.
The hematocrit and hemoglobin levels were also increased by krill meal supplementation. Hemoglobin content of fish blood is an indicator of oxygen-caring capacity, while hematocrit is a primary diagnostic parameter for fish health. This study revealed that the increased values of hemoglobin and hematocrit in the krill meal groups (versus the low fish meal group) indicates that the krill meal could improve fish health similarly to a fish fed a high fish meal diet.
The plasma triglyceride level of the six percent krill meal diet was significantly higher as compared to the low fish meal group. In addition, digestibility of dietary protein and dry matter was also increased as a result of the krill meal three-to-nine percent diets, and were more similar to the high fish meal diets.
Krill meal supplementation in low fish meal diets also revealed significant improvement in innate immunity and condition factor of the olive flounder, within the three-to-nine percent krill meal diet groups. Most notably the group fed six percent krill meal group performed significantly better than the high fish meal group in superoxide dismutase (SOD) activity.
Despite the fact that this experiment did not analyse the direct effect of krill meal on the fish immune system, this study did evaluate resistance to Edwardsiella tarda, which can be indicative of overall immune system response. The results show that the astaxanthin and phospholipids present in krill have immune-stimulatory effects.
Other factors leading to enhanced digestibility and growth performance
This study also showed that goblet cell counts and villi length were positively impacted by all krill meal diets in the experiment. Goblet cell plays an important role in digestion processes and in the protection of the digestive tract as it synthesises and secretes mucus. The krill meal groups show significantly higher goblet cell counts when compared to the low fish meal group, confirming previous results.
It's also notable that the three percent, six percent and nine percent krill meal groups all had higher goblet cell counts than the high fish meal group. In terms of the villi length, this is a useful way to explain the development of intestinal morphology. Longer villi can indicate greater absorption of nutrients through increasing the surface area.
This study revealed increased villi length of the krill meal fed groups. This suggests that krill meal can be beneficial to the development of intestinal morphology, though a direct mechanism was not revealed in this study. That said, this intestinal morphology, as well as the increased villi length and the increased goblet cell counts may be the cause behind the enhanced digestibility and growth performance of the krill meal fed groups.
Higher survival against the E. tarda pathogen
One particularly interesting result from the study show that olive flounder fed with dietary krill meal showed considerably higher survival against the E. tarda pathogen, which is a well-known bacterium in the Asian feed industry. The researchers behind this study were unaware of any available data on disease resistance of fish fed krill meal diets.
This study showed that the krill meal fed group had a much higher survival rate than those in the low fish meal fed group. This is in line with results showing a krill hydrolysate supplementation in a low fish meal diet improved the survival of olive flounder when infected with the E. tarda pathogen. This krill hydrolysate supplement significantly increased the survival of olive flounder and red seabream against E. tarda. The higher survival rate of the olive flounder can be explained due to the improved and/or boosted innate immune system that the krill meal stimulates in the diet.
The olive flounder experiment has clearly revealed that krill meal supplementation in low fish meal diets is increasing growth, as well as improving diet digestibility, intestinal development and function and innate immunity as well as disease resistance of olive flounder.
The positive growth effects on the olive flounder may also result from the feed attractant properties of krill meal, leading to a higher appetite among the test groups consuming the krill supplemented diet. Krill meal contains high levels of phospholipids and astaxanthin, which are considered to be stimulators of fish growth. Based on positive effects of krill meal supplementation in this experiment, the recommend level of krill inclusion is between six-to-nine percent krill meal when the fish meal feed inclusion is low.