by Prof. David Huyben & Mya Gowanlock, University of Guelph, Ontario, Canada
One of the challenges of aquaculture is finding the ideal food for a new species of fish to raise that would optimise development, increase resistance to illness, and yield the most return for the farmers who raise it. Farmers have begun to raise lake whitefish to protect the wild populations that occur in the Great Lakes region, which includes Ontario, Canada. Our research will help to define the optimal diet for lake whitefish, allowing farmers to produce them in an inexpensive and productive manner.
The Lake whitefish
Lake whitefish is a cold-water species belonging to the salmon family. They can reach 65cm (26 inches) in length, weigh up to 1.8kg (four pounds) and are native to North America’s Great Lakes. Lake whitefish have a mild taste and are most commonly eaten in restaurants around Georgian Bay in dishes such as fish and chips. They are also a healthy meal with high levels of protein and omega-3 fats.
Lake whitefish is a newly farmed fish species in Ontario, Canada. Interest in aquaculture production has increased in recent years due to the limited catch from wild fisheries as lake whitefish are considered threatened. Farming lake whitefish will reduce the fishing pressure on wild stocks while maintaining the supply of lake whitefish for human consumption.
Very limited nutritional studies have been done for lake whitefish, and no commercial feed is available in Canada. They are commonly fed diets that are made for other salmonid species like rainbow trout. Not having feed that is specifically meant for lake whitefish may be contributing to low growth rates and poor feed conversion, resulting in inefficient farming of this species.
Dietary protein & fat levels
Omega-3 polyunsaturated fatty acids are used as signalling molecules involved in the immune response. Inadequate and imbalanced levels of total dietary fats and fatty acids can affect the fish’s immunity due to damaged cell membranes. Impaired immune cell membranes can cause the immune system to malfunction, causing problems like autoimmune diseases. For example, whirling disease that affects the fish’s nervous system, leading to the characteristic erratic whirling behaviour of the infected fish.
Optimising protein to fat ratios in fish diets supports immune system function, growth performance, and disease resistance. Dietary fats are an important nutritional element that influences the immune system since immune cells are made up of fats, as evidenced by the fat bilayer of cell membranes.
Further research is needed to improve our understanding of nutrition for lake whitefish and to create better baselines for this fish’s nutritional needs. The aim of our study was to investigate the impact of protein and fat levels as well as insect meal as a feed addition on the growth and immune function of lake whitefish.
Fish feeding trials
This research was funded by the Ontario Agri-Food Innovation Alliance, a collaboration between the Government of Ontario and the University of Guelph and took place at the Ontario Aquaculture Research Centre (Elora, ON, Canada), which is owned by Agricultural Research and Innovation Ontario and managed by U of G through the Alliance. Students Rebecca Lawson, Yubing Chen and Elizabeth Gillians custom-made diets with low and high levels of protein (48 percent and 54 percent) as well as low and high levels of fat (12 percent and 18 percent) using the feed lab facilities at the Animal Biosciences Department of the University of Guelph (Guelph, ON, Canada).
Fifteen tanks of lake whitefish (30 fish in each tank) were fed twice per day for four months. There were five diets included in the trial having three duplicates per diet. Fish and feed were weighed to assess growth and their livers were collected at the end of the trial.
Growth improvement
The commercial and high protein, high fat diets significantly improved fish growth performance compared to other diets. The fish fed on the commercial diet had a higher body fat, indicating negative impacts on the farmer and the consumer due to the reduced fillet yield. Fat deposits in the body cavity of the fish are not ideal because they are removed during fish processing (degutting). Ideally, fat deposition in the muscle is the best result to increase the fillet yield.
No effects of protein levels were found among the diets when evaluating growth performance and feed conversion. However, weight gain varied significantly between the high protein, high fat diet and the low protein, high fat diet. The decreased growth performance in the low protein, low fat group indicates that the fish fed low protein (48 percent) and low fat (12 percent) levels were insufficient to sustain the best growth for lake whitefish.
Reduction in Blood plasma levels
The low-fat diets resulted in higher phosphorus in the fish’s blood, which could be due to the lower weight gain and less energy-dense protein being metabolised rather than fat.
Low fat diets had harmful effects on blood levels of potassium and aspartate aminotransferase, which are signs of sickness and liver damage. The low-fat diet also resulted in higher plasma potassium concentrations; a secondary indicator of damage due to stress. Lake whitefish should not be fed low-fat diets with 12 percent inclusion because of their slower growth and negative effects on their blood.
Descending Stress Levels
The expression of one of the heat shock proteins was increased in the liver of the lake whitefish fed the low-fat diet (12 percent), which indicates oxidative stress and cellular repair. In contrast, a decrease of the heat shock protein was shown in lake whitefish fed the high fat diet (18 percent). At the same time, lake whitefish fed a high protein, high fat diet grew faster compared to other diets, and the feed conversion ratio significantly improved. This suggests that the fish focused their energy on growth and had less oxidative stress.
This study found no ill effects on growth or immunity of including insects (black soldier flies) as a protein replacement in diets for farmed lake whitefish. Replacing fishmeal with insects reduces the use of wild fish and improves the sustainability of farming fish. Based on our results, we recommend feeding high protein, high fat diets for the best growth and immune function of lake whitefish based on our results. This study helped establish a baseline for the future farming of lake whitefish, while more studies are needed to further research and develop this species as a key food fish in the aquaculture industry.
Published in the October 2024 issue of International Aquafeed.
