Novel feeds for rainbow trout - the partial substitution of fishmeal with insects
by Federico Melenchón Ramírez, Agro-Technological Institute of Castilla y León, Segovia, Spain
Due to the facts that fish need high amounts of protein to meet their nutritional requirements, that a high proportion of this protein has been traditionally satisfied with the use of fishmeal (an ingredient that usually comes from extractive fishing practices), and that aquaculture is one of the fastest growing food industries, it is easy to draw a line that links all of these points together.
Indeed, during the last few decades, aquaculture has been facing a big sustainability problem due to the overexploitation of sea resources, motivated by this dependence on fishmeal. In this way, both research and industry have invested great effort in the development of alternatives to this ingredient.
There are several factors to have in mind among these alternatives. For example, some of the most known of them, such as soybean meal, have a vegetable origin, and many vegetable ingredients are known to produce undesired effects in fish gut.
However, there are others like insectmeals that have a promising future due to the advantages they show. It is known that insects can grow and reproduce quickly, that they can feed on several different substrates, and that they require very little energy and space investments for their production, making them a very interesting choice when it comes to thinking on sustainable alternatives.
Furthermore, it seems that insects could be considered functional ingredients as well, since several studies have proven that the inclusion of insects in fish feed may be involved in the stimulation of the immunological response and/or the enhancement of the antioxidant system, ultimately related to animal welfare.
These last points, however, still remain in a thin layer of uncertainty; even though it is usually said that chitin, a natural component of the exoskeleton of insects, could be the main element responsible cause for these body function enhancements, the truth is that the real mechanisms are still unknown, so more research is needed on the matter.
The partial substitution of fishmeal with insect meals
The Agro-technological Institute of Castilla y León (Spain), within the Insectmeal project, has been working on three experiments that involved the partial substitution of fishmeal with insect meals for diets of rainbow trout (Oncorhynchus mykiss). The first experiment, carried out on early stages of fish, had the objective of testing the viability of two insect meals (black soldier fly, Hermetia illucens, and yellow mealworm, Tenebrio molitor) at two inclusion levels (5 and 10%).
As main conclusions, the experiment followed the line of the current literature suggesting that small inclusions of insect meals do not impair rainbow trout growth, and that insect meals tend to enhance the previously mentioned: immunological response and antioxidant status.
It was also highlighted that the intrinsic composition of insects, which are low on ω-3 fatty acids, tends to decrease the amount of these same ω-3 fatty acids in fish fillets.
However, even after this decrease, the ω-3/ω-6 ratio (still higher than 1.0) seems to be enough to consider the fillets as healthy for human consumption.
The second of these experiments acted as an extension of the previous one. Being also carried out on early stages of fish, the inclusion of insect meals was forced a little bit further (up to 18% of the feed formulation). In this case, and even though all fish had similar daily feed intakes, the control diet and the one with yellow mealworm showed quite similar results for growth performance and protein use, while the black soldier fly treatment left lower numbers.
Also, a histomorphology analysis highlighted an overall better status of the distal intestine microvilli on control and yellow mealworm treatments, while fish fed with black soldier fly showed shorter microvilli.
Put together, these results make sense due to the fact that longer microvilli lead to a higher absorption surface in the distal intestine, and a higher absorption surface would lead to a higher digestibility, since there would be more contact with the digested feed.
Yellow mealworm could be a better option
This does not mean that black soldier fly is a 'bad' alternative to feed rainbow trout, since all growth and protein use parameters were still within reasonable numbers. However, a big part of the current scientific literature supports the idea that yellow mealworm could be a better option for a partial replacement of fishmeal when it comes to feeding rainbow trout.
Last but not least, the third experiment, carried out on late stages of rainbow trout (grown up to 420 g), only tested different combinations of yellow mealworm. One of the experimental diets used defatted insect meal to reduce the involvement of insect fat, while other two diets had different levels of an experimental algal oil rich on ω-3 fatty acids.
The main objectives of this experiment were to try and solve the problem with ω-3 fatty acids highlighted in the first experiment through different strategies, and to evaluate the viability of the fish fillet.
The first part is still a work in progress, and we expect to get interesting results in future trials. The second part, however, already dropped some data through both raw and cooked fillet analyses. No significant differences were found on either instrumental or sensorial tests, which is a very positive conclusion; it reinforces the idea that yellow mealworm does not modify the organoleptic characteristics of rainbow trout.
Encouraging further research
Going further on this topic, we would like to take advantage of this opportunity and talk briefly about other preliminary results of the third experiment of the Insectmeal project. A similar situation to the previous experiences was described in this case, with almost no changes on growth, protein use, or distal intestine health.
It is known that evolutionary mechanisms tend to compensate the least digestible feeding habits (typically, the case of herbivorous animals) with longer digestive tracts. In this way, we wanted to test the possible adaptability of the digestive tract inside a single species.
The results were very vague on this topic, but we could spot one coincidence between a lower intestine-somatic index, and a diet with a higher protein digestibility, all within a general and very slight tendency that matches this.
The truth is that the current literature does not give much information on this precise topic. That said, considering that viscera is a considerable proportion of the fish weight (around 10-15% of wet weight depending on the species), we would like to drop this idea to encourage more research on the matter.
Taking some additional measures during a typical sampling such as intestine length and weight is not too time-consuming, and if it helps to create a link between feed digestibility and the length/weight of the intestine, it would be worth the while.
The findings of this study in five key points
- An addition of T. molitor or H. illucens, up to a 10 percent of the formulation to a balanced rainbow trout diet does not impair growth.
- An addition of H. illucens up to 18 percent seems to go over the limit for this ingredient, possibly due to a slight damage on intestine microvilli. However, the addition of T. molitor can reach these numbers without growth problems.
- Insectmeals seem to enhance the immunological and antioxidant responses of rainbow trout, but the mechanisms that cause this still remain uncertain.
- The fatty acid composition of insectmeals tends to be reflected on the composition of the fillet.
- The organoleptic characteristics of rainbow trout fillets fed with diets based on T. molitor are unaffected.
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