Insects reared on seaweed as novel feed ingredients for Atlantic salmon (Salmo salar) : Investigating the transfer of essential nutrients and undesirable substances along the seaweed-insect-fish food chain

Abstract

Traditionally, major sources of protein and lipid in aquaculture fish feeds have been fish meal and fish oil. However, fish stocks used for fish meal and fish oil production are fully exploited, therefore prices of these ingredients continue to increase. In recent years, substantial progress has been made by the research community and feed producers to test novel sources of protein and lipid to replace marine feed ingredients in aquaculture. Insects have been identified as feed ingredients of great potential for farmed fish. In particular, being high in energy and protein content, they seem a good source of ingredients in compound feeds for Atlantic salmon (Salmo salar). However, insects reared on terrestrial feedstuff are not a source of the essential marine omega-3 fatty acids, which Atlantic salmon has a dietary requirement for. The AquaFly project aimed to develop novel insect feed ingredients for Atlantic salmon, contributing essential nutrients to produce robust and healthy fish. To achieve this, tailoring of the nutrient composition of the insect feed ingredients towards fish nutrient requirements was investigated through the use of seaweed as feeding substrate for the insects. Seaweeds are known to contain marine omega-3 fatty acids and essential minerals (like iodine) which are generally absent in terrestrial feedstuff for insects. At the same time, seaweeds can contain undesirable substances, especially heavy metals and arsenic, which could be transferred to the insects, therefore enter the food production chain. The focus of this PhD project, as part of AquaFly, was to evaluate the suitability and safety of the seaweed-insect-fish food production chain, by studying the transfer of both nutrients and undesirable substances along the food chain. Several species of seaweeds from Norwegian waters were screened for their chemical profile. The seaweed species studied contained both nutrients and undesirable substances (heavy metals and arsenic); the concentrations thereof were highly dependent on species and taxonomic group. Based on the data obtained, the brown alga Ascophyllum nodosum was chosen for insect rearing. This seaweed species showed the highest lipid content and the lowest concentrations of undesirable substances among the species investigated. In the insect feeding trial, larvae of the black soldier fly (Hermetia illucens) were fed plant-based media enriched with A. nodosum in increasing percentages (from 0 to 100 % seaweed inclusion). The larvae could grow on such media, up to 100 % seaweed. However, the best growth performance, nutrient utilization and retention were seen up to 50 % seaweed inclusion in the media. Transfer of heavy metals and arsenic from seaweed to larvae occurred, with higher concentrations of these undesirable substances in larvae fed higher inclusion of seaweed in the media. Based on the results, large batches of insect larvae fed 50 % seaweed in the media were grown to produce insect meal and insect lipid for the fish trials. Two fish feeding trials were conducted with i) freshwater-phase juvenile fish and ii) seawater-phase post-smolt fish. In the freshwater trial, fish were fed six diets: a control diet with protein and lipid from traditional ingredients (fish meal, fish oil, plant-based protein and vegetable oil) and five experimental diets where insect meal replaced 85 % of the dietary protein and /or all the vegetable oil was replaced by insect lipid. In this trial, the inclusion of insect feed ingredients in the fish diet did not affect the overall growth parameters, nutrient digestibility and whole fish nutrient composition. In the seawater trial, insect meal was used as replacement for fish meal in the feed (33, 66, 100 % replacements). This did not impact growth performances of the fish, nutrient utilization or fillet composition. In both fish trials, low levels of heavy metals were found in feed and fish. In the seawater trial, transfer of arsenic from feed to fish occurred; however, levels of arsenic in the fillet decreased in fish fed increasing insect meal in the diet. Based on the results of this PhD work, it can be concluded that the seaweed-insect-fish food chain holds great benefits for farming Atlantic salmon by producing nutritious feeds and healthy, robust fish. Moreover, it is an overall safe approach, in terms of both feed and food safety.