The effects of replacing 40% of dietary fish meal (FM) in a reference diet (REF) with either mussel meal (MM), zygomycete fungi (ZYG), extracted baker’s yeast (EY), or non-extracted baker’s yeast (NY) on the lipid and metabolic profile of Arctic charr (Salvelinus alpinus) were investigated. After a 14-week feeding trial, liver and muscle tissues were collected for lipid (lipid content, lipid class, fatty acid composition) and ¹H NMR-based metabolomics analyses (aqueous and chloroform phases). Lipid analyses showed that fish fed ZYG diet had lower liver lipid content and thereby 10% higher level of docosahexaenoic acid compared with REF. Metabolomics analyses showed that on the one hand fish fed NY diet affected liver metabolites (2–3 fold higher concentrations of e.g., n,n-dimethylglycine and betaine) compared with REF, while, on the other hand, the muscle metabolic fingerprint was mainly affected by EY. In general, affected metabolites (e.g., alanine, anserine, betaine, hydroxyproline, isoleucine, malonate, n,n-dimethylglycine, proline, succinate, and valine) in fish fed test diets suggested that the test meal ingredients caused mainly a response in muscle metabolism. Fish metabolism was least affected by MM, which suggests that it may be suitable to replace fish meal in Arctic charr diets.

A nutritional bottleneck in salmonid aquaculture is the procurement of marine‐derived compounds, such as essential amino and fatty acids, including omega‐3 fatty acids, lysine, and methionine. Therefore, insects containing these compounds are highly promising as feed ingredients. The present study evaluates larvae of a “marine” insect (Coelopa frigida, the bristly‐legged seaweed fly larvae, SWFL) reared on brown algae side streams as a feed ingredient for rainbow trout (Oncorhynchus mykiss). SWFL contained, on a dry matter basis, 60% crude protein (CP), 3.5% lysine, and 1.5% methionine, as well as 17% lipids, including 4% eicosapentaenoic acid and docosahexaenoic acid. Four isoenergetic (ca. 23 MJ kg⁻¹ gross energy) and isoproteic (ca. 45% CP) feeds were evaluated in a 10‐week feeding trial. The diets included a control containing 25% fish meal, a commercial reference, and two diets substituting 40% fish meal with either SWFL or partially defatted black soldier fly larvae (BSFL) meal (Hermetia illucens). SWFL‐fed fish displayed higher specific growth rates (SGR) compared to BSFL‐fed fish and similar SGR compared to control and reference diet‐fed fish. Feed intake in SWFL‐fed fish was higher than for the control and BSFL diets and similar to the reference diet. The feed conversion ratio of fish fed the BSFL and SWFL diets was higher compared to the control, suggesting lower bioavailability of both insect meals compared to fish meals. No difference in intestinal health parameters was noted between the insect diets and the control diet, indicating good intestinal health across all treatments. However, changes in electrogenic intestinal transport were observed between the SWFL and BSFL diets, illustrating the heterogeneous effect of different insect products. Overall, SWFL meal is a promising alternative marine feed ingredient, compatible with circular production systems, as it can be efficiently cultivated using marine side streams.

The objective of the present study was to investigate the optimal dietary protein requirement and the effect of varying protein levels on the growth and health of juvenile, wild‐caught Atlantic wolffish, Anarhichas lupus, a promising candidate for cold‐water aquaculture diversification. Six iso‐energetic (ca. 18.3 MJ kg⁻¹), fish meal‐based experimental diets were formulated with crude protein levels ranging from 35% to 60%, with graded increments of 5% in a 12‐week feeding trial in a recirculating aquaculture system (RAS). Weight gain, specific growth rate (SGR), and condition factor (K) were evaluated in response to dietary protein levels. Liver, muscle, and blood parameters were assessed for possible changes in protein and lipid metabolism and welfare. Overall growth was highly variable throughout the experiment on all diets, as expected for a wild population. The feed with highest in protein (60%) inclusion resulted in the highest growth rates, with an average weight gain of 37.4% ± 33.8% and an SGR of 0.31% ± 0.2% day⁻¹. This was closely followed by feeds with 55% and 50% protein inclusion with an average weight gain of 22.9% ± 34.8% and 28.5% ± 38.3%, respectively, and an SGR of 0.18% ± 0.3% day⁻¹ and 0.22% ± 0.3% day⁻¹, respectively. Fish fed the high protein diets generally had increased hepatic lipid deposition (17%–18%) and reduced free fatty acid levels (3.1–6.8 μmol L⁻¹) in the plasma relative to fish that were fed the lower protein diets (35%–45%). No effects of diet were found on plasma protein levels or muscle protein content. Furthermore, stress parameters such as plasma cortisol and glucose levels were unaffected by diet, as were plasma ghrelin levels. Overall, these results suggest that a high protein inclusion in the diet for Atlantic wolffish is required to sustain growth with a minimum protein level of 50%.

Integrated multi‐trophic aquaculture (IMTA), defined as the co‐production of species at different trophic levels, is conceptually appealing. By utilising complementary functions of different species, it holds opportunities for lower environmental impact, higher productivity, and increased profitability in comparison with conventional monoculture. However, whether those advantages materialise in practice remains unclear. Here we review available life cycle assessment (LCA) and cost–benefit analysis (CBA) studies of IMTAs to further the current understanding of environmental and economic performance. We find that, overall, LCAs conclude that IMTAs investigated have not been able to deliver on environmental promises, while CBAs generally find that IMTAs have both better financial performance and lower environmental externalities than monoculture equivalents. The main reasons for these contrasting findings include the nature of the systems assessed so far—often theoretical or experimental set‐ups with non‐optimised designs—combined with methodological choices in assessments. The findings of the review thus suggest that it is still unclear to what extent IMTA can deliver on its promises in practice, and that IMTA is no panacea for aquaculture but requires careful design to be effective for its intended use. Integrating robust LCAs and CBAs early in the development phase can help identify viable IMTA set‐ups based on key drivers of environmental and economic performance and, in this way, minimise the risks of misaligned practices. A first step towards realising the potential of IMTA is to carry out robust assessments that avoid key mistakes in both LCAs and CBAs, to which end we provide hands‐on recommendations for future assessments.