ORIGINAL ARTICLE Effects of three commercial diets on growth, cataract development and histopathology of lumpfish (Cyclopterus lumpus L.) Albert K D Imsland 1,2 | Patrick Reynolds 3 | Thor Magne Jonassen 4 | Thor Arne Hangstad 4 | Tor Anders Elvegård 5 | Tonje Cecilie Urskog 6 | Bjørn Mikalsen 7 1 Akvaplan‐niva Iceland Office, Kópavogur, Iceland 2 Department of Biology, High Technology Centre, University of Bergen, Bergen, Norway 3 GIFAS AS, Inndyr, Norway 4 Akvaplan‐niva, Framsenteret, Tromsø, Norway 5 Nordlaks Oppdrett AS, Stokmarknes, Norway 6 Grieg Seafood Finnmark AS, Alta, Norway 7 Lerøy Aurora, Tromsø, Norway Correspondence Albert K D Imsland, Akvaplan‐niva Iceland Office, Akralind 4, 201 Kópavogur, Iceland. Email: albert.imsland@akvaplan.niva.no Funding information Norges Forskningsråd, Grant/Award Number: 256199 Abstract Three duplicate groups of individually tagged lumpfish (mean initial weight: 20.0 ± 4.0 g) were fed three commercially available diets (A, B and C) for a period of 147 days. There were significant differences in growth rates between the groups with fish fed diet A having the highest growth rates whilst fish fed diet C achieved the lowest growth. Lumpfish fed diets A and B had lower prevalence and severity of cataracts compared with fish fed diet C, suggesting a possible dietary effect on cataract, alone or in combination with other environmental factors. There was a sig- nificant difference in the degree of liver vacuolization between the three dietary treatments at the end of the study period with fish fed diets A and B showing mod- erate to severe vacuolization compared with liver tissue from fish fed diet C, which had milder vacuolizations. There was little or no inflammation in pyloric caeca, mid- gut and hindgut and similarly for epithelial necrosis or vacuolization for all three dietary treatments at the end of the study. Fish fed diet A had the longest intestinal folds whilst fish fed diet C had the shortest. KEYWORDS benchmark, cataract, diets, growth, gut histology, lumpfish 1 | INTRODUCTION The biological control of sea lice in Atlantic salmon farming using “cleaner fish” has recently become a feasible alternative due to the increased occurrence of resistant lice, the reduced public acceptance of chemotherapeutic use in food production, and the urgent need for an effective and sustainable method of parasite control in Atlan- tic salmon aquaculture (Boxaspen, 2006; Denholm et al., 2002; Trea- surer, 2002). As a cold‐water cleaner fish, the common lumpfish (Cyclopterus lumpus) has been suggested and initial results are very promising with up to 93%–97% less sea lice infection (adult female lice) in sea pens with lumpfish (Imsland, Reynolds, Eliassen, Hangstad, Foss et al., 2014; Imsland et al., 2014a,b, 2015a,b, 2016a, b). However, efficient use of lumpfish in terms of the proportion of fish that graze sea lice is dependent on the establishment of healthy and robust populations with their nutritional requirements fulfilled. Lumpfish in sea pens are strongly opportunistic and the fish do not restrict themselves or rely on a single food source if others are pre- sent (Imsland et al., 2015a) and a very high proportion of the lump- fish in salmon sea pens are found with salmon feed pellets in their stomach (Imsland, Reynolds, Eliassen, Hangstad, Foss et al., 2014; Imsland et al., 2015a, 2016b). Thus, it is becoming increasingly evi- dent that the supplementary feeding of cleaner fish deployed within commercial salmon pens is necessary (Imsland et al., 2018; Leclercq, Graham & Migaud, 2015) to maintain the nutritional condition, wel- fare and efficacy as biological delousers. With the dramatic rise in Equal authorship between: Imsland and Reynolds. Received: 20 February 2018 | Revised: 20 April 2018 | Accepted: 12 June 2018 DOI: 10.1111/are.13776 Aquaculture Research. 2018;1–11. wileyonlinelibrary.com/journal/are © 2018 John Wiley & Sons Ltd | 1