Assessment of growth and sea lice infection levels in Atlantic salmon stocked in small-scale cages with lumpfish Albert K. Imsland a, ⁎ ,1 , Patrick Reynolds b,1 , Gerhard Eliassen b , Thor Arne Hangstad c , Ane Vigdisdatter Nytrø c , Atle Foss d , Erik Vikingstad d , Tor Anders Elvegård e a Department of Biology, University of Bergen, High Technology Centre, 5020 Bergen, Norway b GIFAS AS, Gildeskål, 8140 Inndyr, Norway c Akvaplan-niva, Framsenteret, 9296 Tromsø, Norway d Akvaplan-niva Bergen, Thormøhlensgate 53 D, N-5006 Bergen, Norway e Nordlaks Oppdrett AS, Post box 224, 8455 Stokmarknes, Norway abstract article info Article history: Received 7 April 2014 Received in revised form 30 May 2014 Accepted 12 June 2014 Available online 21 June 2014 Keywords: Lumpfish Atlantic salmon Duoculture Growth Sea lice Growth and performance of Atlantic salmon in the presence of lumpfish were assessed in two studies. In the first study, six sea cages (5 × 5 × 5 m) were each stocked with 120 Atlantic salmon with a mean (±SD) weight of 619 (±49) g and reared for 159 days. Control cages were without lumpfish, while two of the cages were stocked with 12 lumpfish (10% density), and two with 18 lumpfish (15% density) with a mean (±SD) weight of 54.0 (±7.2) g. The lumpfish were removed from the cages after 56 days, but feed consumption and growth of the salmon were monitored for another 12 weeks until 17 November 2012 to investigate possible long term effect from the presence of lumpfish on growth performance of the salmon. In the second study, six sea cages (5 × 5 × 5 m) were each stocked with 80 Atlantic salmon with a mean (±SD) weight of 2400 (±220) g and reared for 60 days. Two of the cages were further stocked with 4 lumpfish (5% density), and two with 8 lumpfish (10% density) with a mean (±SD) weight of 360 (±30) g. Two cages without lumpfish acted as controls. In both studies sea lice infes- tation levels were recorded every other week. In the study with small salmon and lumpfish, the presence of lump- fish did not have any negative short- or long-term effects on feed conversion ratio (FCR) or specific growth rate (SGR) in salmon. However, when large salmon were reared together with large lumpfish, FCR was lower and SGR higher in the control cages compared to the two lumpfish treatments. Significantly lower sea lice infection levels were seen on Atlantic salmon when reared together with small lumpfish compared to the control group with- out lumpfish, whereas this trend was not as clear when reared with larger lumpfish. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Duoculture of Atlantic salmon and other fish species has mainly been investigated along two lines. Firstly, there have been trials with Atlantic salmon in duoculture with other salmonoids in an attempt to decrease aggression and enhance growth (Holm, 1989; Nortvedt and Holm, 1991). More recently Atlantic salmon are being reared with other fish species as as cleaner fish to delouse the salmon. The biological control of sea lice through the use of cleaner-fish has recently becomes a feasible option due to the increased occurrence of resistance towards medical treatments in salmon lice, Lepeophtheirus salmonis (Lees et al., 2009; Torrissen et al., 2013), the reduced public acceptance of chemo- therapeutic use in food production and the urgent need for an effective and sustainable method of parasite control in Atlantic salmon aquaculture (Denholm et al., 2002; Treasurer, 2002). Today, cleaner-fish present the only environmentally friendly alternative to chemical de-lousing of salmonids (Skiftesvik et al., 2013; Treasurer, 2002). Imsland et al. (2014a) investigated the possible use of common lumpfish, Cyclopterus lumpus L., as a cold-water cleaner fish. They found clear signs of lumpfish grazing on sea lice, with significantly lower average numbers of pre-adult, mature males and females stages of lice per salmon. There was clear evidence of grazing from the results of gastric lavage, with 28% of all lumpfish found to have ingested L. salmonis on the last sampling day. Overall, the results indicated that lumpfish is a suitable cold-water option for biological delousing of Atlan- tic salmon. Up to now wrasse species have been used as cleaner fish for Atlantic salmon (Skiftesvik et al., 2013). Of the four wrasse species used as cleanerfish, only two grow large enough to delouse salmon during their second year in net pens: corkwing wrasse Symphodus melops and ballan wrasse Labrus bergylta. The ballan wrasse is the largest and hardiest of these two species and, therefore, the most valuable for the industry (Skiftesvik et al., 2013). Ballan wrasse has been found to efficiently clean sea lice off Atlantic salmon at high temperatures. At a ratio of 5% Aquaculture 433 (2014) 137–142 ⁎ Corresponding author at: Akvaplan-niva Iceland Office, Akralind 4, 201 Kópavogur, Iceland. E-mail address: albert.imsland@akvaplan.niva.no (A.K. Imsland). 1 Equal authorship between: Imsland and Reynolds. http://dx.doi.org/10.1016/j.aquaculture.2014.06.008 0044-8486/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online