To assess the efficacy of lumpfish grazing on attached sea lice on Atlantic salmon, six large-scale sea cages, (130 m circumference, 37,688 m³ volume) each stocked with approximately 200,000 salmon 0+ smolts, were stocked with a 4, 6 and 8% density (8000, 12,000 and 16,000, respectively) of lumpfish. The sea cages without lumpfish acted as controls. Sea lice infestation levels on the salmon were monitored weekly and bi-weekly from 6 October to 17 May the subsequent year. Mortality of the lumpfish rose with decreasing sea temperatures to around 0.8% week⁻¹ and did not vary between the lumpfish groups. There were clear signs of lumpfish grazing on sea lice, with significantly lower average levels of chalimus, pre-adult and adult female Lepeophtheirus salmonis and Caligus elongatus sea lice per salmon. Lumpfish in the high density (8%) group reduced the mature female L. salmonis to levels equal to or lower than the counts recorded prior to the start of the study. Overall, the present results indicate that lumpfish are a suitable cold-water option for biological delousing of Atlantic salmon in large-scale production conditions.

Growth, feed conversion e⁄ciency and frequencies of skeletal deformities were studied in juvenile Atlantic cod (Gadus morhua )t hat had been startfed on either rotifers (rotifer group) or zooplankton (zooplankton group). After metamorphosis, the ¢sh were reared at four constant temperatures (7, 10, 13, 161C) or moved successively from 16 to 13 and 101C (T-step, average 13.2 1C). The zooplankton group had a consistently higher growth rate at all the temperatures studied. Further, the zooplankton group had higher food in- take (20%) and higher feed conversion ratio (1.65 vs. 1.31). In addition, a signi¢cantly higher incidence of skeletal deformities was found in the rotifer group (14.2%) compared with the zooplankton group (4.1%). After termination of the laboratory study, the ¢sh were reared in sea pens under ambient condi- tions for 17 months. Final weights of the zooplankton group were consistently larger (between 12% and 14% larger depending on original temperature groups). To verify the growth results, we conducted a follow-up study where a single egg group was divided into two parts and fed either on rotifers or zooplank- ton. This study indicated similar growth di¡erences as found in the ¢rst study. Overall, our data suggest that di¡erent startfeeding diets may be important for long-term growth, incidence of deformities and qual- ity of juvenile cod. The use of zooplankton can greatly improve long-term growth and quality of cod juve- niles. The study also highlights the advantage of using elevated temperatures in the juvenile phase as this will lead to signi¢cantly higher ¢nal weights in the adult stage.

In this review, we have systematized current knowledge about the effect of stocking lumpfish (Cyclopterus lumpus) as cleaner fish to control Lepeophtheirus salmonis infestations on farmed Atlantic salmon (Salmo salar L.). The review was prompted by recent reports in which the usefulness of lumpfish has been doubted, and the urgent need to investigate whether common lumpfish can be used to reduce L. salmonis numbers on farmed Atlantic salmon by active grazing on this species. Available published data clearly indicate that lumpfish graze on L. salmonis, and can significantly lower the lice burden in Atlantic salmon farming. It is possible to enhance the lice grazing behavior of lumpfish with the assistance of live feed conditioning prior to sea pen transfer, and with selective breeding. Data indicate that lice grazing of lumpfish is size dependent, and grazing effect is low for lumpfish larger than 200–250 g. Observations from large-scale rearing of Atlantic salmon in open sea cages in Norway, Iceland, the Faroe Islands, and Scotland also indicate that lumpfish can be effective in lowering infestations of L. salmonis on salmon. Overall, this present review reveals that lumpfish can actively contribute to lower numbers of L. salmonis on farmed Atlantic salmon.