By Beate Hoddevik Sunnset
On a recently completed mission, the “G.M. Dannevig” did 15 trawls in the fjord for wild smolt. 45 smolt were caught, which between them had a total of five lice. Seven sea trout were also caught, which had 54 lice between them, although 36 of these were on a single individual. In addition, smolt cages were deployed at selected locations. Preliminary results show that there are few lice on the fish. Since all of the fish will be examined under microscope to register all lice, these figures are only provisional.
– Lice numbers at the moment are fairly similar to last year, when there were few lice in May, followed by a big increase over the summer and autumn, says researcher Lars Asplin. Developments over the next few weeks are dependent on the temperature and on the success of delousing campaigns at aquaculture facilities. The currents in the fjord can also affect the situation, but it is uncertain how big an impact they have.
Cold water is helpful
Cold water in the winter and spring limits any increase in the number of sea lice, and egg production is reduced to a minimum. At low temperatures, it takes longer for the eggs to hatch than at higher temperatures. As the temperature rises over the summer, egg production therefore increases. Consequently, the number of sea lice multiplies, potentially causing problems for sea trout in later months, as they stay in the fjord throughout the summer. Salmon smolt migrate out of the fjord in the spring, which means that they may get out before the number of lice increases, but the smolt migration is also controlled by the temperature, and this year’s migration looks likely to take place later than last year.
Annual sea lice studies
Each year, The Institute of Marine Research studies the sea lice level in Hardangerfjorden by trawling for fish and deploying smolt cages. The smolt cages are deployed in the same locations each year, and the fish stay in the cages for around 14 days before being replaced by new smolt. This year the first of three groups of smolt was deployed on 10 May, and the last one will be removed at the end of June. We also trawl for wild salmon smolt in the fjord, to compare the number of lice on them with the salmon in the cages. The second round of trawling starts tomorrow, Thursday 3 June, and will continue over the weekend. All of the smolt are inspected visually after they have been taken out/caught, which is how we know that there are currently few sea lice in Hardangerfjorden. Subsequently each fish will be studied under a microscope to determine the exact number of lice.