With the increasing interest and activities regarding seaweed cultivation in Europe, it is becoming crucial to utilize the opportunities that lies within environmental resources, such as light and nutrients, to produce biomass of a high yield and quality. The chemical composition of seaweed varies between seasons and depths as an effect of resource supply and environmental conditions. These factors are particularly important to assess for economically feasible species like the kelp Saccharina latissima. This paper examines how differences in light conditions and nutrient availability affect the growth and intracellular nitrogen in S. latissima. This was done through cultivating sporophytes in land-based tanks with four different combinations of high/low light and high/low nutrient supply over an experimental period of 20 days, with measurements of growth rate and subsequent analysis of tissue nitrogen metabolites. The results revealed that the mean growth rate and the intracellular nitrogen components of the sporophytes were positively related to the external nitrate concentration during the experimental period, indicating that S. latissima requires high nutrient concentration to maintain a rapid growth.
The present article reports the densities of planktonic sea lice (Lepeophtheirus salmonis and Caligus elongatus) in three Atlantic salmon (Salmo salar) localities, and the relationship between the abundance of adult sea lice on the salmon and the densities of planktonic sea lice stages, during a complete production cycle followed by a fallowing period. Samples were taken downstream inside and immediately outside of cages, at one locality with lice skirts and two localities without lice skirts. There were no differences in densities of planktonic sea lice in samples taken from the inside or the outside of cages for any of the localities. However, the proportion-non-zero of planktonic sea lice samples taken from inside the cage was higher during months with a temperature above 9°C (mean abundance: 0.40–2.5 individuals m–3) than months with temperature below 9°C (mean abundance: 0.02–0.21 individuals m–3, odds ratio of the proportion-non-zero: p < 0.01). Densities of planktonic sea lice correlated most strongly with temperature in the first year (τ = 0.44–0.57, p < 0.05). A significant correlation between the number of adult female lice on salmon and average density of plankton sea lice was found in the locality with lice skirts during the second year (τ = 0.43 inside cages, τ = 0.58 outside cages, both p values < 0.05). Background levels of planktonic sea lice in the succeeding fallowing period showed neither L. salmonis nor C. elongatus planktonic sea lice, suggesting that there was successful reduction of the densities of planktonic sea lice for this area during the fallowing period.
The aims of the present study were to describe the salmon lice (Lepeophtheirus salmonis Krøyer, 1837) situation in an intensive salmon production area in mid‐Norway and to consider implications of changing practices of how salmon lice infestation can be controlled. The results in this study suggest that there are steps that can be carried out to keep salmon lice under control even during years when the temperature facilitates a quick salmon lice development. The present work indicates that the use of cleaner fish can delay the time it takes adult female lice to reach 0.1 per salmon in the beginning of a production cycle. It suggests that the timing of cleaner fish deployment into salmon cages can influence its effectiveness in controlling salmon lice. It also gives caution to letting salmon lice develop unchecked, even at levels far below the current lice limit, because of the difficulties to control salmon lice when the external infection pressure is too high. This study took place during a rapid change in delousing methods, in an area with coordinated salmon production. Despite its exploratory nature, this study offers insights into the salmon lice fluctuations in relation to efforts aimed at controlling it.
The objective of the current study was to determine the extent to which planktonic sea lice (Lepeophtheirus salmonis Krøyer, 1838 and Caligus elongatus Nordmann, 1832) were present in the biofouling on open-sea net pens used for commercial rearing of Atlantic salmon (Salmo salar L.), and to assess if biofouling organisms on nets act as barriers similar to salmon lice skirts. We have examined two possible interactions of biofouling and planktonic sea lice, the first was if biofouling could function as a microhabitat for planktonic sea lice, with resuspension of sea lice during net cleaning operations. The second interaction was if biofouling may cause a retaining effect on the transport of planktonic stages out of the net pen. These interactions were investigated at different commercial salmon farms. With only one sea louse found among the biofouling on nets and cleaner fish shelters, we found no indication that sea lice utilized biofouling as a reservoir. This was further supported by the lack of impact on the proportion of samples with sea lice and the average density of sea lice in the water following the release of biofouling material during in situ net cleaning. Furthermore, the presence of biofouling had no effect on the proportion of samples with sea lice or the average density of planktonic sea lice in the net pens. The presence of a lice skirt resulted, however, in a significantly higher proportion of samples with planktonic sea lice inside the net pen in one of the two sites utilizing lice skirts. The results of our study suggested that the presence of biofouling has no influence on the average density and proportion of samples with planktonic stages of sea lice and that planktonic sea lice do not inhabit biofouling.
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