The use of a general linear model to identify epidemiological factors affecting the abundance of chalimus stages of the sea louse (Lepeophtheirus salmonis) on Scottish salmon farms

In this communication chalimus lice numbers in the second, third and fourth quarters of the production cycle are examined in relation to a range of prevailing risk factors using an approach based on that already taken for mobile stages (Revie et al. in press). A total of 54 two-year production cycles from 29 farms on the West Coast of Scotland were analysed. Chalimus levels in the second six-month period of the first year of the production cycle were principally explained (adjusted R2 = 56% of the variation) by:the number of veterinary medicine treatments administered, the site hydrography and current speed characteristics. Sites with a high chalimus abundance attracted more treatments as did fish located in sites with low current speeds and an oscillatory hydrography. In contrast, chalimus levels in the third six-month period of the production cycle were principally explained (adjusted R2 = 65% of the variation) by: the abundance of mobile and chalimus stages in the preceding six-month period and treatment. It would appear that there are both management and environmental factors which affect the number of chalimus sea lice on fish in Scottish salmon farms and these have an impact on overall sea lice abundance

Changes in epidemiological patterns of sea lice infestation on farmed Atlantic salmon, Salmo salar L., in Scotland between 1996 and 2006

Analyses of a unique database containing sea lice records over an 11 year period provide evidence of changing infestation patterns in Scotland. The data, collected from more than 50 commercial Atlantic salmon farms, indicate that both species of sea lice commonly found in Scotland, Lepeophtheirus salmonis and Caligus elongatus, have declined on farms over the past decade. Reductions for both species have been particularly marked since 2001 when more effective veterinary medicines became available. Treatment data were also available in the database and these show a growing trend towards the use of the in feed medication emamectin benzoate (Slice), particularly in the first year of the salmon production cycle. However, this trend to wards single product use has not been sustained in 2006, the latest year for which data are available. There is some evidence of region to region variation within Scotland with the Western Isles experiencing higher levels of infestation. However, compared to the levels observed between 1996 and 2000, all regions have benefited from reduced lice infestation, with the overall pattern showing a particular reduction in the second and third quarters of the second year of production

Study reveals regularity of Caligus elongatus infection

abstract: This article presents the seasonal and annual patterns of C. elongatus infestation on farmed salmon in Scotland, UK. The effect of management factors and veterinary treatments on infestation and differences in abundance patterns of C. elongatus from Lepeophtheirus salmonis are also described

Identifying epidemiological factors affecting sea lice (Lepeophtheirus salmonis)abundance on Scottish salmon farms using general linear models

The variation in Lepeophtheirus salmonis sea lice numbers across 40 Scottish salmon farm sites during 1996 to 2000 was analysed using mean mobile abundance for 3 important 6 mo periods within the production cycle. Using statistical regression techniques, over 20 management and environmental variables suspected to have an effect on controlling lice populations were investigated as potential risk factors. The findings and models developed provide a picture of mobile L. salmonis infestation patterns on Scottish farm sites collectively. The results identified level of treatment, type of treatment, cage volume, current speed, loch flushing time and sea lice levels in the preceding 6 mo period to be key explanatory factors. Factors such as stocking density, site biomass, water temperature and the presence of neighbours, previously cited to be important correlates of sea lice risk from analysis of individual sites over time, were not found to be important. Variation in mobile abundance in the first half of the second year of production could be adequately explained (adjusted R2 between 55 and 72%) by the recorded data, suggesting that there is scope for management to control L. salmonis abundance, though much of the variation remains unexplained

Analysis of bacterial populations in the gut of developing cod larvae and identification of Vibrio logei, Vibrio anguillarum and Vibrio splendidus as pathogens of cod larvae

Poor larval survival rates currently limit production in aquaculture and a better understanding of the bacterial flora in the larval digestive tract is essential to improving survival in hatchery reared larval fish. Identification of bacteria alone is insufficient, as the concentrations of key bacterial species may be important in determining survival. Here, we analysed bacteria, principally vibrios, in developing cod larvae from a Scottish hatchery. Six tanks of larvae were sampled weekly in parallel to monitor the microbial populations in larvae successively fed on rotifers and Artemia before weaning onto dry feed. When cod larvae were fed on rotifers the digestive tract microbiota was dominated by Ruegeria/Roseobacter, Pseudoalteromonas, and Microbacterium but when the diet was changed to Artemia over 90% of the microbiota consisted of vibrios. Polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) analysis was used to identify the vibrios present and this showed that the microbiota was affected by changes in larval diet. Whilst feeding on Artemia, the larval digestive tract microbiota was dominated by Vibrio alginolyticus, but the population rapidly switched to one dominated by V. splendidus when larvae were weaned onto dry feed. Other Vibrio species were detected at lower levels at various times. During the survey, one tank experienced heavy mortalities associated with an elevated level of Vibrio anguillarum. The V. anguillarum isolates differed from those typically associated with mortalities in gadoid fish. Apparently healthy larvae from the tank experiencing heavy mortalities had a culturable microbiota almost equally represented by V. anguillarum and V. logei. Both species, as well as a V. splendidus strain isolated from turbot larvae, were pathogenic to cod larvae in laboratory experiments, and these vibrios could be targets for intervention to control the microbial flora of the larval digestive trac

The epidemiology of the sea lice, Caligus elongatus Nordmann, in marine aquaculture of Atlantic salmon, Salmo salar L., in Scotland

Although Caligus elongatus is one of two major species of sea lice that parasitize farmed salmon, its epidemiology has not been extensively studied. In this communication, the abundances of the adult stage of C. elongatus in salmon populations from 33 farms in the West of Scotland between 1997 and 2000 have been analysed for evidence of seasonal and annual patterns. The findings indicate that the pattern of C. elongatus is remarkably consistent from year to year, and directly opposed to that reported for Lepeophtheirus salmonis, the other major caligid species. In particular, adult infestations rapidly increase from the start of July each year, and are more prevalent on salmon in the first year of production than the second year. Treatment is seen to have an effect on levels of infestation but it is not clear why this species should have significantly lower levels of abundance in the second year of production. There is also evidence that fallowing has no effect on abundance. Strategic management programmes for the control of sea lice on salmon farms, which are increasingly effective in controlling L. salmonis, may also have to give greater consideration to C. elongatus