K L Hammell

Detection of emamectin benzoate tolerance emergence in different life stages of sea lice, Lepeophtheirus salmonis, on farmed Atlantic salmon, Salmo salar L

Emamectin benzoate has been used to treat sea lice, Lepeophtheirus salmonis, infestations on farmed Atlantic salmon, Salmo salar. Recent evidence suggests a reduction in effectiveness in some locations. A major challenge in the detection of tolerance emergence can be the typically low proportion of resistant individuals in a population during the early phases. The objectives of this study were to develop a method for determining differences in temporal development of tolerance between sea lice life stages and to explore how these differences might be used to improve the monitoring of treatment effectiveness in a clinical setting. This study examined two data sets based on records of sea lice abundance following emamectin benzoate treatments from the west coast of Scotland (2002-2006) and from New Brunswick, Canada (2004-2008). Life stages were categorized into two groups (adult females and the remaining mobile stages) to examine the trends in mean abundance and treatment effectiveness. Differences in emamectin benzoate effectiveness were found between the two groups by year and location, suggesting that an important part of monitoring drug resistance development in aquatic ectoparasites may be the need to focus on key life stages.

Effectiveness of emamectin benzoate for treatment of Lepeophtheirus salmonis on farmed Atlantic salmon Salmo salar in the Bay of Fundy, Canada.

Emamectin benzoate (an avermectin chemotherapeutant administered to fish as an in-feed treatment) has been used to treat infestations of sea lice Lepeophtheirus salmonis on farmed Atlantic salmon Salmo salar in the Bay of Fundy, New Brunswick, Canada, since 1999. This retrospective study examined the effectiveness of 114 emamectin benzoate treatment episodes from 2004 to 2008 across 54 farms. Study objectives were to establish whether changes in the effectiveness of emamectin benzoate were present for this period, examine factors associated with treatment outcome, and determine variables that influenced differences in L. salmonis abundance after treatment. The analysis was carried out in 2 parts: first, trends in treatment effectiveness and L. salmonis abundance were explored, and second, statistical modelling (linear and logistic regression) was used to examine the effects of multiple variables on post-treatment abundance and treatment outcome. Post-treatment sea lice abundance increased in the later years examined. Mean abundance differed between locations in the Bay of Fundy, and higher numbers were found at farms closer to the mainland and lower levels were found in the areas around Grand Manan Island. Treatment effectiveness varied by geographical region and decreased over time. There was an increased risk for unsuccessful treatments in 2008, and treatments applied during autumn months were more likely to be ineffective than those applied during summer months.

A fixed-dose approach to conducting emamectin benzoate tolerance assessments on field-collected sea lice, Lepeophtheirus salmonis.

In New Brunswick, Canada, the sea louse, Lepeophtheirus salmonis, poses an on-going management challenge to the health and productivity of commercially cultured Atlantic salmon, Salmo salar. While the in-feed medication, emamectin benzoate (SLICE® ; Merck), has been highly effective for many years, evidence of increased tolerance has been observed in the field since late 2008. Although bioassays on motile stages are a common tool to monitor sea lice sensitivity to emamectin benzoate in field-collected sea lice, they require the collection of large numbers of sea lice due to inherent natural variability in the gender and stage response to chemotherapeutants. In addition, sensitive instruments such as EC(50) analysis may be unnecessarily complex to characterize susceptibility subsequent to a significant observed decline in efficacy. This study proposes an adaptation of the traditional, dose-response format bioassay to a fixed-dose method. Analysis of 657 bioassays on preadult and adult stages of sea lice over the period 2008-2011 indicated a population of sea lice in New Brunswick with varying degrees of susceptibility to emamectin benzoate. A seasonal and spatial effect was observed in the robustness of genders and stages of sea lice, which suggest that mixing different genders and stages of lice within a single bioassay may result in pertinent information being overlooked. Poor survival of adult female lice in bioassays, particularly during May/June, indicates it may be prudent to consider excluding this stage from bioassays conducted at certain times of the year. This work demonstrates that fixed-dose bioassays can be a valuable technique in detecting reduced sensitivity in sea lice populations with varying degrees of susceptibility to emamectin benzoate treatments.

Randomized clinical field trial of a bacterial kidney disease vaccine in Atlantic salmon, Salmo salar L.

A randomized, blinded clinical trial was performed to assess the relative effectiveness of five commercial and one experimental vaccine in a population of farmed fish experiencing a bacterial kidney disease (BKD) outbreak that occurred in one study cage that was part of a larger clinical field trial. A total of 6000 uniquely identified Atlantic salmon S1 presmolts were randomly assigned to vaccine groups in the hatchery and transferred to a commercial marine aquaculture site. Repeated sampling events to evaluate growth, inherent physical conditions and health status were carried out over the entire production cycle. During the second summer at sea, the study cage developed an outbreak of BKD that lasted approximately 240 days. The effectiveness of the selected vaccines was evaluated using survival analysis methods. The sole vaccine group offering protection for BKD was found to significantly decrease the hazard of dying (hazard ratio, HR = 0.68, P = 0.018) during the outbreak, compared to the industry standard, vaccine group. Additionally, during the outbreak, fish with a shortened operculum had a significantly decreased hazard (HR = 0.38, P = 0.033) compared to those fish with a normal operculum, while fish with jaw deformities had a significantly increased hazard (HR = 2.55, P = 0.001) compared to fish with normal jaw status.

Sea lice monitoring on Atlantic salmon farms in New Brunswick, Canada: comparing audit and farm staff counts

Sea lice audits were performed by the Atlantic Veterinary College on commercial aquaculture sites in New Brunswick, Canada, in 2011. Although the primary objective was to verify that farms were reporting similar lice counts to third-party counts, more detailed comparisons were made to identify when lice counts were more likely to differ between the audit team and farm employees. A total of 28 sea lice audits were conducted on 16 sites between June and December 2011. During each audit, 10 cages were evaluated per site where possible, with ten fish per cage being evaluated by an audit technician and a further ten by a farm employee. Data analysis included descriptive statistics of lice counts by stage and limits of agreement plots. A random effects negative binomial model that accounted for clustering of cages within sites was applied to assess the effect of counter type and season on lice counts by stage. The results indicate that farms counts were generally in agreement with audit counts. However, when the average counts for chalimus and preadult (male and female) and adult male lice stages were high, farm counters were more likely to report a lower value. Higher lice counts were observed during autumn compared to summer especially for the adult female stage. Finally, there was a significant clustering effect for site and cage, with most of the variation attributable to site.

Risk factors associated with time to first clinical case of Bacterial Kidney Disease (BKD) in farmed Atlantic Salmon (Salmo salar L.) in New Brunswick, Canada

Infection with Renibacterium salmoninarum, the cause of Bacterial Kidney Disease (BKD) occurs in salmon populations in many locations, including the east coast of Canada. However, information about risk factors for BKD and their effects in the saltwater phase of the salmon aquaculture industry in the region is inadequate. We conducted a retrospective observational cohort study using industry health records in which BKD was recorded in New Brunswick, Canada, between 2006 and 2012. Several risk factors for BKD, such as stocking season, mortality percentage in the first four weeks, food conversion ratio (FCR), lice treatment, Bay Management Area (BMA), and production year were analyzed in a survival analysis using Cox proportional hazards models with cross-classified random effects to account for the structure of the data. The models incorporated effects on two different time scales, time since stocking and calendar time. The risk period was from stocking in salt water to first occurrence of clinical BKD in a pen. Results were time varying. Stocking season had a pronounced effect on time to clinical BKD after middle October of the first year after stocking, with clinical cases occurring less frequently in fall/winter-stocked fish compared to summer and spring-stocked fish; for example, in middle October, the Hazard Ratio of spring- compared to fall/winter-stocked fish was 15.8 (95% CI; 1.05, 354). Differences lasted until June and July of the second year after stocking. Effects of final hatchery before transfer to seawater, and egg source were not detected, but a limitation of this study was that this information was not available for 44.3% of the fish groups in our dataset. BKD status of a site/pen before fallow period and distance to nearest site with BKD were also not detected. Feed conversion ratio and mortality during the first four weeks affected BKD, indicating that better performing fish have a reduced hazard for BKD or vice versa, and implying that good general husbandry practices and BKD are correlated.

Evaluation of external operculum loop tags to individually identify cage-cultured Atlantic halibut Hippoglossus hippoglossus in commercial research trials.

The growth, survival and tag retention of double-tagged [external FT4 lock-on (FT4) and internal passive integrated transponder (PIT)-tagged] Atlantic halibut Hippoglossus hippoglossus were compared to internal PIT-tagged controls in a randomized trial. The objective was to assess the suitability of these tags for monitoring the performance of individual fish in longitudinal trials under commercial cage-culture conditions in the lower Bay of Fundy, New Brunswick, Canada. The FT4 tags were chosen due to their similarity to tags used by investigators to track H. hippoglossus in the wild. A subset of the population randomly received an external FT4 tag inserted through the operculum and were monitored over a 1105 day period. The specific growth rate of FT4-tagged fish was significantly reduced in the first sea summer with no significant difference observed for the remainder of the trial. The differential growth in the first sea summer created a relative size advantage, permitting controls to increase in size significantly faster than FT4 fish in all subsequent periods. The FT4 tags did not significantly influence survival under normal commercial cage-culture conditions. Results, however, suggest that the survival of FT4-tagged H. hippoglossus may be compromised during stressful handling events. Tag retention of FT4 tags was acceptable with 76% of tags remaining at the end of the 1105 day trial. FT4 tags proved to be an effective method to identify individual H. hippoglossus, with the caveat that they seriously bias productivity measures in commercial research trials.