Overview
Sea lice are common parasitic copepods that have the potential to affect both farmed and wild fish stocks. The Ministry of Agriculture and Lands (MAL) has been actively monitoring the status of lice infections on British Columbia (BC) salmon farms since 2003.
A lice management strategy is integral to a Fish Health Management Plan (FHMP) and the lice audits target active Atlantic salmon farms of BC. As part of the reporting requirement of the FHMP, industry information is provided to government monthly where it is posted to the BCMAL Fish Health website.
In addition, the Ministry conducts audits to verify the accuracy of the counts. In 2008, Ministry fish health staff conducted 71 random farm audits and assessed over 4,200 live Atlantic salmon for sea lice.
Provincial sea lice monitoring
There are two components to the lice monitoring programme:
- Industry’s on-farm monitoring and reporting, and
- BCMAL’s audit of these procedures.
BCMAL requires the industry to conduct lice assessments at each active Atlantic salmon farm on a monthly basis and report that monthly data (in an aggregated form) from each sub-zone, with the exception of sub-zone 3.1.
A ‘Trigger level’ of lice abundance has been established to minimise the potential accumulation and amplification of salmon lice on farms. The salmon lice trigger level is set at three motile lice year round. Corresponding management actions are species-specific and outlined below. The industry on-farm sampling programme is based on internationally accepted standards for sea lice monitoring.
Table 1 compares the trigger levels in salmon for various countries.
Industry monitoring and sampling
Protocols industry veterinarians responsible for the health management of farmed fish oversee the information collected at farms and evaluate the need for intervention. These health professionals are responsible for the management and treatment of fish raised under their care.
The lice monitoring programme assesses the abundance and life stages of two types of sea louse found on farmed fish: the ‘salmon louse’, Lepeophtheirus, and the ‘herring louse’, Caligus, with awareness of the differences in fish susceptibility to these lice types.
Atlantic salmon farms
Industry lice counts are conducted once a month within most coastal sub-zones (unless an acceptable reason for not sampling was provided). The frequency of monthly sampling is increased to twice monthly should the trigger level of three motile lice (salmon lice) per fish be reached anytime. During the out-migration of wild juvenile salmon (March to July), should a farm reach that same trigger level, the lice management strategy outlines additional action, such as treatment or harvest, be adopted to reduce the average abundance of lice on that farm.
Continuous review of the sea lice data from wild and farmed fish stocks may lead to refinement of the lice control strategies in various farming sub-zones.
How is sampling carried out?
25 per cent of active Atlantic salmon farms is the target for selection for lice audits each quarter. However for the second quarter (April, May, June> the audit and monitoring frequency doubles to 50 per cent to correspond with the period of the wild smolt out-migration.
At each farm, monthly assessments are conducted using three pens; 20 live fish per pen are anaesthetised and examined (farm total = 60 fish). Pens chosen for assessment include one reference or index pen (i.e. first pen stocked at the farm, or the pen with the highest likelihood of having lice, based on historical counts). The reference pen is sampled each month. Two additional pens may be selected by farm staff either by rotation or convenience.
During the gathering procedure, hundreds of fish are typically captured using a seine net, box seine, or other methods that ensures representative sampling of the population. The method of capture is recorded by staff. Twenty fish are dip-netted into an anaesthetic bath although, on occasion when other tests are underway, farms choose to humanely euthanize the fish before examination. Handling of the live fish is minimised to avoid dislodging lice. The fish are examined for the presence of lice regardless of the health status of the fish (i.e. robust or moribund).
Reporting
All farms report count numbers are submitted to the BCMAL as monthly reports by sub-zone. If the trigger level is reached from March to July either harvest or treatment is undertaken to reduce lice concentrations per fish. For the remainder of the year management action includes more frequent counts (i.e. two per month) in addition to other husbandry considerations and management efforts.
Provincial Audit of Industry
The sea lice audit programme is designed to verify the industry reported results and provide government with up-to-date knowledge of lice levels on BC farmed salmon.
Summary of industry sea lice results 2008
Abundance of lice on farmed fish in 2008 during the out-migration period of wild fry (March to July) was well below the trigger level of 3 motile lice per fish in all sub-zones.
In most cases the lice abundance on the salmon farms in late 2007 and early 2008 had declined or been managed such that fewer than 2 motile lice per fish were present by April 2008. That abundance of motile lice remained low, typically for five or six months. In other words, no within-farm recruitment of lice populations was evident between March and August 2008.
The trigger level of three motile lice per fish continues to be a conservative monitoring and management objective.
Sea lice are natural marine parasites of fish in all regions. There is no indication in the sentinel Atlantic salmon population of BC farms of ill health even when afflicted by higher numbers of lice observed each autumn.
Lice abundance varies between year classes.
The overall abundance of lice on juvenile Atlantic salmon is generally lower in their first year of sea water compared to 2nd year fish (adults).
Lice abundance can vary substantially between areas.
Data collected by industry on a farm-by-farm basis and submitted to government clearly shows that there are areas where lice abundance has consistently been very low for years. Sub-zone 3.1 (Sechelt) has not had its lice abundance approach the trigger level since monitoring began whereas other areas experience increases in lice abundance each autumn. With the exception of the autumn and winter months in 2008, most sub-zones showed a louse abundance that averaged less than 1.0 motile louse per fish.
Abundance of lice varies naturally from year to year.
Sea lice data have been collected and reported consistently for more than five years in BC (2004 -2008 inclusive) using a standardised protocol and reporting structure. Annual comparisons interest some people but direct comparisons are difficult because the location of ‘active’ and reporting farms change from year-to-year. An annual fluctuation in average lice abundance in all sub-zones is to be expected.
Sea lice are naturally occurring parasites of fish.
Data collected from wild stocks shows that returning adult salmon carry high numbers of sea lice. Undoubtedly this host-parasite relationship is a natural phenomenon of salmon.
Marine conditions can affect the occurrence and abundance of lice on farms.
Information on environmental conditions and the impact on salmon and lice survival and reproduction is well documented.
Conclusion
The objective of the sea lice audit is to ensure that on-farm counting protocols are followed and to verify the state of lice infestations on BC Atlantic salmon farms. The industry has embraced the sea lice management strategy and full compliance with the Ministry’s requirements for monitoring occurs. Overall, lice abundance on Atlantic salmon farms in 2008 was the lowest on record, and springtime averages in all regions were well below the trigger of three motile lice per fish.
Further Reading
 |
- | You can view the full report by clicking here. |
