Making lice counts count

“Many farmers have probably noticed that the numbers go up and down from week to week in the same cage. Obviously this does not mean that the sea lice population actually goes up and down in a few days…it is almost always due to the selection of fish at the count, the quality of the census and how representative the sample is,” Rune Stigum Olsen, of Benchmark Animal Health, told Kyst.no

According to Olsen, the fish with the most lice are often found deeper in the pen, especially when it’s dark, and are therefore less often caught when a sample is being taken – a theory he says that has been backed up by a study conducted by Samantha Bui of IMR last year.

“Fish do not have a homogeneous distribution in cages. When an animal gets a lot of parasites its behaviour changes. It's always good to get experience confirmed by researchers, such as Bui,” Olsen reflects.

As a result, he argues, the impact of lice treatments is hard to quantify.

“There is no doubt that it is difficult to evaluate a treatment outcome, but something can be done. And although the industry has many skilled and experienced workers, one sees that there are different practices and that there is almost always room for improvement,” he says.

“Many of these measures leading to safer evaluation take time to implement, but by doing a good job of sampling, so you can also spare treatments. It could in fact prove that the effect of delousing is far better than first anticipated. Or the opposite,” Olsen observes.

In order to improve lice counts he suggests a number of tips, including:

• Do not feed the fish to count, as this does not provide a representative sample.
• For the last count before a treatment, if possible, take two samples – one in the morning and one later in the day, as you are more likely to get substantially different fish from the cage and thus the figure will be more representative.
• Sample more fish than the mandatory amount – 50-100 per cage if one has the opportunity.
• Note the poor performers – the worst performing fish often have fewer lice in cages with cleanerfish as they are more likely to be grazed on, while in cages with no cleanerfish they tend to have more than the average lice numbers.
• Have a proper system for counting lice that fall off the holding tub. Strain this water. Lice in here may account for over 20% of the number.
• If possible, use the same - well-trained - people to do all the lice counts.
• Be sure to have people with good eyesight conducting the counts.
• Do not leave the fish too long in the landing net.
• Wait 5-7 days after treatment before conducting a count. Be consistent on this. Some lice cling on for several days after dying.

Olsen believes that following these instructions should help ensure a more consistent count.

“I have seen examples of the treatment effect may vary from 60% to 90% depending on how the evaluation before and after is completed. It is also likely that in some can occur far more lice in fish group than was thought they had. It is possible this may explain the sudden explosions of lice that are sometime reported, especially in the summer,” he warns.