Sea lice skirt questions answered
How long should the lice skirts be?
There is no ‘optimum’ length. Skirts are usually produced to a length that goes around the entire cage, but it is also possible to have two or more skirts covering the area. For steel cage systems, it is usual to encapsulate each individual cage.
As most of the suppliers work on completely bespoke solutions, the length of skirts will always be decided on following consultation with the customer. Skirts can be delivered with overlap and either with or without closure.
How deep should the lice skirts be?
Typically, they would be produced at standard depths of either six or nine metres, although most suppliers can also cater for other depths on request. The site’s depth, flow, oxygen conditions, freshwater flow, cage and mesh type are all contributing factors that can determine the exact length of the skirt. However, it would be highly unusual to deliver a skirt outside of the range of 3-15m deep.
How much flow is needed at the top?
A typical flow channel would be 160mm x 50mm. This would give a buoyancy of 8kg per metre. A skirt with a flow and 4kg download/m would be approximately self-flowing depending on the prevailing current flow conditions. A floating element at the top of the skirt facilitates the fitting at the location, although it is also possible to mount a skirt without flow.
How much down-soldering should the skirts have?
In order for the skirt to hang straight down and not to bow it is necessary to have some down-soldering of the skirts.
Again 4 kg download/m would be a standard measurement. Typically, down-soldering would be done using steel balls spun with polypropylene and polyethylene. The advantage that this has in front of the chain, is that spun down down-soldering gives fewer wear points, while at the same time being more compact and flexible than the chain itself.
Is a splash edge recommended to prevent waves from breaking over the float?
Some fish farmers find that they need a splash edge to prevent waves from
washing over the float/skirt and into the cage. Most lice skirts can be supplied with a splash edge to reduce the impact of wave water. This would usually be in the form of a conventional gusset attached to the skirt at a height of 50cm, with slots for the groove attachment.
Do skirts affect the oxygen levels in the cage?
Testing has shown that lice skirts reduce the water exchange in cages. The oxygen level is therefore lower within the shielded volume, although the fish tend to stand deeper if they have space available underneath the skirt. The fact that the fish are deeper can, of course, help to prevent sea lice onset. However, this can present a challenge during periods of low oxygen levels. A lift curtain system can be introduced to increase water exchange inside the cages and, recently, new oxygen extractors have been tested in Canada producing Ultra Fine Bubbles (UFB) that are so small they lack buoyancy. Initial results have been promising, with better water dissolution reducing oxygen consumption.
What is the maximum flow condition that the skirts can withstand?
Testing suggests that lice skirts start to creep upwards when the current exceeds 80cm/s, with deeper skirts more affected than shallow ones.
Should the skirt be mounted on the outside or the inside of the float ring?
This is a matter of choice, although most suppliers would recommend mounting inside. For interior mounting, the best results are achieved by doing it before mounting the net. It is easier to keep the inside of the skirt free from growth when mounted on the inside. The skirt is washed at the same time as the net. Potential problems with external mounting include gnawing from the main foot attachments, as well as a greater chance of the skirt getting caught in the propeller.
How often do the skirts need to be washed?
As stated in the previous answer, when the skirt is mounted inside the float ring it is cleaned at the same time as flushing the net. If mounted externally, the skirts can be washed in the sea, with the frequency depending on how exposed the site is to growth. When the skirts are picked up by the sea, they can be laid out for drying or they can be sent to the net laundry for cleaning.
What is the shelf life of a lice skirt?
This will depend on several variables such as power, the exposure level of the cages, and how well the skirt is maintained. Typically, skirts should last for between one and three seasons.
Finally, how do lice skirts work alongside the use of cleaner fish?
Typically, no sea lice remedy will work in isolation. A combination of methods will always produce the best results. Lice skirts combined with cleaner fish and expert cleaning methods are likely to produce a perfect cocktail.
With thanks to FiiZK
“A combination of methods will always produce the best results”