An overall perspective on in-situ cleaning of nets
The in situ cleaning of nets is an effective method that has been increasingly used in salmon farming over the last few years. This increase is a result of the more frequent use of wrasse to keep salmon lice under control. In situ cleaning is often used in combination with anti-fouling products, coating or on untreated netting.
The technology is based on remote-controlled rigs that remove fouling using rotating discs with high pressure nozzles. Water pressure and flow vary, but it appears that the use of very high pressure (over 150 Bar) is becoming less common and it is more usual to apply lower pressure and with higher volumes of water.
Net wear and tear
- high pressure and broken filaments
Observations made on fish farms show that high pressure in situ cleaning (over 150 Bar) or prolonged in situ cleaning in one place can damage the netting. The water flow can stretch the tight weave of the netting and damage the individual fibres by exposing them to a too powerful or prolonged water flow.It is common to have to repair erosion damage caused either by the actual water flow or by the in situ cleaning disc rotating for too long against the net (abrasion damage).
The latter leads to different type of damage in that it causes wear and tear to the netting without any major stretching of the fibres.
Stretched netting makes it easier for species such as hydroids, mussels and skeleton shrimps to attach themselves and exploit new and well-suited
areas for further growth.
In situ cleaning dislodges considerable amounts of biological material that is then crushed into small and large fragments. This material creates a slowly sinking cloud that can be a problem for the fish’s gill health. The main problem is caused by polyps, or the tentacles of hydroids. Hydroids are basically jelly fish on stalks and have a large number of polyps, each of which is covered with stinging cells. The stinging cells shoot out harpoon-like needles that pump poison into whatever tissue is being attacked. Many people are familiar with this feeling after close contact with stinging jelly fish. The polyps float around in the cage that is being flushed and easily spread into other cages. Investigations carried out on fish farms show that the number of polyps in the water increases to 300 per m3 during in situ cleaning where there were none before in situ cleaning.
It has been shown that gill damage is caused by contact with stinging cells that have been spread through in situ cleaning. The damage is characterised by inflammation and slime formation in the gills, which can lead to other problems, both as a substrate for AGD and for gill related problems during de-lousing.
Other fragments consist partly of relatively harmless algae, but can also contain sharp fragments of mussels and skeleton shrimps. The algae can cause discomfort and avoidance behaviour, but it is clear that hard and sharp fragments of mussels and skeleton shrimps can exacerbate the problem caused by the stinging cells.
Re-colonisation of a newly washed net
After a successful and professional in situ cleaning procedure the net appears clean and free of fouling. This is generally true with the exception of one species, hydroids. Hydroids have a root like network that they wind in and around the netting filaments. The hydroids can take over and create monocultures where they out-compete all other species. In situ cleaning will increase the number of hydroids in the area, and this can easily affect other facilities as viable fragments and juveniles get carried by the currents and attach themselves to suitable surfaces. With increased recruiting and the absence of competition, the hydroids will be able to expand and be the dominant species on the net within a short space of time.
Let knowledge create value
Based on findings in the available literature and the experience shared from people in the industry, it is possible to draw the following conclusions. Nets should be protected against the forces of an in situ cleaning rig. It is easy to end up in a situation where the netting is weakened despite the fact that all the procedures were followed.
Furthermore, it would appear advisable to monitor the gill status before and after in situ cleaning. This is the only way to study the possible after-effects of in situ cleaning and carry out the necessary changes in routines. An evaluation should also be made as to whether in situ cleaning one week or less before de-lousing or AGD treatment is a good idea, again based on an accurate monitoring of gill status before and after in situ cleaning.
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About the author:
Björgólfur Hávarðsson is a fisheries biologist from the University of Bergen. Björgólfur started working in the aquaculture industry in Iceland in 1987 and in Norway from 1991. His experience ranges from stroking to slaughtering and from counting and size measurement to the optimisation of feeding and feeding technology. At Steen-Hansen Björgólfur uses his industry expertise and background in biology to contribute to the development of better anti-fouling products, customer support and training programmes.
Björgólfur is also a quality manager in the company and maintains a full overview of our ISO certifications that comprise both ISO 9001 and ISO 14001. Björgólfur‘s favourite food is soya-marinated salmon with lime pepper and rice. In his free time he likes to fish for salmon and take photographs. He really likes pictures of himself with salmon!
Would you like more information? Contact Bøggi:
email@example.com / +47 92212727
John is Sales Manager at Steen-Hansen. He has worked with nets for an entire generation, among other things as general manager of a service station.
Discussions with John are not limited to the protection of nets and reduction of fouling. He also has a broad knowledge about nets, net materials and the construction of nets.
firstname.lastname@example.org / +47 94010670
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