Dr Oppedal, who is based at IMR in Bergen, outlined a variety of research projects – undertaken from 2001 right up to the present day – relating to trying to ensure salmon spend less time at the top 10 or so metres of the water column, which is the area where lice are most prevalent.
In order to decrease the risk of the parasites attaching themselves to the fish, reflected Dr Oppedal, the salmon can either be encouraged to spend more time at greater depth by voluntary means (such as by installing subsurface lights or subsurface feeding systems to attract the fish deeper), or by forced means, (such as submerging the tops of the salmon cages to physically prevent them from reaching the surface layers).
While all of the trials he mentioned in his talk were successful at significantly reducing lice counts, some of them also had other impacts that were less beneficial to the health of the fish. For example, a trial which involved installing a net which prevented the salmon from entering the top 10 metres of the cage resulted in an 83% reduction in sea lice after 43 days, but the fish were found to be in poor condition – with significantly lower weights than those in control cages – prompting speculation that it would be beneficial to allow them access to the surface to refill their swim bladders at least once a week.
Trials such as this resulted in the development of technologies such as air domes and cages with ‘snorkels’ which keep the majority of fish below the lice layer but could still allow them to refill their swim bladders. He described a number of trials that have been done with a range of snorkel depths, which restrict most of the fish to depths down to 16m, but allow them access to the surface, through the opening of the snorkel, when they need to adjust their swim bladders. One IMR-led study he used to illustrate his talk was undertaken in 2015 and involved the use of 4m snorkels in 12x12x12m cages, each stocked with 3500 salmon. Results after 3, 6, 9 and 12 weeks showed lice levels in the snorkel cages were 65, 24, 43 and 56% lower, respectively, compared to the control cages.
However, at least one trial he mentioned showed that, while snorkel systems proved effective at reducing lice levels, they also made the fish more susceptible to AGD. This was somewhat surprising given that a University of Melbourne-led study published last year suggested that restricting access to the surface should help to reduce AGD, due to a tendency of its causative agent, Neoparamoeba perurans to congregate in the surface layer at times of great abundance.
The current trial he is involved in features three commercial cages with snorkels and three control cages and he said that the feedback from the farmers involved had been both positive and negative.
“They say that, initially, using the new equipment is a challenge in terms of gear and resources, but as you learn you can deal with it. They also say that the increased work associated with snorkels is recouped through needing to spend less time and effort on lice treatments. They also say the snorkel cages improve the welfare of the fish. Lastly, more impact from AGD was seen, but this can be alleviated by applying fresh water at the snorkel’s surface,” he reflected.
His concluding advice was, whether or not you choose to invest in such equipment, it was clearly beneficial to encourage fish to spend more time at greater depths and the simplest means to do this is by feeding them in the correct manner and by placing anti-maturation lights deeper down.
“Make sure the fish are not hungry, so they are less likely to spend time at the surface. Camera technology allows you to see when they’re hungry before they need to move to the top of the cage – you don’t want to be the guy on the feed barge who’s watching with a pair of binoculars for the fish to break the surface during most of the feeding time,” Dr Oppedal concluded.