EXPERT TOPIC SALMON
INTRODUCTION SALMON EXPERT TOPIC
Salmon (salmonids) are one of the most important species of fish in aquaculture. In the US, Chinook salmon and rainbow trout also are most commonly farmed salmonids for recreational and subsistence fishing. In 2007, the aquaculture of salmonids was worth US$10.7 billion globally. The production has grown 10-fold during the 25 years from 19822007 and has continued to rise exponentially ever since. A challenge that the industry is working together to overcome at the moment is the problem of sea lice. This month’s ‘Expert topic’ examines the relationship between the most commonly farmed fish and the parasite that is threatening to decimate entire stocks of the fish from farms all over the world from Norway to Chile to Scotland. The lice attach themselves to the fish and feed on them, similarly to ticks or nits on humans, this action however kills the stock or renders them unsuitable for sale to consumers. The lice can grow to roughly the size of a pea and lay thousands of eggs in their brief lifetime. When in the wild, diseases and parasites are normally at low levels and kept in check by natural predation. However, in crowed pens they can easily become epidemics. These diseases and parasites also are known to transfer from farmed to wild salmon. According to “It’s all about salmon – Salmon Aquaculture”, by the Seafood Choices Alliance, Spring 2005, described links between the spread of parasitic sea lice from river salmon to wild pink salmon in the same river, the study was performed in British Columbia. In 2002 the European Commission (2002) concluded, “The reduction of wild salmonid abundance is also linked to other factors but there is more and more scientific evidence
by Zasha Whiteway-Wilkinson, Production Editor, International Aquafeed establishing a direct link between the number of lice-infested wild fish and the presence of cages in the same estuary. Finally, it has also been reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms (“Declining Wild Salmon Populations in Relation to Parasites from Farm Salmon”). These predictions however were disputed by other scientists and recent harvests have indicated that the predictions were in error. In 2011, Scottish salmon farming introduced the use of farmed wrasse for the purpose of cleaning aquacultured salmon of the ectoparasites. Wrasses are a family, Labridae, of marine fish, many of which are brightly coloured. The family is large and diverse with over 600 species in 82 genera, which are divided further into nine subgroups or tribes. They are typically small fish, most of them less than 20cm (7.9 inches) long, although the largest, the humphead wrasse can measure up to 2.5m (8.2ft). They are efficient carnivores, feeding on a wide range of small invertebrates. Many smaller wrasses follow the feeding trails of larger fish, picking up invertebrates disturbed by their passing. The initial method of dealing with the lice on salmon problem was by feeding fish a pesticide with the active ingredient of emamectin benzoate, but around 2009 the lice appeared to develop a resistance to the chemical and have spread globally since. More recent methods of control include using new closed in pens that resemble eggs to standard mesh, another is a device known as a Thermolicer to warm the water and detach the lice, there are also of course a technology known as “Optical delousing”, which shoots the sea lice with a laser gun to kill the parasite. You can read more on this method and others on the next page in “Lice and laserbeams” written by Vaughn Entwistle, Features Editor at International Aquafeed.
38 | May 2018 - International Aquafeed