By Asbjørn Bergheim*
Another study traced 80 % of all salmon in some Norwegian rivers back to fish farms. Yet another report refers to a study indicating that adult farm salmon have been shown to be 84 % less successful than native fish at reproducing in rivers. However, so-called precocious male parr raised in farms were four times more successful than their wild counterparts at the spawning grounds.
Selective breeding for intensive farming since the early 1970’s has resulted in genetic differences between wild and farmed salmon, such as different growth and behavior. Offspring of farmed salmon also display reduced survival in the wild compared with wild salmon. The potential for interbreeding between escaped farmed and wild salmon in the rivers is the basic motive for introduction of sterile salmon in aquaculture. A couple of years ago, about one million of sterile smolt were stocked in cages along the Norwegian coast.
A recent study, however, demonstrated that use of triploid salmon in commercial aquaculture may cause implications at the spawning sites in the rivers. Triploid males can display the full range of spawning behavior similar to wild males, and wild females will spawn with triploid males, even in the absence of a diploid male.
Triploid salmon and trout can be produced by exposing eggs to high pressure. This treatment causes the offspring to receive the usual one set of chromosomes from its father, but two from its mother. The method evolved in the 1980s and was easily introduced in rainbow trout in Scotland and Tasmania. However, salmon offspring of such treatment frequently suffered from skeletal deformities and cataracts. Due to new knowledge, the method has re-emerged as a potential way even to produce sterile salmon without such infirmities. Use of modified feed, for instance by adding extra phosphorous to prevent skeletal problems, and more insight into optimal environmental conditions are contributing major factors to successful farming of triploid salmon. However, sterile salmon are still more vulnerable to high summer temperature and low dissolved oxygen concentration than normal salmon.
According to Troutlodge some (out of many) benefits of utilizing triploid trout are:
- They can be released without any risk of genetic impact of wild stocks.
- Improved growth to large size, as their energy is transferred to growth rather than reproduction.
- In the early life stages, the growth is comparable to diploid fish.
- Better flesh quality as compared to diploids.
Some years ago, a transnational project in NW Europe studied the feasibility of commercial triploid Atlantic salmon production. A series of experimental and full-scale trials were conducted at all stages of production. Triploids outperformed their diploid siblings and the rate of deformity was minimal. Both out-of-season (0+) and natural (1+) triploid post-smolts demonstrated encouraging performance during the first months after transfer to sea.
BioMar Norway claims that a full production cycle can be done up to 4-6 weeks faster using triploids compared to diploid salmon due to higher growth rate. The growth pattern of triploids is quite different from diploids. This company, BioMar, launched a new feed line (Tri-X) adapted to production of triploids. According to Dan Leeming, BioMar: “Basically we had to redesign the composition of our salmon feeds completely to match the needs of triploids in the different growth stages.”
As mentioned previously, the introduction of triploid salmonids initially caused severe deformity problems. Based on comprehensive research, addition of a probiotic bacteria strain improves the mineralization and at the same time reduces inflammatory conditions, which are main reasons for vertebral deformities in sterile fry according to the experts.
Dr. AsbjØrn Bergheim is a senior researcher in the Dept. of Marine Environment at the International Research Institute of Stavanger. His fields of interest within aquaculture are primarily water quality vs. technology and management in tanks, cages and ponds, among others.