Genetic diversity

It has been reported that the tuna culture technology has advanced to a point where gene manipulation and selective cross-breeding can be applied to establish a race with reduced mortality of tuna at the early life stage and with high growth during farming. This raised one concern: the possibility of losing genetic diversity.
It is well known that repeated inter- breeding within a same stock of animals may accumulate recessive genes, leading to the development of genetic defects. This means that species diversity is lost. In addition to the accumulation of recessive genes, the stock which has poor diversity in genes has less adaptability to, and a higher risk of damage by the change of environment
The loss of genetic diversity is one of the major problems which fish cultures and restocking the wild fauna have to overcome. The restocking of salmon (by hatching and releasing salmon smolt or post larvae to rivers) is a good example. Wild salmon originating from a stock of a river would return to the same river of origin after spending a few years at sea. However, there are many sub-stocks even in the same river, and their spawning grounds scatter among many tributaries. Also, there are sub-stocks which differ in spawning time and in number of years which they spend at sea, even if they originate from the same spawning grounds. Such diversities are very important for the continuation of a species. For instance, if a salmon spawning ground in one tributary is damaged by a flood in one year, the sub-stock in another tributary would compensate the shortage of reproduction. Furthermore, fish from the same sub-stock may return to that spawning ground in the following year if a part of the fish spends one more year at sea. That is how the sub-stock can thus be sustained.
At present, at least in Japan, most of salmon is mainly supported by human stocking at the hatcheries. The salmon stocks would probably not be sustainable if they were to be left only for wild spawning, particularly with its inevitable loss of diversity. None the less, in the salmon hatcheries, a great number of spawners are used so that genetic diversity may be maintained to some extent. However, in case of tuna, only a very limited number of spawners are used. Therefore, fish from those limited spawners would be genetically more uniform. If selective breeding and gene manipulation are carried out under such limited circumstances, it is questionable just how much genetic diversity can be ensured. While those fish are kept in pens for farming, this will not have an effect on the wild stocks. However, should re-stocking be planned in the future using those “improved races”, their possible effects on the wild stocks have to be well-evaluated with simulations before they are actually released.