Conservation of Atlantic salmon - gene banking: principles and considerations

Executive Summary

Atlantic salmon in Scotland face a number of external stressors, which has meant that, in many cases, populations are in decline. Such pressures risk loss of genetic diversity both within and across populations and an associated loss in adaptive ability in the face of continued environmental change and other stressors. One technique that may be used to preserve genetic diversity is gene banking. Gene banking can be achieved through the maintenance of breeding population/s in a hatchery (Live Gene Banking), and/or the cryopreservation of gametes (Cryobanking/Cryopreservation).

All gene banks are types of ex situ biorepositories that seek to sample and retain, as much as is feasible, the existing genetic diversity of a population at the time of sampling, with the aim of preventing further loss of genetic diversity in a declining population and/or the expiration of the population. Each technique comes with its own opportunities and risks.

Live gene banking can be very expensive and, impacted by the various risk factors of running any hatchery, including small population size, inbreeding depression, domestication selection, and loss of local adaption. However, the approach has been shown to be successful for addressing targeted local issues, when specific stressors can be identified and ameliorated relatively quickly. A key advantage of live gene banks is the ability to restore the full genome of populations of concern to a point in time. Live gene banking may thus provide a useful tool in specific time-limited local situations; however, it would seem that it would not be practical at large scale.

Cryopreservation of gametes has been shown to be an effective method for preserving the genetic profile of many species of animal. Cryobanking seeks to maintain, preserve and eventually re-establish the genetic integrity of threatened populations through the freezing and retention of gametic material. In salmonids, cryopreservation is based on sperm, as successful freezing of eggs or embryos is not possible, due to ice crystal formation resulting from their relatively large size. This method has a number of important considerations that need to be addressed, such as the loss of mitochondrial DNA from females, the design of the sampling programme, the development of preservation protocols, the requirement for genetic screening of samples to avoid inbreeding and introgression, the costs of sampling, and the setup and running of a database. Notwithstanding these issues, cryobanking also comes with a number of opportunities. It is cost effective when compared to other options, it provides the preservation of gametes across both time and space; samples can be stored almost indefinitely, large numbers of samples and populations can be retained, and the running of any cryobank can be performed with relatively small resource inputs. Thus, it would seem to offer one of the few practical tools that could be used for large scale application. Gene banking is not a substitute for maintaining viable local populations in the wild, which may evolve in response to environmental change and invasions within a metapopulation. Rather, it offers a safety net for retaining genetic diversity that may then be applied in a restoration process.