Genetic Monitoring for Managers


Types of GEM

Category II GeM Project Example: Effective Population Size

Long term genetic monitoring of brown trout in Sweden- the Bävervattnen Lakes Project.

Laikre at al. 1998, Jorde and Ryman 1996, Palm et al. 2003

The Bävervattnen Lakes project used genetic methods to monitor natural and experimentally released brown trout populations in several systems of small mountain lakes and creeks in the Hotagen Nature Reserve in Jämtland county, central Sweden (Fig. 1).

Figure 1. Hotagen Nature Reserve study area in north-central Sweden.

The objectives of the Bävervattnen Lakes Project included:
• Estimating and monitoring genetically effective population sizes (Ne).
• Monitoring spatial structure and intraspecific genetic composition.
• Monitoring rates and patterns of gene flow from releases into native populations.
• Monitoring establishment of genetically and ecologically divergent brown trout populations in a new enviroment.

Brown trout tissue for genetic analyses has been collected annually since the 1980s. An existing tissue bank comprised of approximately 25,000 individual fish is added to annually. Samples include cohorts from 1972 - present. These samples are available for future genetic monitoring studies as new genetic markers and questions emerge. Genetic markers that have so far been used include allozymes, mtDNA, microsatellites, and MHC.

Major findings include:

• Effective population sizes of natural brown trout populations are surprisingly small considering the observed levels of genetic variation (Fig. 2).
• The genetic composition of both within and between levels of variation is stable over the time periods investigated.
• Point estimates of Ne fluctuate for individual populations, but the general trend indicates stability (Fig. 2).

Figure 2. [Ne for 2 creeks] Monitoring estimates of Ne at two sampling localities in a creek. Filled symbols are the point estimates, and open symbols represent the moving averages over five consecutive years. Strippled lines indicate the Ne estimate over the total time period (Palm et al. 2003).













• Comparing female to total effective population size indicate that males and females contribute equally to the gene pool.
• Released populations can establish quickly in a new environment.


Figure 3. Location, type, and magnitude of releases.

• The pattern of establishment appears to be affected by genetic background of the released stock (Fig. 3).
• Genes from released stocks have spread rapidly to native populations about 15 km from the release site and are becoming established in the native populations (Fig. 3).

Allele frequency differences between consecutive cohorts were significant in all populations studied. There were no indications of natural selection, and the authors concluded that random genetic drift is the most likely cause of temporal allele frequency shifts at the loci examined. The estimated effective sizes of the four populations range from 52 to 480 individuals, suggesting that the effective size of natural brown trout populations may differ considerably among lakes that are similar in size and other apparent characteristics. In spite of their different effective sizes all four populations have similar levels of genetic variation (average heterozygosity) indicating that excessive loss of genetic variability has been retarded, most likely because of gene flow among neighboring populations.