Types of GEM
Long term genetic monitoring of brown trout in Sweden- the Bävervattnen
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
The objectives of the Bävervattnen Lakes Project included:
Estimating and monitoring genetically effective population
Monitoring spatial structure and intraspecific genetic
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,
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