Loss of genetic variation and effective population size of Kirikuchi charr: implications for the management of small, isolated salmonid populations

Abstract

Small, isolated populations may face extinction due to a combination of inbreeding depression and other threats. Effective population size (N e ) is one comprehensive measure that allows us to evaluate the genetic status of a population, and to make management decisions regarding genetic viability. We simulated loss of genetic variation and estimated N e for two small, isolated populations of Kirikuchi charr Salvelinus leucomaenis japonicus, the endangered, southernmost local populations of the genus Salvelinus in the world, using VORTEX, an individual-based stochastic PVA model. Approximately half of the genetic variation was lost over 200 years regardless of census population size and demographic parameters, and N e estimates were roughly 50 in each of the two populations, suggesting the possibility of inbreeding depression. The target population size of N e 4500, by securing long-term viability, is several times that of the present size of each of the populations studied, and no local habitats maintaining such a target number are considered to exist. The results strongly indicate a need for recovering natural connections and potential gene flow among local populations. However, the impending threat to these populations from non-native charr widely distributed throughout the drainage has prevented the recovery of the connections. Given the small N e of the two populations, it would be necessary to retain gene flow artificially within or across local populations. This will be true of many other salmonid populations that have been isolated or fragmented recently.

Population trends and vulnerability of humpback dolphins Sousa chinensis off the west coast of Taiwan

“…The accuracy of the trend projection and risk assessment presented here may be further affected by impacts of inbreeding, genetic drift and loss of genetic diversity in a small population (Souléa & Simberloff 1986, Lacy 1993, Mills & Smouse 1994, Sato & Harada 2008. A minimum population size, at least 250 adults (Shaffer 1981, Nunney & Campbell 1993 but usually up to thousands of adults (Harcourt 2002, Brito & Figueiredo 2003, Reed et al 2003, Traill et al 2007, is needed in mammals to resist the minimal level of stochastic genetic diversity loss.…”

Human‐induced population fragmentation and management of small, isolated Kirikuchi charr populations

“…In contrast, no genetic differentiation was detected (F ST = 0.0097; T. Sato et al, unpubl. data) in the two Kirikuchi charr populations considered in Sato & Harada (2008). In the past, Kirikuchi charr populations persisted as a large metapopulation in the drainage.…”

“…Given the relative risks of inbreeding and outbreeding depression, and the evolutionary history of the Kirikuchi charr, we feel that artificially retaining gene flow between the two populations, pointed out in Sato & Harada (2008), becomes a management option, at least given the present situation of the Kirikuchi charr. Nevertheless, as Vincenzi et al (2009) pointed out, we agree that a more conservative strategy is more appropriate as a first step.…”

“…Contrary to this, the fragmented distribution of the Kirikuchi charr populations has obviously resulted from anthropogenic factors, as described above. Specifically, the two isolated Kirikuchi charr populations considered in Sato & Harada (2008) have persisted, although their evolutionary history is not natural.…”

“…In a recent correspondence, Vincenzi et al (2009) raised concerns about mixing populations of Kirikuchi charr, Salvelinus leucomaenis japonicus, as a strategy to reduce the risk of inbreeding depression, which we previously suggested as a necessity (Sato & Harada, 2008). In summary, their concerns are that (1) outbreeding depression should be considered more seriously before mixing populations; (2) small, isolated populations with low genetic variation may be well adapted to their environments and not exposed to serious inbreeding depression; (3) the extinction of small, isolated populations (e.g.…”