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<br />Squawfish Population Viability Analysis --July 1993 <br /> <br />Page 25' <br /> <br />4NmNf <br />Ne = N + N <br />m f <br /> <br />Tyus (1990) observed fewer.females than males in samples in breeding <br />areas. He obtained an M:F ratio of 5: 1 to 15: 1. Plugging these values into <br />the above equation lowers Ne relative to census N by 50 to 75%, which <br />brings the current estimated population size into the range of the Ne = 500 <br />threshold. However, these ratios cannot be accepted uncritically. <br /> <br />The correction for long-term population fluctuations is <br /> <br />1 1 "'( 1 1 1 <br />N = T L.J N + N + ... + + N ) <br />e 1 2 T <br /> <br />which is an harmonic mean. It is speculative to suggest that the squawfish <br />has had low population sizes in the past. But huge earth quakes that <br />temporarily (say, for a decade, i.e., a generation) create lakes that interrupt <br />dispersal could cause extreme drop offs in population size. This would <br />historically explain current low Nes, but would not be of importance in <br />recovery planning. <br /> <br />2.11 Inbreeding and Inbreeding Depression <br /> <br />Soule (1980) suggested, based on a rule of thumb of animal breeders, that <br />an Ne of 50 was a limit below which one might expect inbreeding <br />depression to become a problem. Inbreeding depression is thought to <br />happen most often when a recessive, deleterious allele fluctuates to high <br />frequency through the random sampling (genetic drift) associated with <br />very small populations. The Florida panther (discussed in Discover [July <br />1992]) is a striking example of inbreeding depression. Its populations, <br />around 35 adults, are in the range where inbreeding is extreme. <br />Physiologically and morphologically, the panther is now starting to exhibit <br />a number of fitness-reducing traits: many of the animals now have a <br />defective heart valve. Some adults are cross-eyed. Adult males have a <br />very high frequency of malformed sperm. <br /> <br />If the Colorado squawfish populations are divided into many isolated small <br />populations, such inbreeding depression could happen. However, it takes <br />an extremely small amount of gene flow to counteract inbreeding in local <br />populations. A robust magic number is 1 migrant per generation. That is, <br />if one adult Colorado squawfish breeds outside of its natal population in a <br />