When herds are small, their sizes are relatively (but never very) easy to track. This often involves finding 1-2 groups in a short time period and knowing a number of individuals by natural markings or, occasionally, collars. When numbers increase it means that more groups have to be found in a short time period to develop a reliable count. The probability of having that success in the field drops steeply with increasing herd sizes. We are now in that stage of increasing difficulty. When the native herds were at their recent largest size in the 1970s and 1980s, we could develop good counts when the sheep were most concentrated on low elevation winter rangers. It ws the abandonment of those winter rangers in the 1980s that brought about the recent severe ppulation declines. Some herds have shown recent notable increases in use of such low elevations in winter; but, for some this remains inconsistent and has not yet occurred for others. The result is that population monitoring continues to occur substantially at hgh elevations in summer and fall when sheep are often dispersed over considerable area. Monitoring efforts have been focursed on females, the reproductive base of populations. While this helps limit the hunt, it is still difficult. To the field observations we now also add laboratory genetic analyses for some herds to determine how many different lambs are present each year.
This past year has been particularly difficult. Unlike the previous few years, winter began early in late November. It brought many sheep from the herd at Wheeler Ridge just north of Bishop to their winter ranger by early December. If winter had continued similarly, those numbers would probably have gronw steadily through winter and eventually provided good counting opportunities. Instead, winter largely ended after December and good counting opportunities never materialized. Despite these difficulties, there are some groups of females for which we obtained good information in the past year. This includes all the sheep in the Mono Basin, which have been of great concern because of low numbers. In 2001, five lambs were present in those herds, of which 4 were documented as yearlings this year. Three of those were female, which is good for the reproductive base. Thisyear the number of lambs dropped to 4; but all are female, which will give the reproductive base an added boost if they survive to adulthood.
While we lack good counts for a number of herds, what we have seen throughout the Sierra Nevada this past season has been continued evidence of very high overwinter survival of lambs and continued good production of lambs. There is no question that the herds have all continued increasing. It is safe to project that the total number of sheep in the Sierra Nevada is now about 300. What is remarkable about that figure and the 250 figure from a year ago is that they match past landmark population levels since reliable counts began in the late 1970s. The first such good count was 250 in 1978 when these sheep were distributed as the thre surviving native herds the Mount Williamson, Mount Baxter, and Sawmill Canyon herds. Then years of reintroduction efforts began in 1979 and total numbers increased to about 300 by 1985 before ten years of decline began. What is notably different about the 300 sheep today is a changed distribution. In 1985 most sheep were still in the Mount Baxter and Sawmill Canyon herds, whereas today they are more evenly distributed across all herds. Today the Wheeler Ridge herd, which was the first reintroduction, is the larges herd and numbers about 100 sheep. We are hoping for good winter sheep counting conditions this year so we can report more refined information next year.
See also Sierra
Nevada Bighorn Sheep: a Brief History
by John Wehausen
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