All About The Numbers

How many are there? That may be the most asked question in the history of ecology and wildlife management (at least one of them). Unfortunately, it is not an easy question to answer with any certainty. Animals don’t line up and volunteer to be counted, and thus biologists must find ways to estimate population sizes rather than having absolute and verifiable counts. Many hours of research (link) and multiple volumes (link) have gone into designing and describing ways of estimating populations sizes, and there are still many unsolved problems that are being worked on all the time.

Estimating population size for animals like fishers is difficult because they are relatively rare and spread over large areas that may be difficult to access and survey. Trapping is one method that we can use to make estimates of fisher population sizes using mark-recapture techniques. Additionally, trapping gives us insight into the sexes and ages of the animals what we capture. Because we attempt to survey as much of the area where fishers might live as we can it requires a lot of traps spread out over a big area (see the map below). It’s a busy time and a bit frenzied (hence “Fisher Frenzy”).

Map showing distribution of traps that captured (red dots) and did not capture (blue dots) captures during fall of 2012

Map showing distribution of traps that captured (red dots) and did not capture (blue dots) fishers during fall of 2012

Kevin gave some basic counts of the animals we captured but I thought I would provide a little more detail about the trapping, and some of the implications of the results. Of the 29 fishers we captured 14 were males and 15 were females. The majority of animals we captured were animals that were born on Stirling and that we estimate at 6-19 months old. You can see the percent of animals that we captured from each cohort in the top panel of the figure below. Though we captured more young animals there are still substantial number of adult animals from each of the first 3 years of the translocation. To date, we can confirm that 11 translocated fishers have died, and a minimum of 23 animals were born on Stirling – a net gain of 12. Unfortunately  we do not know the fates of 14 translocated fishers (lost collars etc) meaning that some, or all, could have died. Still, the number of  young fishers we’ve captured is nearly the same as all translocated animals that died or have gone missing.

Percent of fishers captured (top panel) and percent of the known alive population (bottom) by their release or capture cohort

Percent of fishers captured (top panel) and percent of the known alive population (bottom) by their release or capture cohort. Juv 2011 and 2012 are those animals less then 2 years old born on Stirling and capture in those respective years. AdY1, AdY2, and AdY3 are adult animals released onto Stirling in Year 1, Year 2, and Year 3 of the translocation

Another point to consider is that we know we do not capture all fishers in our efforts. In fact, because we radio-track animals, we know of at least 6 fishers that were alive and on the study area that we did not capture. Though we capture a relatively high percentage of the animals that are on our study site, we know we do not get them all – adults or juveniles. Thus, it is very likely there are more young fishers out there than we can document. I have included a secondary figure (bottom panel) that shows the percent of animals in each cohort, and by sex, relative to all animals we can document (captured or otherwise).

So there is still uncertainty to the question of how many fishers exist in the northern Sierra. At a minimum we know that 38 fishers were alive at the beginning of our trapping (those captured + those not captured but known through telemetry). This is 2 fewer than the 40 total we released. Preliminary mark-recapture analyses gives a population estimate of 29-45 fishers, and this seems to make sense given our other information. For now, fishers seem to be doing well enough to survive and reproduce. The spring of 2013 is the first year that we will be tracking female fishers, born on Stirling, that will be able to den. Meaning, next fall is the first year we can capture the progeny of fishers born in the northern Sierra Nevada.Documenting this will be another important milestone and indication of how the population might fair in the future.  Hopefully, we’ll see you all next fall for the Fisher Frenzy.


Categories: Updates | Tags: , , , , , | 1 Comment

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One thought on “All About The Numbers

  1. dlclifford

    Great to see how much area we covered during trapping – its really impressive. Kudos to you and your team for all that hard work. and yes— its hard to answer that question. How many are there? The good news is the data indicates there are many!

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