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Thread: A primer on aerial population surveys

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    Member bushrat's Avatar
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    Default A primer on aerial population surveys

    Since there is a thread recently on aerial surveys I thought I'd jot down some words on how aerial population estimates (known as "density estimates") work, a bit of the science behind them, and why they are so expensive.

    I'll just deal with moose density estimates for now.

    Most density estimates are usually done in early November after the first snowfall because we need snow on the ground to see tracks and for the contrast needed to spot moose as well. We also need to do it while bulls still have their antlers. So what we have to have for an accurate density estimate is good flying weather and that snow on the ground.

    What was typically done first was to use larger faster planes (Cessna 185s or 206s or equivalent) to stratify certain areas within a unit. This stratified random sampling and spotting determines low-density and high-density moose areas based on the amount of moose spotted. We don't want to fly just the high density areas, or the low density areas, but a mix of both. So after that is done those areas are mapped out for the actual survey, and coordinates chosen for line transects to fly.

    Now comes the super cubs with spotters (or equivalent slow-flying aircraft), typically there are several aircraft involved in the survey. This requires hiring extremely experienced pilots, and also having experienced spotters. Both pilots and passengers do the spotting and counting, and they also try to determine bulls, cows, and calves/yearlings. So the day arrives and the planes head out. Prior to a newer method we used what is known as the Gassaway method (named after the Alaskan biologist who perfected it). The Gassaway method involved flying a certain amount of time over individual survey units (SUs), around 4 minutes per square mile, and counting moose. Recognizing that with that amount of time over an SU not all the moose there may be spotted, Gassaway method involved re-flying some of the SUs for longer and recounting to come up with a "sightability correction factor" (SCF). This SCF was then taken into account afterwards and utilized in the data to correct for errors and improve the "confidence interval" (CI), which is basically a margin of error. Most reports you read will have a 90% CI, or a 95% CI. The higher the CI the less margin of error.

    Now all this usually involved flying over several days by several aircraft. A typical moose density estimate for a certain area can be done for around $30,000-40,000 at the cheapest. If we want to cover larger areas and not extrapolate as much it can cost a lot more. If it's done right, in good conditions, the resulting data is very sound. There are times when a survey was all planned and pilots and aircraft in place, and the weather turned crappy (high winds, poor visibility), but we have still done them. The results from those estimates can be pretty meaningless, as spotters are puking in the back seat, planes have to fly higher, it is turbulent etc etc. I think we realize now it is best to call off surveys when we don't have the right conditions, rather than spend all that money for unsound data. Unfortunately though, this can prevent surveys for a year or more if we don't get the snow we need and the right flying weather at the right time.

    We do things a bit differently now. We mostly use the GSPE (geo-spatial-population-estimate) method. This method was perfected by another ADFG employee, a biometrician. They are like the math geeks <grin>. The new GSPE method recognized that instead of re-flying certain survey units to correct for any missed moose, that if we spent a longer time flying each SU, there was no need to come back and re-fly and recount, so the GSPE method typically requires 8 minutes over each square mile chosen to survey.

    Now even using highly experienced pilots for this, it isn't without risk. Many of you have heard of a "moose stall." That is when circling to look at moose or count them the plane can go into a stall. We've lost some extremely good pilots and biologists this way, though it has been rare. We've had some planes go down with no injuries. The most important thing when we do these surveys is to hire the most experienced pilots we can get, and often these pilots are guides and/or air-taxi operators with very high time in their specific aircraft.

    That's the basics. I would like folks to recognize that we can do very accurate density estimates from the air, but that the results don't always agree with what hunters are seeing on the ground. And in many cases hunters then don't "believe" what ADFG is saying because the area they hunt either doesn't have that many moose or an area they are familiar with has less moose than in the past. What we need to recognize is that it is impossible to gauge a population based on what you are seeing in a specific place, or even if you are a pilot and fly the same exact line out to your cabin and in the past saw more moose than you see now, that somehow means the ADFG data is flawed. These are fairly large areas we are talking about and we need to take in the whole area rather than a few specific places.

    This kind of thing happened quite a bit in the 20A and surrounding areas; hunters didn't believe there were as many moose as ADFG data showed. It's good to question data and ask how we arrived at it, and what the confidence intervals are etc. I think, however, that there is a wave of hunters who are discounting good data out of hand more and more, and this is causing some difficulties. Most of it is education related I think; if hunters knew more how these surveys were conducted and the science behind them I think they'd trust the data more. Which is why I'm typing this up this morn. I could have some of the particulars wrong, this is all from memory, but wanted to get the basics down. If you got this far then you get a gold star <grin>.

  2. #2

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    Good stuff....thanks for the effort.


    Quote Originally Posted by bushrat View Post
    Since there is a thread recently on aerial surveys I thought I'd jot down some words on how aerial population estimates (known as "density estimates") work, a bit of the science behind them, and why they are so expensive.

    I'll just deal with moose density estimates for now.

    Most density estimates are usually done in early November after the first snowfall because we need snow on the ground to see tracks and for the contrast needed to spot moose as well. We also need to do it while bulls still have their antlers. So what we have to have for an accurate density estimate is good flying weather and that snow on the ground.

    What was typically done first was to use larger faster planes (Cessna 185s or 206s or equivalent) to stratify certain areas within a unit. This stratified random sampling and spotting determines low-density and high-density moose areas based on the amount of moose spotted. We don't want to fly just the high density areas, or the low density areas, but a mix of both. So after that is done those areas are mapped out for the actual survey, and coordinates chosen for line transects to fly.

    Now comes the super cubs with spotters (or equivalent slow-flying aircraft), typically there are several aircraft involved in the survey. This requires hiring extremely experienced pilots, and also having experienced spotters. Both pilots and passengers do the spotting and counting, and they also try to determine bulls, cows, and calves/yearlings. So the day arrives and the planes head out. Prior to a newer method we used what is known as the Gassaway method (named after the Alaskan biologist who perfected it). The Gassaway method involved flying a certain amount of time over individual survey units (SUs), around 4 minutes per square mile, and counting moose. Recognizing that with that amount of time over an SU not all the moose there may be spotted, Gassaway method involved re-flying some of the SUs for longer and recounting to come up with a "sightability correction factor" (SCF). This SCF was then taken into account afterwards and utilized in the data to correct for errors and improve the "confidence interval" (CI), which is basically a margin of error. Most reports you read will have a 90% CI, or a 95% CI. The higher the CI the less margin of error.

    Now all this usually involved flying over several days by several aircraft. A typical moose density estimate for a certain area can be done for around $30,000-40,000 at the cheapest. If we want to cover larger areas and not extrapolate as much it can cost a lot more. If it's done right, in good conditions, the resulting data is very sound. There are times when a survey was all planned and pilots and aircraft in place, and the weather turned crappy (high winds, poor visibility), but we have still done them. The results from those estimates can be pretty meaningless, as spotters are puking in the back seat, planes have to fly higher, it is turbulent etc etc. I think we realize now it is best to call off surveys when we don't have the right conditions, rather than spend all that money for unsound data. Unfortunately though, this can prevent surveys for a year or more if we don't get the snow we need and the right flying weather at the right time.

    We do things a bit differently now. We mostly use the GSPE (geo-spatial-population-estimate) method. This method was perfected by another ADFG employee, a biometrician. They are like the math geeks <grin>. The new GSPE method recognized that instead of re-flying certain survey units to correct for any missed moose, that if we spent a longer time flying each SU, there was no need to come back and re-fly and recount, so the GSPE method typically requires 8 minutes over each square mile chosen to survey.

    Now even using highly experienced pilots for this, it isn't without risk. Many of you have heard of a "moose stall." That is when circling to look at moose or count them the plane can go into a stall. We've lost some extremely good pilots and biologists this way, though it has been rare. We've had some planes go down with no injuries. The most important thing when we do these surveys is to hire the most experienced pilots we can get, and often these pilots are guides and/or air-taxi operators with very high time in their specific aircraft.

    That's the basics. I would like folks to recognize that we can do very accurate density estimates from the air, but that the results don't always agree with what hunters are seeing on the ground. And in many cases hunters then don't "believe" what ADFG is saying because the area they hunt either doesn't have that many moose or an area they are familiar with has less moose than in the past. What we need to recognize is that it is impossible to gauge a population based on what you are seeing in a specific place, or even if you are a pilot and fly the same exact line out to your cabin and in the past saw more moose than you see now, that somehow means the ADFG data is flawed. These are fairly large areas we are talking about and we need to take in the whole area rather than a few specific places.

    This kind of thing happened quite a bit in the 20A and surrounding areas; hunters didn't believe there were as many moose as ADFG data showed. It's good to question data and ask how we arrived at it, and what the confidence intervals are etc. I think, however, that there is a wave of hunters who are discounting good data out of hand more and more, and this is causing some difficulties. Most of it is education related I think; if hunters knew more how these surveys were conducted and the science behind them I think they'd trust the data more. Which is why I'm typing this up this morn. I could have some of the particulars wrong, this is all from memory, but wanted to get the basics down. If you got this far then you get a gold star <grin>.


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    Moderator LuJon's Avatar
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    Default

    Great post BR

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    Default Great Information

    Thanks for the insite into the moose counting. One thing that might be better would, to get the U2's to do some filming for us. Problably would even be better for Caribou.

    Terry

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    Default Terry...

    Terry, we're doing photographic census' of some caribou populations now, pretty high tech camera mounted on aircraft. It can work well with large populations like that when they congregate in a swath or area, and then you do the counting from the photos.

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    Member Vince's Avatar
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    hey Mark great job here i have a question on it though...


    MY EXPERIENCE. in 20A is this that once a year Moose come out of the wood work and travel many miles to participate in the rut.. and being that they do not travel BACK to where they came from until well after the rut..

    I can go entire months with out fresh sign on the trial and then in sept here they come.

    so how accurate can counting in the snow be when they are pulled into certain geographical areas...

    i can look at the 20A zone map for the bull hunts and see where the moose head off to for the rut buy the number of tags given.

    shouldn't it be counted when the moose are pre rut and in the valleys they live in,,, when one valley has 50 moose and the five nearby have none thats 10 moose per... an average of 10 square miles or so.. so what is the formula.. and the average acceptable population per s mile for moose. what is the estimated range of the bulls cows....

    what is the favorable bull to cow ratio?

    Thanks
    "If you are on a continuous search to be offended, you will always find what you are looking for; even when it isn't there."

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    Member bushrat's Avatar
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    Default more info forthcoming

    Vince, gotta get out of here right now, will post more info on this that I have, will see if I can get some of the ADFG data uploaded, may be too large of files.

    We can't do reliable counts while leaves are still on the deciduous trees and there is no snow on the ground. Too much error there in missed moose. So we can't do pre-rut counts with any efficacy.

    Bios are aware of moose movements and I have all that on file here and will try to see if I can upload some of it tomorrow or whenever I get the next chance. Keep in mind the initial stratified random sampling was/is done over very large areas to determine where certain high-density and low-density areas are in late October or early November. There are definitely some post-rut concentration areas and winter concentration areas we know about, so again, all that is taken into consideration.

    Moose migration patterns and home ranges vary considerably depending on a number of factors. Some moose early in life had to deal with deep-snow winters that may have affected their movements so they may travel farther because that was what they learned to do. And of course they learn from their mothers too. Some moose have very small home ranges, some have larger home ranges. I'll try to get the ADFG map up that shows all this; it's pretty interesting.

    Right now we're trying to bring the moose pop. in 20A and adjacent areas down. The average acceptable density per square mile depends on available habitat/browse. Browse in 20A and surrounds is very bad off and health of the herd very poor; we can use twinning rates and indicators like rump fat depth and prevalence of disease to determine if we have overpopulations, which is the case in 20A and adjacent areas. We can also do browse surveys to determine what is going on habitat wise. 20A has the highest density of moose in all of North America. Don't recall the figure offhand, think it is around 2moose/mi2. Don't recall the bull:cow or calf:cow ratios offhand, just keep in mind that what may be "favorable" now has more to do with concerns on trying to lower the population rather than raise it, so what may be favorable numbers there may not be favorable elsewhere.
    Later,

  8. #8
    Member Vince's Avatar
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    thanks mark look forward to it... some of this i have been trying to get out of the Bio. but with out luck
    "If you are on a continuous search to be offended, you will always find what you are looking for; even when it isn't there."

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    Default Aerial Survey manual is online

    I posted some answers to Vince's question, plus some ADFG data, into a separate thread. If anyone would like more info on aerial survey techniques and the science behind it you can jot down the link in the pic below and paste into your browser:


  10. #10
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    http://winfonet.alaska.gov/sandi/moo...ionsManual.pdf

    heres the link so you don't have to write it down...
    "If you are on a continuous search to be offended, you will always find what you are looking for; even when it isn't there."

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    Member Vince's Avatar
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    Thanks again for that link mark... to put it in layman's terms this link DOES NOT show the surveydata... and is used to show HOW the data is collected,sorted and all added up...

    been awhile since i have had to wrap my mind around some of those numbers...
    "If you are on a continuous search to be offended, you will always find what you are looking for; even when it isn't there."

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