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Thread: Do decreased numbers of mature rams result in low pregnancy rates?

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    Default Do decreased numbers of mature rams result in low pregnancy rates?

    Hello All. I received an email last night asking this question, and instead of responding to just one person, thought that everybody might appreciate seeing what the data shows for the research project over the last five years.


    Here you go....and, by the way, thanks for the email, it's a great question.



    We have been checking pregnancy rates through the course of my research. On the 13D project, we’ve been simultaneously tracking pregnancy rates and conducting summer surveys since 2009. We did not survey in summer 2011 but other than that it’s been flown consistently.


    The question you are asking is one that we asked right as we started the project; that is “Do low ram numbers result in depressed pregnancy rates?”

    At this point in the project I’m pretty confident that the problem is nutritional restriction rather than lack of rams. I’ll lay out the case for what I’ve seen and then add a few caveats at the end of the discussion.

    First, keep in mind that the test itself will not tell us if the ewes were or were not being bred (potentially as a result of low ram numbers). It just measures the level of progesterone in the blood which is elevated in pregnant animals. The test is extremely precise, and I am told that it is sensitive enough that it will show a negative within about a week to ten days after the ewe has lost the pregnancy.


    The 13D study area (between the Matanuska and Tazlina glaciers in western 13D) has had extremely low hunter harvest rates as a result of the initially conservative strategy employed in the DS 160/260 any ram drawing permit hunt which was instituted beginning with the 2008 season. It was harvested heavily up until 2008 however.

    Here are the numbers and ages of the rams harvested in the study area since the drawing permits were instituted: 3 in 2008 (ages 6, 6, and 4) 3 in 2009 (ages 6, 6, and 9); 3 in 2010 (ages 7, 7, and 10); 3 in 2011 (ages 10, 8, and 9); 3 in 2012 (ages 5, 8, and ); and 6 in 2013 ages (5,9,9, 5, 3, and 9). As an aside, there were additional animals taken on the 160/260 permit in 2012 that were not harvested in the study area (in the permit unit but outside the study area boundaries)

    Over the last five years, surveys show steady increases in the ratios of both legal and sublegal rams per 100 ewes. In 2009 we recorded 3.8 legals and 27.5 sublegal rams/100 ewes; in 2010, 5.1 legals and 32 sublegals/100; in 2012, 7.5 legals and 34.0/100 sublegals; and a slight drop in legals to 6.5/100 but a continued increase to 39.9/100 sublegals in 2013.

    Overall population numbers increased slightly from 2009-2011 but then decreased somewhat in 2012-13, probably as a result of the extremely heavy snowfall in 2011-2012. The late spring in 2013 didn’t help. 2009 – 574 sheep counted; 2011 – 708 sheep counted, 2012 – 491 sheep counted, and 2013 –486 sheep counted.

    Pregnancy rates have remained low in the collared ewes in the 13D study area with the exception of two of five years. 13D ewes had a 62% pregnancy rate in 2009, increased to 88% in 2010, dropped again to 69% and then 21% in 2011 and 2012 respectively, and then rebounded to a more normal 91% in 2013.

    For comparison, Steve Arthur reported typical pregnancy rates of 85-100% during his 1999-2003 project in the AK range, with only one low year at 74%. And, Canadian biologist Mari Wood, working on Stone’s sheep in BC, reported rates of 95-100%, testing 45-50 ewes annually over a four year project.

    I suspect if ram numbers were the limiting factor we would have seen a steady increase in pregnancy rates as the ram/100 ewe ratios increased between 2008-2013 rather than the back and forth shift between low and higher rates. I suspect that the back-and-forth shift is indicative of animals lambing, taking a year off, lambing again, taking a year off, and so forth which is supported when we look at the individual reproductive history of collared ewes. This is also a typical strategy we have observed in adjacent moose populations when they are nutritionally stressed. I also suspect that the 91% rate we recorded last year is a rebound effect as most ewes did not have lambs in summer 2012 and thus had a bit of a nutritional surplus going into 2013 and were able to maintain a pregnancy.

    Finally, and most importantly, these animals are very skinny when we capture them. A good description would be “skin and bones”. We know from animal physiology that animals below a certain threshold of body fat reserves will not maintain pregnancy, or may not enter estrus in the first place.


    However, as I mentioned there are a couple caveats that need to be acknowledged.

    First, keep in mind that these sheep surveys cover most of the core sheep habitat, though some area is not surveyed. Also, observers, pilots, and counting conditions may vary from year to year, meaning that survey data should be referred to as minimum counts, and taken as an indication of population trend rather than an exact population census. Even under the best conditions, we will never see all the sheep in the unit. Composition data (rams / 100 ewes) from these surveys are generally considered fairly accurate representations of the population.

    Second, rams produce viable sperm at 18 months. So, a shortage of mature rams may not ever actually result in a lowered pregnancy rate. Along the same lines, there’s no way to definitively state that that population did or did not experience a low enough ram numbers that could have resulted in decreased pregnancy rates. I’ve heard—though have not ever seen scientific documentation for—that male:female ratios below 20/100 are suspected to be approximately the threshold for decreased pregnancy rates in some other ungulate populations.

    Third, there are several hypotheses that are largely untested regarding the value of maintaining older rams in a population. These range from Geist’s dominance related mortality, to others supported anecdotally by historical reports of movement patterns and range use that suggest that these older rams are critical to teach younger rams travel routes, and winter and summer ranges, to older animals serving as a sort of ‘buffer’ in case of a hard winter or increased predator load. So, keep in mind that harvesting all –or even a majority of –the mature rams in a population could have effects that we cannot quantify at this point in time.

    And, finally, keep in mind that I’m not saying that this sheep population doesn’t have problems. I’m just saying that I don’t think that depressed ram numbers are the cause of low pregnancy rates we’ve been observing.

    I’m totally swamped right now trying to get geared up for captures, scientific meetings, Board of Game meetings, etc. So, I’ll do my best to answer questions but can’t promise that I’ll be able to answer all of them in a reasonable time frame.

    I am planning on attending the forum gathering on the 25th in Eagle River for an informal Q&A, and Brian M. is currently locking in a venue and a time for me to present the complete research talk, at which point I’ll make sure we have a long time window available for Q&A after the talk. Keep an eye out for details.


    Tom Lohuis

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    Thanks for the info Tom and your time to post this.

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    Member Yellowknife's Avatar
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    Filed away. Thanks Tom.

    Yk

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    It's these types of posts that I hope shed light on just what our biologists are doing out there. It pains me to see denigrating comments about them when I know the hard work they are putting in. We are lucky to have objective scientist out there doing meaningful work to help us understand exactly what we are up against. We are lucky to have folks like Tom in the states employ! I will be at both the pizza man gathering and the Chugiak High presentation and look forward to hearing more.

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    Thank you Tom for sharing some of your data and insight. I look forward to hearing more at Pizza Man.

    Scott

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    Tom,

    Let me summarize what I think I should be getting from your research:

    1. The decline in sheep populations in the Chugach and perhaps elsewhere appears to be most directly linked to low pregnancy rates in ewes, subsequently leading to poor recruitment.

    2. The low pregnancy rate appears most likely to be linked to poor nutrition.

    3. The ram/ewe ratio does not appear to be a significant factor in this low pregnancy rate.

    4. The harvest or lack of harvest by hunters is somewhat unlikely to serious effect the pregnancy rates which is the primary reason for the depressed population.

    5. It is possible however the removal of a large percentage of the oldest rams may have other more subtle and difficult to measure impacts on survival.

    Do I have that more or less straight?

    Yk

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    Thanks for posting that information Tom!

    I logged this article a while back and reading your report made me dig it up and review...
    http://media.nwsgc.org/proceedings/N...9-Heimer-2.pdf

    I have a question. Do you think the "alternate-year reproduction syndrome" as described in the article [and sited below] may have something to do with declining sheep populations elsewhere.

    The article describes the problem as going away when the legal limit was increased from 3/4 to 7/8 curl. Do you think its possible that we're seeing the same thing, and the 7/8 curl change was just a coincidence? 7/8 curl is basically a 7yo sheep. Is it possible that we've depressed the averag age of rams harvested to a low enough level that we're seeing this "syndrome" pop up? My estimation shows the average age of rams harvested in unit 20A hovering around 8.3yo for the last 5-6 years. The study also points to the lack of mature rams having an impact on the survival of the younger rams.

    Sorry didn't mean to derail your discussion.

    As increasing hunter interest during the late 1960s and 1970s drove Dall ram harvest pressure upward in Alaska, accessible populations were harvested to the legal ¾ curl limit (Heimer 1980b). That is, virtually every legal ¾ curl ram in these populations was harvested each year. Subsequent field research where marked ewe reproductive success was monitored over the course of about 20 years revealed consecutive-year reproduction was rare in these populations (Heimer 1978). A high percentage of ewes (almost 25 percent) bred at 18 months of age, ewes maintained lambs on milk throughout winter rather than weaning in October, and ovulation rate was low. The cumulative effect was an alternate-year reproductive success syndrome (Heimer and Watson 1990). Nutritional and body composition studies (Heimer 1983) showed no difference between populations exhibiting the alternate-year syndrome and comparable populations where consecutive-year reproductive success was high (but yearling ewe breeding was virtually absent (Heimer and Watson 1986a)). The obvious difference between these radically different populations was mature ram presence, which was linked with overall higher ram abundance. Subsequent adjustment of harvest regimes from a lower legal limit of ¾ curl upward to 7/8 curl eliminated the alternate-year reproductive syndrome among ewes (Heimer and Watson 1990), but did not result in the anticipated increase in ram harvests.

    Subsequent analysis of marked sublegal ram survivorship from the poor quality population indicated harvests were compromised because physically and socially immature rams were paying the mortality cost of dominance beginning at age three instead of at social and physical maturity at age eight (Heimer and Watson 1986b). That is, mortality rates among marked sublegal rams equaled those calculated by Deevey (1947) from Murie’s (1944) age distribution at death. The difference was that, in the absence of dominant rams, the accelerated mortality portion of Deevey’s survivorship curve began at age three instead of age eight. These findings were consistent with predictions from Geist (1971 and pers commun.). Increasing the legal horn size minimum from 7/8 to full curl resulted in highly significant (mean 35 percent increase, p<0.001) increases in realized harvests from this population (Heimer and Watson 1990).

    With the exception of the divided Northwest Territories, thinhorn sheep are now harvested at legal minimums defined as full-curl throughout their distribution. The extent of actual horn development differs between Alaska and the Canadian jurisdictions (Heimer 1990, Barichello and Carey 1990, Case, 1990, A. Veitch pers commun.), but intent is to restrict non-subsistence hunting to mature rams.

    Summary: Mature rams are the only clearly surplus animals in thinhorn populations. Harvest by humans should be limited to fully mature rams if population maintenance and maximized human benefit are management goals.

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    Member Yellowknife's Avatar
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    Quote Originally Posted by Heimer View Post
    Increasing the legal horn size minimum from 7/8 to full curl resulted in highly significant (mean 35 percent increase, p<0.001) increases in realized harvests from this population (Heimer and Watson 1990)..
    I've wondered about this and several similar statements in Heimers papers for a while now. I'm not a biologist so I'm sure I'm not qualified to question his findings, but...

    In 1990, the full curl law was based on the theory that harvesting only the mature rams would in fact provide a long term "increase in realized harvest" as he puts it. 24 years later we have a LOT more years worth of information and in my mind that has proven to be anything but true. Sheep harvest peaked about 2 years after the FC law was implemented, and has been sliding down hill ever since. In 2013, we set a new low for number of sheep harvested by resident hunters going back to at least the 70's. We are now taking less than half of what we were in the early 90's. In large part that is because the statewide population of both sheep and sheep hunters is a only a portion of what it was during the days of 3/4 and 7/8 curl laws.

    Was the FC law the cause of the declines? Highly unlikely, and not something I'm trying to say. (Although, if I didn't know better, looking at a graph of sheep populations and harvest stats from the last 30 years would certainly suggest it.) However now that we have long term data, it appears that moving to full curl was not the silver bullet cure to reproductive rates and ram survival as was suggested by those early studies.

    I wonder if it's not time to do another study on the effect of ram harvest given the changes in the sheep population landscape in the last 20+ years. We now have a variety of FC, Any Ram, and completely protected populations to look at. Maybe everything is well with the way we currently do it.... and maybe we should be changing things to match today's conditions.

    Yk

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    Quote Originally Posted by Yellowknife View Post
    ....and maybe we should be changing things to match today's conditions.

    Yk
    Imo, no maybes about it. And yet again, the thought that something should have been done some time ago reoccurs. I hate to sound like a broken record......
    Sheep hunting...... the pain goes away, but the stupidity remains...!!!

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