Efficacy of calf:cow ratios for estimating calf production of arctic caribou
Caribou (Rangifer tarandus granti) calf:cow ratios (CCR) computed from composition counts obtained on arctic calving grounds are biased estimators of net calf production (NCP, the product of parturition rate and early calf survival) for sexually-mature females. Sexually-immature 2-year-old females,...
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doaj-c30f8e997bd548818317d31969ebddfb2020-11-24T22:08:52ZengSeptentrio Academic PublishingRangifer1890-67292013-04-0133210.7557/2.33.2.25272339Efficacy of calf:cow ratios for estimating calf production of arctic caribouRaymond D. CameronBrad GriffithLincoln S. ParrettRobert G. White Caribou (Rangifer tarandus granti) calf:cow ratios (CCR) computed from composition counts obtained on arctic calving grounds are biased estimators of net calf production (NCP, the product of parturition rate and early calf survival) for sexually-mature females. Sexually-immature 2-year-old females, which are indistinguishable from sexually-mature females without calves, are included in the denominator, thereby biasing the calculated ratio low. This underestimate increases with the proportion of 2-year-old females in the population. We estimated the magnitude of this error with deterministic simulations under three scenarios of calf and yearling annual survival (respectively: low, 60 and 70%; medium, 70 and 80%; high, 80 and 90%) for five levels of unbiased NCP: 20, 40, 60, 80, and 100%. We assumed a survival rate of 90% for both 2-year-old and mature females. For each NCP, we computed numbers of 2-year-old females surviving annually and increased the denominator of CCR accordingly. We then calculated a series of hypothetical “observed” CCRs, which stabilized during the last 6 years of the simulations, and documented the degree to which each 6-year mean CCR differed from the corresponding NCP. For the three calf and yearling survival scenarios, proportional underestimates of NCP by CCR ranged 0.046–0.156, 0.058–0.187, and 0.071–0.216, respectively. Unfortunately, because parturition and survival rates are typically variable (i.e., age distribution is unstable), the magnitude of the error is not predictable without substantial supporting information. We recommend maintaining a sufficient sample of known-age radiocollared females in each herd and implementing a regular relocation schedule during the calving period to obtain unbiased estimates of both parturition rate and NCP. https://septentrio.uit.no/index.php/rangifer/article/view/2527composition countserrorsfecundityherd productivity |
collection |
DOAJ |
language |
English |
format |
Article |
sources |
DOAJ |
author |
Raymond D. Cameron Brad Griffith Lincoln S. Parrett Robert G. White |
spellingShingle |
Raymond D. Cameron Brad Griffith Lincoln S. Parrett Robert G. White Efficacy of calf:cow ratios for estimating calf production of arctic caribou Rangifer composition counts errors fecundity herd productivity |
author_facet |
Raymond D. Cameron Brad Griffith Lincoln S. Parrett Robert G. White |
author_sort |
Raymond D. Cameron |
title |
Efficacy of calf:cow ratios for estimating calf production of arctic caribou |
title_short |
Efficacy of calf:cow ratios for estimating calf production of arctic caribou |
title_full |
Efficacy of calf:cow ratios for estimating calf production of arctic caribou |
title_fullStr |
Efficacy of calf:cow ratios for estimating calf production of arctic caribou |
title_full_unstemmed |
Efficacy of calf:cow ratios for estimating calf production of arctic caribou |
title_sort |
efficacy of calf:cow ratios for estimating calf production of arctic caribou |
publisher |
Septentrio Academic Publishing |
series |
Rangifer |
issn |
1890-6729 |
publishDate |
2013-04-01 |
description |
Caribou (Rangifer tarandus granti) calf:cow ratios (CCR) computed from composition counts obtained on arctic calving grounds are biased estimators of net calf production (NCP, the product of parturition rate and early calf survival) for sexually-mature females. Sexually-immature 2-year-old females, which are indistinguishable from sexually-mature females without calves, are included in the denominator, thereby biasing the calculated ratio low. This underestimate increases with the proportion of 2-year-old females in the population. We estimated the magnitude of this error with deterministic simulations under three scenarios of calf and yearling annual survival (respectively: low, 60 and 70%; medium, 70 and 80%; high, 80 and 90%) for five levels of unbiased NCP: 20, 40, 60, 80, and 100%. We assumed a survival rate of 90% for both 2-year-old and mature females. For each NCP, we computed numbers of 2-year-old females surviving annually and increased the denominator of CCR accordingly. We then calculated a series of hypothetical “observed” CCRs, which stabilized during the last 6 years of the simulations, and documented the degree to which each 6-year mean CCR differed from the corresponding NCP. For the three calf and yearling survival scenarios, proportional underestimates of NCP by CCR ranged 0.046–0.156, 0.058–0.187, and 0.071–0.216, respectively. Unfortunately, because parturition and survival rates are typically variable (i.e., age distribution is unstable), the magnitude of the error is not predictable without substantial supporting information. We recommend maintaining a sufficient sample of known-age radiocollared females in each herd and implementing a regular relocation schedule during the calving period to obtain unbiased estimates of both parturition rate and NCP.
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topic |
composition counts errors fecundity herd productivity |
url |
https://septentrio.uit.no/index.php/rangifer/article/view/2527 |
work_keys_str_mv |
AT raymonddcameron efficacyofcalfcowratiosforestimatingcalfproductionofarcticcaribou AT bradgriffith efficacyofcalfcowratiosforestimatingcalfproductionofarcticcaribou AT lincolnsparrett efficacyofcalfcowratiosforestimatingcalfproductionofarcticcaribou AT robertgwhite efficacyofcalfcowratiosforestimatingcalfproductionofarcticcaribou |
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