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Efficacy of calf:cow ratios for estimating calf production of arctic caribou
- Source :
- Rangifer; Vol 33 (2013): Special Issue No. 21; 27-34, Rangifer, Vol 33, Iss 2 (2013)
- Publication Year :
- 2013
- Publisher :
- Septentrio Academic Publishing, 2013.
-
Abstract
- 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.
- Subjects :
- education.field_of_study
composition counts
biology
Ecology
fecundity
Population
herd productivity
Ice calving
General Medicine
biology.organism_classification
Fecundity
Rangifer tarandus granti
Animal science
Arctic
Herd
errors
Sexual maturity
lcsh:Animal culture
education
Survival rate
lcsh:SF1-1100
Subjects
Details
- Language :
- English
- ISSN :
- 18906729
- Database :
- OpenAIRE
- Journal :
- Rangifer
- Accession number :
- edsair.doi.dedup.....86466f29c9f9890b45c0850ee8a8cfd4