Estimating pregnancy rate from blubber progesterone levels of a blindly biopsied beluga population poses methodological, analytical and statistical challenges

L-A Renaud; X Bordeleau; N M Kellar; G Pigeon; R Michaud; Y Morin; S Lair; A Therien; V Lesage

doi:10.1093/conphys/coad075

Estimating pregnancy rate from blubber progesterone levels of a blindly biopsied beluga population poses methodological, analytical and statistical challenges

Conserv Physiol. 2023 Sep 26;11(1):coad075. doi: 10.1093/conphys/coad075. eCollection 2023.

Authors

L-A Renaud¹, X Bordeleau¹, N M Kellar², G Pigeon³, R Michaud⁴, Y Morin¹, S Lair⁵, A Therien¹, V Lesage¹

Affiliations

¹ Department of Fisheries and Oceans Canada, Maurice Lamontagne Institute, P.O. Box 1000, 850 Route de la Mer, Mont-Joli, Québec, G5H 3Z4, Canada.
² Southwest Fisheries Science Center, National Marine Fisheries Service, P.O. Box 271, La Jolla, California 92038, USA.
³ Institut de recherche sur les forêts, Université du Québec en Abitibi-Témiscamingue, Rouyn-Noranda, Québec, J9X 5E4, Canada.
⁴ Groupe de recherche et d'éducation sur les mammifères marins (GREMM), 108 de la Cale-Sèche, Tadoussac, Québec, G0T 2A0, Canada.
⁵ Faculté de médecine vétérinaire, Université de Montréal, P.O. Box 5000, 3200 Rue Sicotte, St-Hyacinthe, Québec, J2S 7C6, Canada.

Abstract

Beluga (Delphinapterus leucas) from the St. Lawrence Estuary, Canada, have been declining since the early 2000s, suggesting recruitment issues as a result of low fecundity, abnormal abortion rates or poor calf or juvenile survival. Pregnancy is difficult to observe in cetaceans, making the ground truthing of pregnancy estimates in wild individuals challenging. Blubber progesterone concentrations were contrasted among 62 SLE beluga with a known reproductive state (i.e. pregnant, resting, parturient and lactating females), that were found dead in 1997 to 2019. The suitability of a threshold obtained from decaying carcasses to assess reproductive state and pregnancy rate of freshly-dead or free-ranging and blindly-sampled beluga was examined using three statistical approaches and two data sets (135 freshly harvested carcasses in Nunavik, and 65 biopsy-sampled SLE beluga). Progesterone concentrations in decaying carcasses were considerably higher in known-pregnant (mean ± sd: 365 ± 244 ng g^-1 of tissue) than resting (3.1 ± 4.5 ng g^-1 of tissue) or lactating (38.4 ± 100 ng g^-1 of tissue) females. An approach based on statistical mixtures of distributions and a logistic regression were compared to the commonly-used, fixed threshold approach (here, 100 ng g^-1) for discriminating pregnant from non-pregnant females. The error rate for classifying individuals of known reproductive status was the lowest for the fixed threshold and logistic regression approaches, but the mixture approach required limited a priori knowledge for clustering individuals of unknown pregnancy status. Mismatches in assignations occurred at lipid content < 10% of sample weight. Our results emphasize the importance of reporting lipid contents and progesterone concentrations in both units (ng g^-1 of tissue and ng g^-1 of lipid) when sample mass is low. By highlighting ways to circumvent potential biases in field sampling associated with capturability of different segments of a population, this study also enhances the usefulness of the technique for estimating pregnancy rate of free-ranging population.

Keywords: Beluga; biopsies; blubber progesterone; mixture model; pregnancy.

Associated data

Dryad/10.5061/dryad.34tmpg4r5