Contaminants, body condition, and mortality: A comparative study of at-risk St. Lawrence Estuary belugas and Northwest Atlantic harbour porpoises

Marine mammals in the St. Lawrence (Canada) face cumulative anthropogenic stressors, including contaminant exposure. While resident St. Lawrence Estuary (SLE) belugas ( Delphinapterus leucas ; endangered) possess extensive blubber reserves, summering Northwest Atlantic (NWA) harbour porpoises ( Phocoena phocoena ; special concern) operate on high metabolic rates with minimal lipid storage. Contaminants including halogenated flame retardants (HFRs), per- and polyfluoroalkyl substances (PFAS), and trace elements may interfere with energetic metabolism, yet their relationships with body condition remain poorly understood. We evaluated whether these contaminants were associated with body condition, estimated by the Scaled Mass Index (SMI), and causes of mortality in stranded carcasses of SLE belugas ( n = 59) and NWA harbour porpoises ( n = 29) in 2016–2024. Contaminant profiles reflected contrasting habitat use; SLE belugas exhibited higher blubber HFR and liver PFAS concentrations, whereas NWA harbour porpoises presented lower organohalogen levels and higher liver trace element levels. In SLE belugas, SMI was negatively correlated with liver PFAS and total mercury (THg) concentrations, and over the study period. In NWA harbour porpoises, lower SMI was associated primarily with age class (calves) and higher liver THg levels. Multivariate analyses revealed that infectious disease mortality in NWA harbour porpoises clustered with higher trace element concentrations (THg, Se, Zn, Ag), whereas no clear associations between contaminants and causes of mortality were identified within age classes in SLE belugas. Present findings indicate that morphometric-based body condition and specific mortality categories correlate with distinct tissue contaminant profiles, pointing to potential linkages with energetic metabolism dysregulation and disease vulnerability.