Seasonal and developmental patterns of body condition in Arctic seals

Quantifying the body condition of free-ranging marine mammals remains a central challenge in marine ecology, particularly for species inhabiting remote or logistically difficult environments. As rapidly warming conditions threaten the long-term viability of many polar species, such data are urgently needed. Body condition is a key physiological indicator of individual health and informs understanding of population-level responses. Assessments of body condition based on a combination of mass, morphometric, and blubber thickness values can be robust, but are often time-consuming, resource-intensive, and limited to individual sampling events. It is especially difficult to evaluate the body condition of ice-associated Arctic seals given that both access to animals and handling time are highly constrained. Thus, field assessments often rely on simple metrics to infer body condition. Further, most data are collected during discrete periods of annual access, leaving entrained seasonal changes poorly understood. We obtained fine-scale morphometric and blubber depth measurements from captive spotted (Phoca largha; n=4), ringed (Pusa hispida; n=3), and bearded seals (Erignathus barbatus; n=1) to track longitudinal changes in blubber content over seven years. We identified predictable seasonal changes in the blubber content of our seals with mean annual fluctuations between minima and maxima of 27-55% in the spotted seals, 18-28% in the ringed seals, and 18.5% for the bearded seal. We then conducted a site-selection analysis to identify which single blubber depth location best tracked longitudinal changes in blubber content by species and age class. We used linear mixed-effects and generalized least squares models to evaluate the efficacy of commonly used body condition metrics to inform field sampling efforts. Across species and age classes, LMD index values and sternal blubber depth were the best supported proxies for blubber content. Continued work with seals in human care can improve practices for assessing body condition of free-ranging individuals, particularly using emerging technologies that do not rely on animal handling. This study provides fine-scale quantitative data that define intrinsic, predictable seasonal patterns in Arctic seal body condition, inform best practices for field monitoring, and increase our ability to distinguish routine variation from abnormal changes driven by external stressors, underlying pathology, or disease.