Spatial structure of harbour seal populations and the implications thereof

• Published in: Canadian Journal of Zoology
• Main Authors: Härkönen, T, Harding, K C
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2001
• Subjects: harbour seal; Phoca vitulina
• Online Access: http://dx.doi.org/10.1139/z01-172; http://www.nrcresearchpress.com/doi/pdf/10.1139/z01-172

A long-term study of freeze-branded harbour seals (Phoca vitulina) revealed explicit site fidelity. Individuals were followed up to 14 years of age and none of the 163 branded animals were observed to haul out beyond a 32-km distance from the site where they were branded as pups. Within this range, striking spatial segregation by age and sex prevailed. While females' site fidelity increased with age, males spent less time at their natal site with increasing age. These findings have consequences for understanding the population dynamics of harbour seals, since single "colonies" will act as partly isolated "subpopulations" in some contexts but not in others. The differing migration tendencies of the population segments lead to spatially segregated sex and age ratios of subpopulations and create a complex pattern of connectivity among these subpopulations. Ignoring the spatial scale will lead to severe misinterpretations of analyses of basic population-dynamic processes, especially rates of population increase, rates of gene flow, and the dynamics of the spread of diseases. We suggest that when studies have different aims, these should be addressed by encompassing different numbers of subpopulations.