Trends and Patterns of Seal Abundance at Haul-out Sites in a Gray Seal Recolonization Zone
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Trends and Patterns of Seal Abundance at Haul-out Sites in a Gray Seal Recolonization Zone
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    Gray (Halichoerus grypus) and harbor seals (Phoca vitulina) were greatly reduced in numbers along maritime Canada and nearly extirpated from waters along the US Atlantic Coast by the mid-1900s. Harbor seals were first to show substantial increases in US Atlantic waters. Expanding from a rapidly rebuilding residual population off Atlantic Canada, gray seals are recolonizing areas of southern New England. Although their abundance appears to vary seasonally, the presence of large numbers of gray seals raises questions about the potential for competitive exclusion of smaller harbor seals as well as public concern over resource conflicts. We counted gray and harbor seals hauled out on frequently used sites near the southern edge of a gray seal recolonization zone, primarily near Cape Cod, Massachusetts. Counts were made during periodic aerial surveys conducted during 2005-2015 to measure seal occupancy at haul outs and to gauge trends in abundance during that time period. We also used statistical models to characterize this variation. In particular, we characterized the annual cycle of gray seal abundance at haul outs with an expanding sinusoidal function and fit annually varying growth functions from year start to peak abundance to characterize the pattern of peak abundance change among years, relying on empirical Bayes methods to estimate parameters. We limited models of harbor seal counts to testing for change in the upward trend in peak counts. Gray seal peak abundance at haul outs occurred between 15 April and 13 May, and total abundance across surveyed sites increased from 10,847 in 2005 to 23,579 in 2015. Although peak abundance of gray seals in the study area more than doubled during our study, the rate of growth declined over time. Concomitant with increasing gray seal presence at haul outs, peak numbers of hauled out harbor seals detected in the study area increased for about half of the study period and then declined. Our study documents the fluctuating but increasing abundance of gray seals on haul-out sites at the edge of a gray seal recolonization zone during 2005-2015, and we provide further evidence that increasing gray seal populations may reduce use of the region by harbor seals. We did not measure total seal abundance in the region. Our work demonstrated that, despite a lack of knowledge about true availability and therefore total abundance, repeated surveys of haul outs can be informative about trends in seal abundance.
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