Ban and trites: Marine Mammal Research Consortium

For thousands of years, Steller sea lions have occupied some of the most barren and remote coastlines in the North Pacific. A key to their survival has been the careful selection of terrestrial habitats – haulouts for resting, and rookeries for breeding – that offer some protection against wind, weather and predators.

Yet the rationale for selecting and inhabiting some locations, while ignoring others, remains a mystery to scientists. How do sea lions judge a section of shoreline suitable or unsuitable for breeding or hauling out? Do sea lions in the declining western populations select different types of habitat than their thriving eastern counterparts?

In the hands of managers, this information could help conserve dwindling populations by identifying key shorelines or forecasting the effects of climate change.

To address this paucity of information, Stephen Ban and Andrew Trites of the Marine Mammal Research Unit at the University of British Columbia, quantified the types of terrestrial habitat used by Steller sea lions at rookeries and haulouts in British Columbia and Alaska. Their study was recently published in the journal Marine Mammal Science.

Exposed Coastlines
By studying shoreline classification data – detailed descriptions of shorelines issued by the U.S. and Canadian governments to aid in oil spill response – Ban and Trites assessed the general characteristics of some 294 known haulouts and 38 rookeries.

“Classifying and comparing the shoreline type of haulouts and rookeries against sites not used by Steller sea lions showed that they preferentially locate their haulouts and rookeries on exposed rocky shorelines and wave-cut platforms,” the authors write. “Shoreline types used less frequently by sea lions included fine-to-medium-grained sand beaches, mixed sand and gravel beaches, gravel beaches, and sheltered rocky shores.”

While there were differences in the distribution of haulouts and rookeries among shoreline types, Ban and Trites did not find rookeries on sheltered sites as expected where pups would be better protected from storms.

Thus, the relative shelter offered by a site does not appear to be a consideration when colonizing a site. Instead, Ban and Trites suggest that other factors may drive site selection, such as protection from predation by bears and wolves or proximity to key foraging areas.

Range of Habitats
By surveying sites across the entire North American range, Ban and Trites sought to better understand why western populations dwindled while eastern populations held relatively steady. Both populations had access to and utilized similar shoreline types; however, a higher proportion of sites in the western population favored wave-cut platforms, while eastern sites favored exposed rocky shorelines.

Overall, Ban and Trites conclude that site selection likely involves either an optimization or compromise of two factors: the nearness to favorable foraging areas, and the degree of difficulty in entering or exiting the water during different tidal heights.

In providing the first quantitative survey of sea lion habitats, the study also corroborated a number of anecdotal observations offered by other scientists. These findings collectively open the door for future studies with finer-scale topographic information, which may offer further insights into why some coastlines are more popular with sea lions than others. Ultimately, this information will help to protect key habitat and conserve the species.

Publication:

	Quantification of terrestrial haul-out and rookery characteristics of Steller sea lions.
		Ban, S. and A.W. Trites. 2007.
		Marine Mammal Science 23:496-507.
		abstract Steller sea lions (Eumetopias jubatus)are known to have occupied the same terrestrial haul-out and rookery sites across the North Pacific rim for centuries, but it is not known why they choose and stay at these locations, or what defines their preferred habitat. Classifying and comparing the shoreline type of haulouts and rookeries against sites not used by Steller sea lions showed that they preferentially locate their haulouts and rookeries on exposed rocky shorelines and wave-cut platforms. However, no preference was found for selecting rookeries on sheltered shore-types. Shoreline types used less frequently by sea lions included fine-to-medium-grained sand beaches, mixed sand and gravel beaches, gravel beaches, and sheltered rocky shores. Quantifying the shoreline types used by sea lions confirms anecdotal reports of habitat preferences and may prove useful in identifying and protecting sea lion terrestrial habitat, or in forecasting how climate change might affect the distribution of sea lions.

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