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Ecological invasions and colonizations occur dynamically through space and time. Estimating the distribution and abundance of colonizing species is critical for efficient management or conservation. We describe a statistical framework for simultaneously estimating spatiotemporal occupancy and abundance dynamics of a colonizing species. Our method accounts for several issues that are common when modeling spatiotemporal ecological data including multiple levels of detection probability, multiple data sources, and computational limitations that occur when making fine-scale inference over a large spatiotemporal domain. We apply the model to estimate the colonization dynamics of sea otters (Enhydra lutris) in Glacier Bay, in southeastern Alaska.

Steller sea lions Eumetopias jubatus are central-place foragers that forage in the marine environment while using terrestrial sites to rest and care for young. Some terrestrial sites are used seasonally; however, the reasons for doing so are not fully understood. We addressed the hypothesis that seasonal availability of prey influences seasonal abundance and diet of sea lions. We quantified monthly prey availability and sea lion abundance and quarterly diet composition at Benjamin Island in SE Alaska (2001)(2002)(2003)(2004). Large numbers of sea lions occupied Benjamin Island during the nonbreeding season from October to April when Pacific herring Clupea pallasii biomass was highest. Herring was the most common species in sea lion diet (frequency of occurrence [FO] = 90%) and comprised over 81% of the available pelagic prey biomass and 96% of the energy encountered during pelagic surveys. Walleye pollock Theragra chalcogramma accounted for 19% of the available prey biomass but was only slightly less common in sea lion diet (FO = 88%) than herring. Herring biomass was correlated with the number of sea lions; in contrast, there was no relationship between pollock biomass and number of sea lions. Several fish species were found in nearshore areas, but were uncommon in sea lion diet. Sea lions consumed the available pelagic prey but little of the available nearshore prey. The FO of herring and pollock in sea lion diet did not differ significantly between seasons; however, the FO of other seasonal prey species differed between seasons. Seasonal occupation of Benjamin Island by sea lions is influenced by seasonally available, densely aggregated, energy-rich prey.

We hypothesized that: (1) Steller sea lion Eumetopias jubatus diet choice is a function of prey availability, (2) sea lions move to take advantage of times and locations of seasonal prey concentrations and (3) the number present depends on the amount of prey available (numerical response). Over 3 yr, typically on a quarterly basis, in Frederick Sound, SE Alaska, multiple measurements were taken of Steller sea lion abundance (aerial surveys), diet (scats), dive behavior (satellite telemetry) and prey availability and caloric density (nearshore, pelagic and demersal fish surveys). We found that Steller sea lions shifted diet composition in response to changes in prey availability of pollock Theragra chalcogramma, hake Merluccius productus, herring Clupea pallasi and salmon Oncorhynchus spp. They selected intermediate-sized fish and avoided small (<10 cm) and large (> 60 cm) fish, and moved between areas as prey became available seasonally. The number of sea lions present depended on the amount of prey available; a standing biomass of 500 to 1700 t of prey in a nonbreeding area such as Frederick Sound, depending on species composition, can attract and sustain about 500 sea lions. Pollock was more frequent in sea lion diet in inside waters of SE Alaska -including Frederick Sound, Stephens Passage and Lynn Canal -than anywhere else in Alaska and contributed ~1⁄ 3 of the dietary energy in Frederick Sound. This finding implies that a diet with substantial year-round contributions from less nutritious, but abundant prey such as pollock can form part of a healthy diet as long as more nutritious prey such as herring, salmon or eulachon Thaleichthys pacificus also are consumed. Our study supports the conclusion that the Steller sea lion is an opportunistic marine predator with a flexible foraging strategy that selects abundant, accessible prey and shifts among seasonally available species.

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