Scratching the Surface

Ecosystem Models Reveal Subtle Interactions

On the surface, the rugged coast of Alaska may seem to have escaped the environmental changes that transformed the continent during the twentieth century. However, in the latter half of that century, even this remote marine ecosystem saw a revolution in everything from ocean climate to populations of some of its smallest – and largest – inhabitants.

Some changes were due to natural cycles; others were the result of increased human activity. By the 1980s, their collective impact had become apparent among populations of Steller sea lions. Two contrasting trends emerged: a sharp and severe decline in Western Aleutian Steller sea lion populations, and a concurrent increase in populations in Southeast Alaska. This alarmingly visible shift in abundances of a top marine predator was cause for serious concern among scientists.

A number of hypotheses alternately implicated human fisheries, marauding killer whales, a shift in ocean climate, and inter-species competition for prey. Yet no single theory has to date successfully explained the conflicting rise and fall of Steller sea lion populations in each ecosystem, and most research to date could only test one hypothesis at a time.

With technological advances in computer modeling, however, ecosystem changes can now be explored by simulating a number of ecological scenarios. Using such a model, University of British Columbia researchers Sylvie Guénette, Sheila Heymans, Villy Christensen, and Andrew Trites simultaneously evaluated four hypotheses explaining the rise and fall of Steller sea lions: fishing, predation, competition, and ocean productivity. Their study was recently published in the Canadian Journal of Fisheries and Aquatic Sciences.

East vs. West

“We compared two contrasting study areas that differed by the trends of their respective sea lion populations, their fishing histories, and the characteristics of their ecosystems: the Aleutian Islands and Southeast Alaska,” the scientists write. “We modeled the period 1963–2002, which encompasses the [1977] ocean regime shift, the development of the major fisheries, and the major changes in Steller sea lion abundances.”

The model identified the Pacific Decadal Oscillation (PDO) – an ocean climate system that warms or cools the Gulf of Alaska on a decadal basis – as a central player in the abundance of Steller sea lions and other marine life. In 1977, an abrupt shift in the PDO ushered in a new ocean climate regime, decreasing primary production (phytoplankton) and altering the quantity or quality of the marine food web.

“It appears that climate impacted several species in both ecosystems, including species such as Atka mackerel and Pacific herring that form an important part of the Steller sea lion diet,” the authors write. “Thus, change in ocean productivity appears to have had a major, indirect effect on sea lions.”

When modeled in a computer simulation, this shift in ocean productivity impacted the Aleutian model more than the Southeast Alaska model. According to the model’s predictions, the Steller sea lion decline in the Aleutians can largely be explained by the regime shift and by killer whale predation, although the model also suggests that fishing for Atka mackerel and competition between sea lions and large flatfish for prey also played a role.

In contrast, simulations of Southeast Alaska suggested that the rise in Steller sea lion populations was linked to change in ocean productivity and increases in herring and salmon populations (which paralleled the steady rise in Steller sea lion populations from 1963-2002), which together overruled the effect of killer whale predation, and competition for prey from notably arrowtooth flounder.

In describing their findings on fishing, competition and predation, the authors note that fishing could have affected the ecosystem structure by influencing the abundance of Atka mackerel (in the Aleutians) and Pacific herring (in southeast Alaska). Competition for prey against flatfish – Pacific halibut in the Aleutians and arrowtooth flounder in southeast Alaska – was a less significant factor but also appeared to impede sea lion population growth over time in each area.

Finally, the model showed that predation by killer whales was important when sea lions were less abundant (in the 1990s in the Aleutians and in the 1960s in southeast Alaska), but appear to have little effect when sea lion numbers were high.

Overall, the ecosystem modeling exercise succeeded in integrating climate, competition, fisheries, and predation as potential causes of change in each ecosystem. Like all ecosystems, the Alaskan coast is in constant flux, and this study provided valuable information that enables scientists to further explore the subtle changes and interactions beneath the surface of this seemingly timeless coast.

31 January 2007

Publication:
Ecosystem models show combined effects of fishing, predation, competition, and ocean productivity on Steller sea lions (Eumetopias jubatus) in Alaska. Guénette, S., S.J.J. Heymans, V. Christensen, and A.W. Trites. 2006. Canadian Journal of Fisheries and Aquatic Sciences 63:2495-2517. abstract Steller sea lions (Eumetopias jubatus) increased in the eastern portion of their range while declining in the Gulf of Alaska and Aleutian Islands from the late 1970s to late 1990s. We constructed ecosystem models of the central and western Aleutians and of Southeast Alaska to simultaneously evaluate four hypotheses explaining sea lion dynamics: killer whale (Orcinus orca) predation, ocean productivity, fisheries, and competition with other species. Comparisons of model predictions to historical time series data indicate that all four factors likely contributed to the trends observed in sea lion numbers in both ecosystems. Changes in ocean productivity conveyed by the Pacific Decadal Oscillation influenced the abundance trajectory of several species. Fishing could have affected the ecosystem structure by influencing the abundance of Atka mackerel (Pleurogrammus monopterygius) in the Aleutians, and herring (Clupea pallasii) in Southeast Alaska. Halibut (Hypoglossus stenolepis) in the Aleutians and arrowtooth flounder (Reinhardtius stomias) in Southeast Alaska appear to impede sea lion population growth through competitive interactions. Predation by killer whales was important when sea lions were less abundant in the 1990s in the Aleutians and in the 1960s in Southeast Alaska, but appear to have little effect when sea lion numbers were high.