Sea Change: Ocean Climate and the Shifting Fortunes of Alaska's Steller Sea Lions

Nearly 30 years ago, an abrupt change in ocean conditions swept through the North Pacific Ocean, affecting everything from sea-surface temperatures to fisheries. The so-called regime shift of 1976-77 was a natural event in the ocean's climate cycle, but its impacts on Alaska's marine ecosystems are still felt today.

According to a recent study by a team of 30 leading scientists, this single climate event may be the missing link that ties together the various theories behind the decline in western Alaska's Steller sea lion populations, and the curious success of populations to the east.

"The sharp decline of the larger western population through the 1980s was mirrored by population growth in the smaller eastern populations in Southeast Alaska, British Columbia and Oregon," write the authors, led by Dr. Art Miller of the Scripps Institution of Oceanography, Dr. Andrew Trites of the Marine Mammal Research Unit at the University of British Columbia, and Dr. Herb Maschner at Idaho State University. "Much of the search for why Steller sea lions declined in western Alaska has focused on trying to identify a single cause for the changes, rather than recognizing that many of the proposed theories are inter-related."

How can a change in ocean climate unify these seemingly unrelated theories of epidemic disease, killer whale predation and shifts in prey abundance? To find out, the researchers considered sea lion diets, examined the region's physical oceanography and marine ecosystems, and combed through archaeological data spanning 5000 years. In the process, they unravelled a tale of shifting fortunes with not one, but two endings.

Figure 1. Conceptual model showing how sea lion numbers might be affected by ocean climate through bottom-up processes. This hypothesis suggests that water temperatures, ocean currents and other climatic factors determine the relative abundances of fish available to eat, which in turn affects sea lion health (proportion of body fat, rates of growth and at a cellular level – oxidative stress). These three primary measures of individual health ultimately determine pregnancy rates, birth rates, and death rates (through disease and predation). Also shown are the effects of human activities that could have directly or indirectly affected sea lion numbers.

Dietary Dilemma
The nutritional stress hypothesis – also referred to as the junk food hypothesis – is a leading 'best guess' at why western Alaska's sea lions have declined. Simply put, it suggests that sea lions in declining populations shifted to a diet of low-energy prey following the regime shift. But in areas where populations increased, the sea lions consumed a higher diversity of prey and enjoyed a more energy-rich diet.

"Shifting from a high-energy diet (dominated by fatty fishes) to one dominated by lower-energy fish… may have significantly affected young sea lions by increasing the amount of food they would have to consume to meet their daily energy needs," the authors note.
In other words, a young sea lion cannot hold and process enough low-quality food in its small stomach to satisfy its hunger, leaving it weaker and undernourished. Nutritional stress from low-quality prey also affects the reproductive health of adults, translating to lower birth rates.

Figure 2. Diets, population trends and sizes for Steller sea lions at 33 rookeries in Alaska during the 1990s. Estimated numbers of sea lions in 2002 (N) and population growth rates (λ) were determined from linear regressions of log-transformed counts of pups and non-pups conducted from 1990-2002. Trend and numbers were calculated independently for pups and non-pups, and then averaged. Average population trends ranged between 0.85 and 1.05, with values <1 indicating declines. Bar heights are proportional to the maximum average N and λ, with solid vertical lines denoting distinct geographic shifts in population sizes and trends. Diet data for summer (S) and winter (W) are from Sinclair and Zeppelin (1998), with circles representing the split-sample frequencies of occurrence of prey (proportional to area of circle). The frequencies sum to 100% and were calculated for the 9 principal species shown as well as for less important species not shown (grouped into 6 prey types: flatfish, forage fish, gadids, hexagrammids, other, rockfish). The zig-zag lines indicate seasonal geographic changes in diet.

While a change in ocean conditions could indeed force an entirely new diet upon sea lions, the nutritional stress hypothesis does not explain how a regime shift affecting the entire Gulf of Alaska could produce different responses in the eastern and western portions of the basin. In order to answer that question, the researchers had to delve deeper into the Gulf's physical composition.

The Dividing Line
Samalga Pass (169°W), just west of Unmak Island on the Aleutian chain, was an unassuming geographical feature until field research in the early 21st century uncovered its significance: Samalga Pass is a transition point between the eastern waters of the continental shelf and the western waters of the open ocean. The shelf waters east of the pass are governed by the Alaska Coastal Current, while the open waters further west are at the mercy of the Alaskan Stream.

The Alaskan Stream is the continuation of the Alaska Current (a relatively warm, counter-clockwise gyre in the Gulf of Alaska) as it moves west along the southern side of the Aleutian Islands. Mathematical models of sea-surface temperatures (SST) – the most complete set of oceanographic data available – show a strengthening of the Alaskan Stream following the 1976-77 regime shift.

The strengthening of the Alaskan Stream was likely due to a shift in atmospheric circulation patterns over the Aleutian Islands, and may have impacted large ocean eddies that transport nutrients across the Aleutians. While these mechanisms are not fully understood, they suggest that the stronger Alaskan Stream could have fundamentally altered the distribution of energy through the waters of western Alaska, resulting in an increased abundance of low-quality prey such as cod and pollock.

Conversely, in the eastern Gulf of Alaska, the models show nearly unchanged flows in the Alaska Current after the regime shift. The fundamental difference in source waters between the eastern and western portions of the basin – and the degree to which the strengthened Aleutian Low impacted the western waters – set the stage for a dramatic contrast in habitat on either side of Samalga Pass. Sea lions were among the many winners and losers of this ecological shakedown.

Historical Perspective
While eastern populations of sea lions may appear to have avoided the climate-induced poverty of their western counterparts, archaeological data suggests that their good fortune may be temporary. Sea lion populations have varied significantly in Alaska over the past 5000 years, and historic accounts tell of at least one collapse within the last few centuries. Hunting and fishing did not significantly contribute to any of the historical declines.

Figure 3. Long term trends in the percentage of Steller Sea lions harvested by Alaska Peninsula Aleut in relation to their total sea mammal harvest. The Aleut harvested resources in proportion to their actual distribution on the landscape. This chart shows the percentage difference from the mean harvest over the last 4000 years.

"The North Pacific and southern Bering Sea have been dynamic and volatile, and subject to great fluctuations over the last hundreds to thousands of years," the authors write. "The results also provide additional evidence that climate may very well underpin ecosystem restructurings that can be manifested as large, regional changes in Steller sea lion abundance."

The enigmatic 'black box' of ocean climate perhaps more closely resembles a Pandora's Box: a cascade of ecosystem impacts triggered by key events such as the 1976-77 regime shift. By fundamentally reconfiguring the marine environment, these episodes of sea change act as harbingers of feast or famine for sea lions.

One event, one ocean, two outcomes: portions of western Alaska's Steller sea lions face extinction while eastern populations thrive. Each is at the mercy of the ocean's changing fortunes, and whether their fates will reverse is not so much a question of 'if', but 'when'.

5 March 2007

Publication:
Bottom-up forcing and the decline of Steller sea lions (Eumetopias jubatus) in Alaska: assessing the ocean climate hypothesis. Trites, A. W., A. J. Miller, H. D. G. Maschner, M. A. Alexander, S. J. Bograd, J. A. Calder, A. Capotondi, K. O. Coyle, E. D. Lorenzo, B. P. Finney, E. J. Gregr, C. E. Grosch, S. R. Hare, G. L. Hunt, J. Jahncke, N. B. Kachel, H.-J. Kim, C. Ladd, N. J. Mantua, C. Marzban, W. Maslowski, R. Mendelssohn, D. J. Neilson, S. R. Okkonen, J. E. Overland, K. L. Reedy-Maschner, T. C. Royer, F. B. Schwing, J. X. L. Wang and A. J. Winship. 2007. Fisheries Oceanography 16:46-67. abstract Declines of Steller sea lion (Eumetopias jubatus) populations in the Aleutian Islands and Gulf of Alaska could be a consequence of physical oceanographic changes associated with the 1976-77 climate regime shift. Changes in ocean climate are hypothesized to have affected the quantity, quality and accessibility of prey, which in turn may have affected the rates of birth and death of sea lions. Recent studies of the spatial and temporal variations in the ocean climate system of the North Pacific support this hypothesis. Ocean climate changes appear to have created adaptive opportunities for various species that are preyed upon by Steller sea lions at mid-trophic levels. The east-west asymmetry of the oceanic response to climate forcing after 1976-77 is consistent with both the temporal aspect (populations decreased after the late 1970's) and the spatial aspect of the decline (western, but not eastern, sea lion populations decreased). These broad-scale climate variations appear to be modulated by regionally sensitive biogeographic structures along the Aleutian Islands and Gulf of Alaska, which include a transition point from coastal to open-ocean conditions at Samalga Pass westward along the Aleutian Islands. These transition points delineate distinct clusterings of different combinations of prey species, which are in turn correlated with differential population sizes and trajectories of Steller sea lions. Archaeological records spanning 4000 years further indicate that sea lion populations have experienced major shifts in abundance in the past. Shifts in ocean climate are the most parsimonious underlying explanation for the broad suite of ecosystem changes that have been observed in the North Pacific Ocean in recent decades.