In cold blood: evidence of Pacific sleeper shark (Somniosus pacificus) predation on Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska (original) (raw)
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Abundances of marine mammal populations changed dramatically in the Bering Sea and Gulf of Alaska over the past century. Some of these changes are attributed to known ecological or human-caused events, but considerable uncertainty surrounds others, such as the decline of the western stock of Steller sea lions in the 1970s and 1980s. In the case of sea lions, one factor that has been identified but poorly studied is mortality from shooting by fishermen and others. Documenting information about the nature and extent of such shooting, together with its spatial and temporal characteristics, may provide valuable insights into the causes of the decline and the potential for recovery. In addition, commercial fishermen and regional residents are likely to have a great deal of knowledge about historical ecological events and conditions, including climate regime shifts, which have not been documented to date. Ecological knowledge is particularly sparse prior to routine fish stock assessment surveys in the 1970s (Bering Sea) and 1980s (Gulf of Alaska). Interviews that we conducted with selected fishermen and regional residents in the Kodiak, Cold Bay/Sand Point, and Seattle areas have added much information to our knowledge base. A preliminary reconstruction of mortality by fishery based on these interviews suggests that, apart from the Shelikof Strait trawl fishery in the 1980s, shooting practices do not appear to have changed appreciably from the 1950s to the 1970s. Thus, shooting mortality appears to be unable to account for a major portion of the unexplained mortality during 1974-1990. We are currently in the final stage of our project in which we are analyzing fishery data in combination with our newly acquired information on shooting to more fully investigate hypotheses about shooting-related mortality as a potential leading cause of the decline of the western stock of Steller sea lions. The final stage of our analysis was held up by a lengthy delay in obtaining data from the Alaska Department of Fish and Game, which experienced a loss of staff and a crash of the computer system containing the landings database. However, as of mid January 2009, we are in receipt of all requested data and we will complete our project and Final Report in the coming months. As our conclusions are pending the completion of these our analyses, we request that our work should be considered preliminary and not cited.
1 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 The western Gulf of Alaska Steller sea lion (Eumetopias jubatus) population has declined dramatically in recent decades, resulting in their addition to the Endangered Species List in 1997. The reasons for the decline immediately became politicized and debated. Broad and sweeping restrictions on North Pacific fisheries became the favored solutions of policymakers and environmental groups. Aleut fishermen, who were fated to lose large portions of their traditional fishing grounds, were left in a reactionary mode, having never been included in the regulatory or recovery processes. Here we consider the wealth of knowledge of the North Pacific ecosystem that Aleut fishermen and their heritage possess, which places humans squarely upon the landscape, and argue that the inclusion of their traditional knowledge and history is crucial to understanding variations in the North Pacific ecosystem. We show that the Steller sea lion has gone through at least three major declines in the last 1000 years and that these collapses are closely tied to reported regime shifts in the north Pacific ecosystem. The result is a refutation of the direct role of humans in the modern collapse of the Steller sea lion population and extensive support for hypotheses encompassing climate change and regime shifts. But the relationships between people and SSLs are complicated by historical factors reaching back hundreds, if not thousands of years.
PLoS ONE, 2012
The endangered western stock of the Steller sea lion (Eumetopias jubatus) -the largest of the eared seals -has declined by 80% from population levels encountered four decades ago. Current overall trends from the Gulf of Alaska to the Aleutian Islands appear neutral with strong regional heterogeneities. A published inferential model has been used to hypothesize a continuous decline in natality and depressed juvenile survival during the height of the decline in the mid-late 1980's, followed by the recent recovery of juvenile survival to pre-decline rates. However, these hypotheses have not been tested by direct means, and causes underlying past and present population trajectories remain unresolved and controversial. We determined post-weaning juvenile survival and causes of mortality using data received post-mortem via satellite from telemetry transmitters implanted into 36 juvenile Steller sea lions from 2005 through 2011. Data show high post-weaning mortality by predation in the eastern Gulf of Alaska region. To evaluate the impact of such high levels of predation, we developed a conceptual framework to integrate density dependent with density independent effects on vital rates and population trajectories. Our data and model do not support the hypothesized recent recovery of juvenile survival rates and reduced natality. Instead, our data demonstrate continued low juvenile survival in the Prince William Sound and Kenai Fjords region of the Gulf of Alaska. Our results on contemporary predation rates combined with the density dependent conceptual framework suggest predation on juvenile sea lions as the largest impediment to recovery of the species in the eastern Gulf of Alaska region. The framework also highlights the necessity for demographic models based on age-structured census data to incorporate the differential impact of predation on multiple vital rates.
Aquatic Mammals, 2003
The discovery of ipper tags from 14 Steller sea lions (Eumetopias jubatus) in the stomach of a dead killer whale (Orcinus orca) in 1992 focused attention on the possible role of killer whale predation in the decline of Steller sea lions in western Alaska. In this study, mariners in British Columbia and Alaska were surveyed to determine the frequency and outcome of observed attacks on sea lions, the age classes of sea lions taken, and the areas where predatory attacks occurred. The 126 survey respondents described 492 killer whale/sea lion interactions, of which at least 32 were fatal attacks on the sea lion. The greatest rate of observed predation occurred in the Aleutian Islands. The stomach contents of dead and stranded whales also were examined. Stomachs that were not empty contained only sh or marine mammal remains, but not both. This supports earlier evidence of dietary segregation between sh-eating resident and marine mammal-eating transient killer whales in Alaska. Steller sea lion remains were found in two of 12 killer whale stomachs examined from Alaska between 1990 and 2001. Stomach contents from two oVshore killer whales provided the rst direct evidence that this third form of killer whale feeds on sh.
Aquatic Mammals, 2010
Pinnipeds prey primarily on fish and invertebrates; however, several species are known to prey upon other pinniped species. Herein, we document an observation of a Steller sea lion (Eumetopias jubatus) attacking and partially consuming a juvenile harbor seal (Phoca vitulina richardii) in Johns Hopkins Inlet in Glacier Bay National Park, Alaska. Population trends for Steller sea lions and harbor seals contrast dramatically in the Glacier Bay region. Although other marine predators are known to attack harbor seals, it is possible that Steller sea lions could potentially have both a direct and indirect influence on harbor seals in the Glacier Bay region.
The behavioral and predatory patterns of Gulf of Alaska (GOA) transient killer whales (Orcinus orca) were studied between 2000 and 2005 using remote video and vessel-based observations near the Chiswell Island Steller sea lion (Eumetopias jubatus) rookery and in the broader Kenai Fjords (KF) region of the northern GOA. GOA transient killer whales were observed on 118 d over the 6-yr period; the median group size was two (range: 1-9). Nine predation events were observed from vessels and an additional sixteen were inferred from remote video studies; all involved Steller sea lions. Estimates from field observations suggest that fifty-nine sea lions were consumed over the summer seasons of 2002-2005; whereas estimates based on published caloric requirements of transient killer whales would suggest a loss of 103 sea lions over the same time period. GOA transients spent a large proportion (43%) of their time resting which may be a strategy for conserving energy. Predation 1 Current address: Earthwatch Institute, 3 Clock Tower Place, P. O. Box 75, Maynard, Massachusetts 01754, U.S.A. 306 MANISCALCO ET AL.: KILLER WHALE PREDATION ON STELLER SEA LIONS 307
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 eastwest 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.
Behavioral Insights into the Decline and Natural History of Steller Sea Lions
Ethology and Behavioral Ecology of Otariids and the Odobenid, 2021
Two competing hypotheses were proposed to explain why Steller sea lions had declined in the Gulf of Alaska, Bering Sea and Aleutian Islands. One of the theories was that young sea lions were starving because fisheries had reduced the abundance of groundfish—the overfishing hypothesis. The other was that these low-fat species of fish had increased in abundance as the sea lion population declined following the 1976–1977 oceanic regime shift, and were compromising sea lion reproductive and survival rates—the junk-food hypothesis. Behavioral ecologists tested these hypotheses by comparing sea lion behaviors in the declining region (Gulf of Alaska and Aleutian Islands) with sea lion behaviors in an increasing region (Southeast Alaska) to determine whether the populations exhibited behavioral differences consistent with food shortages. These studies involved comparing dive depths, dive durations, time spent foraging, and time spent nursing by regions and seasons. Research also focused on ...