For more than 30 years, certain species of marine mammals and seabirds have been declining in Alaska’s marine ecosystems. Some populations in Alaska have reached such critical numbers that they have now been listed as threatened or endangered under the Endangered Species Act. These include Steller sea lions (Eumetopias jubatus—endangered in their western stock and threatened in their eastern stock) and northern sea otters (Enhydra lutris kenyoni—threatened in the Aleutian archipelago), whose decline has been particularly alarming - over 95% of northern sea otters have disappeared in many parts of southwest Alaska!
Scientists have struggled to understand why populations of harbor seals (Phoca vitulina), northern fur seals (Callorhinus ursinus), sea otters, Steller sea lions, polar bears (Ursus maritimus), and certain cetaceans have declined so drastically. But an even more urgent question has taken precedence—why have these species not recovered from their respective population declines?
Several hypotheses emerged in the early 1990’s to explain why Steller sea lion populations declined and these continue to be relevant today. Because Steller sea lions overlap with many other marine mammal species in range, habitat, and the food they consume, it is likely that these same causes are affecting other species too. What is unknown, however, is the degree to which each factor is affecting each species.
To date, there is little evidence to support a single, overriding cause in any of these declines, with the possible exception of killer whale (Orcinus orca) predation causing the sea otter decline in southwest Alaska. As in any scientific study, it is very difficult to go back in time and collect data that would accurately represent the era when sea lions or other species were rapidly declining. But as more research and evidence mounts each year, scientists are beginning to confirm that it was most likely a combination of top-down (i.e. predation) and bottom-up (i.e. food limitation) factors that contributed to the population declines and their lack of recovery.
For over 15 years, there has been an international, collaborative, and coordinated effort to determine potential causes for decline, and to help conserve and protect these species for future generations. Some of the leading hypotheses that the Alaska SeaLife Center (ASLC) and others are investigating to explain the trends in populations of harbor seals, sea otters, northern fur seals, and Steller sea lions are described below. Certain organizations, ASLC included, have taken the lead in particular areas of research as facilities, expertise, and accessibility differ among institutions. The ASLC has structured its Sea Otter and Pinniped Programs around many of these hypotheses. Acronyms can be found at our Collaborators page and are found below in no particular order.
(Top-Down Hypothesis—ASLC, UAF, UCSC, UBC, DFOC, NMFS, ADF&G, NGOS, Center for Whale Research, FEROP, NRC)
Killer whale predation is thought to be one of the major factors affecting populations of multiple marine mammal species. It is unclear, however, to what degree transient (marine-mammal eating) killer whales preyed upon certain marine mammals during the onset of their declines, but recent evidence suggests that they may be a factor currently preventing species from recovering from their depleted status. To compound things more, transient killer whales are declining in numbers where Steller sea lion and sea otter populations are critical, and abundant where Steller sea lions are showing signs of recovery.
“Junk-Food” and Prey Quality Hypotheses
(Bottom-Up Hypotheses—UBC, NMFS, UAF, ASLC, ADF&G, NRC, U of HI, WSU, APU, UCONN)
As research has mounted, originally distinct hypotheses have merged into larger questions as in the case of the “Junk-Food Hypothesis” and Prey Quality. Scientists are exploring whether changes in prey affected Steller sea lion survivability. These changes in prey may have been in availability of prey type, inter- and intra-specific quality of prey, composition, abundance, and distribution (geographically and in the water column). The theory that perhaps a shift in the prey base in the Gulf of Alaska from fish like herring—a high lipid content fish—to a lower lipid content fish, like pollock, prompted scientists to coin the term “Junk-Food Hypothesis,” which has been one of the most heavily studied aspects to the Steller sea lion decline. Now, many different factors are being explored in prey studies including how Steller sea lions forage and digest their prey.
(Bottom-Up Hypothesis—ASLC, UBC, UAF, APU, U of HI, NRC, WSU, UCONN, ADF&G, NMFS, Aleutians East Burrough)
Every species must gain more energy than it expends, or else physiological compromises occur eventually leading to death. Stemming from the prey studies previously mentioned, it seems likely that if or when prey changes occurred, marine predators such as harbor seals and Steller sea lions could have had a hard time adjusting to that change. As a result, scientists have been investigating how sea lions and harbor seals adjust to nutritional stresses from different prey items using biomarkers such as hormones, which control many physiological processes.
Contaminants & Disease
(Top-Down Hypothesis—ASLC, U of HI, ADF&G, NMFS, MMC, UAF)
The effects of contaminants such as DDT and Polychlorinated biphenyls (PCBs) have been well documented since the 1950’s, but little was known about how these destructive chemicals impacted the physiology of pinnipeds and cetaceans. PCBs and other contaminants disrupt hormonal balances that affect growth and reproduction in certain species. Additionally, they bioaccumulate in the blubber layers of marine mammals as they cannot be digested or broken down. Efforts were then launched to identify potential contaminants and how they impact pinnipeds such as Steller sea lions and harbor seals.
Juvenile Steller sea lions were thought to be the most vulnerable age class during the onset of the population decline. Juveniles are inexperienced and therefore more likely to have difficulty foraging or finding prey and, potentially, adjusting to these nutritional stressors.
(Top-Down and Bottom-Up Hypotheses—ADF&G, MSU, NMFS, UBC, UAF)
Direct and indirect marine mammal mortality can occur due to fisheries activities. Alaska’s has the largest salmon and groundfish fishery in the U.S., and many of these same fish are also prey for marine mammals like Steller sea lions, harbor seals, sea otters, orcas, and northern fur seals. Federal and state agencies have primarily taken the lead in the studies examining the direct (i.e. incidental catch) and indirect sources of mortality associated with fisheries.
An extension of the predation hypothesis, is that after peak commercial whaling, transient killer whales did not have the Great whales (sperm and humpback whales, for example) to prey upon as populations were decimated. This prompted a shift in predation to Steller sea lions in the early 1970’s. The population decline of Steller sea lions, and the slow recovery rate of cetacean populations, caused killer whales to switch again sequentially, to the other marine mammal populations that have declined recently, harbor seals, northern fur seals, and finally, sea otters.
With shifts in the composition of the prey base and in foraging behaviors of marine mammals, is it possible that a larger ecosystem impact may be occurring in the marine ecosystems of Alaska? Scientists have discovered decadal oscillations (changes) in ocean conditions in the North Pacific Ocean that may have contributed to overall ecosystem change. Scientists are investigating how these large-scale changes in oceanic and climatic variables may affect many different marine organisms, but especially fisheries and endangered and threatened species.
On-going research on each of these hypotheses continues to provide insight every day as to how these declines may have occurred and why populations are not recovering. Science has always been predicated upon asking good questions and then finding ways to answer them. For over 20 years, studying marine mammals, especially endangered and threatened species, has helped us unravel the mysteries of complex ocean ecosystems.