Threats to Steller Sea Lions
Steller sea lions are exposed to a variety of human-caused and natural threats.
Sea Level Rise and Ocean Acidification
Sea level rise caused by climate change will directly affect terrestrial rookery and haulout sites currently used by Steller sea lions as well as those that may be used by a recovering population. This may result in more deaths among small pups, and traditional sites on some islands with low relief may be submerged. Decadal scale regime shifts, and shorter change oceanographic anomalies (e.g., El Niños), also can have large effects on distribution and abundance of Steller sea lion prey. Ocean acidification effects on Steller sea lions are uncertain but are likely to include serious impacts on ecosystems and may have adverse effects on specific species prey through food web effects.
Disease and Parasites
Steller sea lions are exposed to a variety of diseases and parasites. Adult females and pups are likely the age-classes most vulnerable to disease and parasitism, this threat occurs often, and there is a medium level of uncertainty associated with our conclusion about the level of threat. Therefore, we believe infectious disease and parasitism have a relatively low impact on the recovery of the Steller sea lion’s western distinct population segment. However, climate-change-related shifts in distribution of other species may expose Steller sea lions to novel disease vectors or parasites that could have large-scale impacts. NOAA Fisheries works with a variety of scientists and veterinarians to monitor disease in animals that are found dead and to sample live animals.
Incidental Take Due to Interactions with Active Fishing Gear
Historically, commercial fishing in the Bering Sea and the North Pacific Ocean killed many Steller sea lions incidentally. Changes in fishing techniques and areas and times fished are thought to have significantly reduced incidental take. Amendments to the Marine Mammal Protection Act in 1988 and 1994 required observer programs to monitor marine mammal incidental take in some domestic fisheries and NOAA Fisheries provides estimates of mortality and serious injury of Steller sea lions due to US commercial fisheries in marine mammal stock assessments for the western and eastern DPSs. These estimates are likely an underestimate of the actual levels, since large segments of the fishing industry, (including fisheries near Steller sea lion rookeries, haulouts, and feeding areas, and fisheries with known past interaction with sea lions) have either no or very limited observer coverage. Thus, we are highly uncertain about the size of this threat.
The relative impact of toxic substances on the recovery of the western DPS is thought to be medium, with a medium feasibility of mitigation. Contaminants that might harm Steller sea lions enter ocean waters from many sources, such as oil and gas activity (tankering and pipeline transport, production, etc.), vessel accidents and sinkings (leading to loss of cargo, fuel, etc.), local industrial development, atmospheric transport, wastewater discharges, at sea processing, runoff, toxic waste sites, nuclear testing, industrial accidents, and natural sources. Once in the environment, some contaminants move up the food chain and accumulate in top predators. Certain pollutants that Steller sea lions are exposed to have been shown to damage the immune and reproductive systems of other mammals and to negatively impact their health and survival. Of particular concern now is information showing that mercury contaminant loads of some western DPS Steller sea lion pups are relatively high in some parts of the range where declines are still occurring. Exposure to organochlorines known to affect health impacts in other species have been found in parts of the western and eastern DPSs, though impacts on Steller sea lions are not clear. Lastly, we have concerns about emerging environmental contaminants, the PBDEs, which are known to bioaccumulate in marine mammals. However, we have little understanding of Steller sea lions’ exposure to these compounds.
Historical accounts document substantial mortality within the western DPS due to illegal shooting, especially associated with fisheries. While there is likely much less shooting than during the period of sharp decline in the 1970s and 1980s, recent documented increases in the Copper River Delta area, as well as fishermen’s reports of greater interactions with fisheries leading to increased anger towards the species, are cause for concern. Most parts of the range of the western DPS are not systematically monitored for carcasses, so there are no current studies on evaluating levels of illegal shooting; large parts of the fishing industry, including fisheries near Steller sea lion rookeries, haulouts, and feeding areas, have either no or very limited observer coverage.
Disturbance of Steller sea lions on their terrestrial sites can lead to individuals fleeing toward the water and to mass stampedes during which pups and other smaller animals may be crushed or injured by larger ones. It can lead to interruption of important behaviors such as nursing, resting, breeding, territory holding, and socializing. Repeated disturbances that cause lactating females to abandon rookeries or use them less could damage the health and survival of pups by threatening normal nursing cycles. Sea lions may be disturbed by vessels approaching from the water, by aircraft, and by approach from the land. Their response is often highly unpredictable. The sexual dimorphism of Steller sea lions, and the fact that they may climb up on rocks or cliff faces on some sites, both increase their risk of injury or death if they are disturbed. Sea lion feeding can also be disturbed by vessel traffic or underwater noise.