It’s all too common to hear about marine mammals being driven towards extinction thanks to the actions of humans. But humans are not the only member of the natural world that can impact on the survival of another species. A new study has discovered that predation of stellar sea-lions is causing the species problems.
Over the last 40 years the number of Steller sea-lions have crashed by about 80%. The study by Oregon State University has discovered that the role of predators in the loss may have been significantly underestimated. [pullquote]Unless predation is lessened, it appears they are in a productivity pit.[/pullquote]
Steller sea-lions an important prey species.
Steller sea-lions are an important prey species for large marine predators such as killer whales and sharks such as the pacific sleeper sharks or salmon sharks. The new research suggests that these top predators are targeting younger or juvenile sea-lions on their hunts. This results in fewer females making it to breeding age and a subsequent reduction in birth rates. As fewer sea-lions are born and prey becomes scarcer so the predators target younger sea-lions so leading to even fewer females making it through to breeding age.
“It is generally accepted that most pinniped populations suffer from high attrition in the juvenile years, but this study suggests that predation accounts for most, if not all of this attrition in the case of Steller sea lions,” said Markus Horning, an Oregon State University marine mammal expert and lead author on the study.
“The focus of predators on juveniles has the end result of heavily capping female recruitment – or the number of females that survive until they are old enough to have pups,” Horning added.
6 year project looked a sea lion predation rates.
The research team included members from the Oregon State University, Coastal Oregon Marine Experiment Station, Alaska SeaLife Center and the University of Alaska Fairbanks. The study looked at the fates of 36 juveniles found in the Gulf of Alaska over a 6 year period (2005 – 2011).
The team used ‘whole life’ transmitters to carry out their study. These are special transmitters that are implanted in to the sea-lions abdomen where they record a range of data including temperature and light. The transmitters remain in the body of the sea-lion until they die when the tag eventually gets released as the body decomposes, breaks up or passes through the digestive system of predators. Once released the transmitters float to the surface or get washed ashore and they transit their date.
“The transmitters are amazing recorders of the life history of the animals, and can tell us in most cases how they died,” Horning said. “Gradual cooling and delayed extrusion are signs of a non-traumatic death, say disease or starvation, or of entanglement, drowning or shooting. When the sensors record precipitous drops in ambient temperature along with immediate sensing of light and the onset of data transmission, it is indicative of acute death by massive trauma – usually associated with dismemberment by predators.”
11 of 12 sea lion deaths caused by predators.
Over the course of the 6 years of the study 12 of the 36 tagged sea-lions died. 11 of the 12 died from predation. Knowing the death rates from predation allowed the research to run population models on the Steller sea-lions and the conclusion was not good. With the current rate of predation and number of predators in the area the Steller sea-lions of the Gulf of Alaska would not be able to survive over the long term.
Other studies have shown that a killer whale needs to consume 2 or 3 sea-lions pups a day or an equivalent 1 adult female to meet its calorific needs. As adult sea-lions reduce in number the predators will take more and more pups leading to an ever decreasing circle.
“Young sea lions spend more time close to shore and the haul-outs where they are suckled by their mothers,” said Horning, an associate professor of fisheries and wildlife at OSU. “They can be found more predictably by predators than can older animals and adult males.”
“As the density of more ‘profitable’ adults declines, more juveniles may be targeted and never grow to adulthood, which makes rebuilding their populations problematic,” Horning added. “Unless predation is lessened, it appears they are in a productivity pit.”
Too few juvenile sea-lions becoming old enough to breed.
The risk to the future of the Steller sea-lion is demonstrated by comparing the figures of when the sea-lions were much more common. 40 years ago the predation of sea-lions composed of pups comprising 7 percent of all predation events, juveniles 46 percent, and adults 47 percent. But when overall populations decline to a level of 20 percent (which is the current level for the western stock), pups comprise 23 percent of the mortality, juveniles 72 percent, and adults just 5 percent.
- PlosOne: Predation on an Upper Trophic Marine Predator, the Steller Sea Lion: Evaluating High Juvenile Mortality in a Density Dependent Conceptual Framework.
- Oregon Stste University.
- Alaska SeaLife Center.
- University of Alaska Fairbanks.