Markus Horning spent the summer of 1998 in the remote Aleutian Islands, conducting research aboard the research ship M/V Tiglax. He spent many late nights pondering the absence of scientific data behind the steep decline of the region’s Steller sea lion populations.
“We needed data from the animals that had disappeared,” says Horning, now based at Oregon State University. “The experiments at the time focused on the animals that were healthy and accessible on land, not the ones that were sick or dying at sea. We rarely saw dead animals on rookeries or haulouts, where most sea lion studies were based.” This led to biased sampling, he explains, which he believes led to skewed scientific advice on the overall health of the population.
Horning’s experiences aboard the Tiglax inspired him to spend the next several years developing a Life History Transmitter that could be harmlessly carried by young sea lions and which would monitor them throughout their life. Most importantly, it would collect valuable information at the end of their lives about when, where and why they died.
Pattern of Predation
In 2012, Horning and co-author Jo-Ann Mellish (Alaska Sea life Centre) published in the journal PloS ONE an analysis of life history data they had collected from 2005-2011. Over that time, 16 of the 36 tagged sea lions had died, which was not a surprise to Horning. What did surprise him was the way in which most had died. “In 14 of 16 cases, the animals were killed by a predator, such as a transient killer whale or a large shark,” Horning explains. “Steller sea lions have a hard life, and many die young—fewer than half make it to reproductive age. But based on our findings, almost all of that mortality may be due to predation rather than starvation, disease, trampling, or competition. We believe none of those factors are nearly as impactful as predation.”
Horning and Mellish’s study challenges a highly cited 2007 study by Holmes et al., who created a demographic model and suggested the decline was largely due to a drop in birth rates. Horning says that model, which was “broadly embraced as the conceptual centerpiece of policy and management,” largely ignored the impacts of predation. In response, Horning and Mellish devised a density-dependent conceptual framework to consider the role of predation in population dynamics.
Conceptual Thinking
“Our framework is really a thinking exercise, a what-if scenario that we play through,” he says. “We’re not trying to work with numbers, we’re simply asking questions like, Is it possible that certain demographic information is driven by predation and not just by the decline in birth rates? In this case, yes, it’s possible that these things are linked, which highlights knowledge gaps and gives us an idea of what to look for.”
How a Life History Transmitter Works
From 2005-11, Horning and Mellish implanted two small Life History Transmitters in 16 juvenile Steller sea lions in a surgical suite at the Alaska SeaLife Center. The second tag was a fail-safe in case the first tag stopped working properly.
Each tag constantly records the surrounding temperature, the amount of light, and the types of tissue surrounding the tag. These and other data are uploaded to a satellite whenever the sea lion surfaces.
When the host animal dies, the tag records a change in the surrounding temperature. In a violent (predatory) death, the tag is instantly liberated as the animal is dismembered, and the tag records a sudden drop in temperature and an increase in light levels.
In a non-violent death, the tag usually stays inside the body and records a gradual cooling. As the body decomposes or is scavenged, the tag is released and floats to the surface where it senses light. “Plugging this information into our conceptual framework, one impact of predation may be to crimp recruitment, or the number of young animals who eventually reach reproductive age,” Horning says. “This leads to fewer adult females in a population who are capable of giving birth. This alone could easily make it look as if the birth rate has gone down.”According to the data collected by Horning and Mellish’s life history transmitters, predation rates on Steller sea lions were higher in the first year after weaning, and declined after that. Predators seemed to take young sea lions more often than older ones.
Even if birth rate was constant, Horning says predation could drive the decline of a population because of density dependence—an ecological concept that states the behavior of an animal may change as the density of its predators or prey changes. This concept allowed Horning and Mellish to explore a number of hypothetical ecological scenarios.
“Imagine if 1,000 transient killer whales ate 5,000 Steller sea lions per year,” Horning explains. “This wouldn’t have much of an impact in a population of 200,000 Steller sea lions. But if the sea lion population declines to 20,000 and the predators’ behavior doesn’t change, those 5,000 Stellers eaten each year now have a huge impact on population trajectory. However, because it is more difficult to find prey in a smaller population, the killer whales would likely start looking for something else. These are the types of questions we ask in a density-dependent framework.”
Advice for Managers
Horning and Mellish reject the recent hypothesis that juvenile survival rates have recovered while birth rates have declined. Instead, they suggest that predation on juvenile sea lions is the single largest impediment to the recovery of the species in the eastern Gulf of Alaska, and they call for demographic models that use age-structured census data, so the effects of predation on different age classes can be understood.
Horning cautions against extrapolating the results of this study too widely, noting: “Our model is not talking about what triggered the decline of Steller sea lions. We’re talking about what is driving the population trajectory right now in the eastern Gulf of Alaska, in the context of this thinking exercise.”
He does hope the density-dependent conceptual framework will jump-start a languishing discussion on how to effectively manage Steller sea lion populations in the face of predation. He notes that fisheries managers often assess the health of a population by looking at birth rate, not death rate—but Horning believes this is an overly simplistic model.
“We can’t point out anything more than saying there’s a lot of predation,” Horning admits, “and the response of fisheries managers could be, so what? But our conceptual model helps to consider the potential impacts of changes in predation, recruitment and the birth rate. For example, our model suggests that simply increasing the birth rate won’t necessarily overcome the impacts of predation—only reducing predation will help to restore a balance.”
PUBLICATION
[webref_pretty title=”Predation on an upper trophic marine predator, the Steller sea lion: evaluating high juvenile mortality in a density dependent conceptual framework”]