Paper in a Nutshell: Bishop, Dubel, Sattler, Brown, Horning. 2019. Wanted dead or alive: characterizing likelihood of juvenile Steller sea lion predation from diving and space use patterns. Endangered Species Research.
Endangered species is a topic that we can all relate to. Whether it is a Blanding’s turtle (Emydoidea blandingii) that is living in your local pond, an Andean mountain cat (Leopardus jacobita) roaming the mountains, or a Blue whale (Balaenoptera musculus) that is crossing the ocean, we can probably all name one or more endangered species. Some of the biggest threats to endangered species are habitat loss, road accidents, invasive species, pollution, and disease. Even the smallest change in the population size of an endangered species can make a huge difference. This is why it is so important for researchers to know why certain age classes or individuals are more vulnerable. Are they older? Weaker? What about their behaviors? These questions can provide vital solutions on how to manage and protect endangered species.
It is already known that often younger members of a species are more vulnerable to predation threats. More recently, scientists have started looking at how behavior can influence the likelihood of survival. This concept has been investigated in laboratory and terrestrial settings, but there is a clear lack of data on this information in marine ecosystems. Considering the vast depths and lengths of the ocean it is a rare occurrence that you will witness a mortality event in the ocean never mind be able to locate and track the animals that you are interested in researching.
In this recently published study, ALSC scientists aimed to combine observations of mortalities and individual behaviors in order to understand the susceptibility to predation of juvenile Steller sea lions. The western population of Steller sea lions has been on the endangered species list since 1997. We know from previous studies that telemetry data shows that 91.7% of juvenile mortalities can be attributed to predation (Horning & Mellish 2009, 2012). So, that team—along with some new postdocs and interns—decided to investigate if behavioral patterns of juvenile Steller sea lions are related to their predation levels.
Between 2005 and 2014, 45 Steller sea lions from Resurrection Bay and Prince William Sound were brought to the Alaska SeaLife Center for a period of temporary captivity. While in captivity the animals were implanted with life history transmitters (LHX tags) and monitored carefully after the implantation. Before the Steller sea lions were released back into the wild, we also equipped them with satellite tracking tags to monitor their behavior over the following months. This new approach of being equipped with the combination LHX and satellite tags allowed ASLC researchers to link the survival of the animal, to the behavior of the animal.
LHX tag (left) and Wildlife computer tag (right).
By 2018, 20 mortalities had occurred. Of those 20 mortalities, 2 of them had insufficient data to determine the cause of death and 18 of those deaths were determined to be by predation. This information allowed researchers to look at differences in diving and horizontal movement data between the sea lions that died, relative to the ones that survived.
Results of whether diving behavior was related to probability of predation were mixed. There was some evidence in this study that suggests individuals that spent more time hauled out, and less time in the water, had a greater chance of mortality. Since Steller sea lions don’t have many (or any) land predators…why were we seeing this pattern? It could be that this pattern reflects that sea lions hauling out on land may have been at risk from specialist predators that are known to target sea lion rookies and haul out spots such as killer whales (Orcinus orca).
We also know that Pacific Sleeper sharks (Somniosus pacificus) are a predator of Steller sea lions. Therefore, one of our hypotheses was that sea lions that dove deeper on average would be more susceptible to predation by the Pacific Sleeper sharks. However, there was no evidence of that shown in the data.
Together, we know that both of these two predators are a threat to juvenile Steller sea lions, so it could be that the small sample size limited our ability to differentiate more patterns, but the initial findings are helpful for understanding the many connections in marine ecosystems.
In this study there were 13 predation events where the specific predator could not be determined. Future research would benefit by developing ways to determine how to differentiate one predator from another. For example, the development of advanced algorithms that can integrate information from multiple tags or sensors could possibly help to determine the species of the predator in a specific mortality. For example, to better understand the role of generalist predators, the Alaska SeaLife Center is actively studying the Pacific Sleeper sharks.
Other literature suggests that resource availability and predation can influence foraging behaviors and the overall health of Steller sea lions. In this study, the majority of mortality events occurred in juvenile Steller sea lions, possibly before the juveniles could learn and develop specific foraging strategies. Is it possible that juvenile sea lions that “settled” into their adult strategy earlier were the ones that were safe? There is a lack of current knowledge on the development of behavioral strategies in Steller sea lions when transitioning from juveniles to adults and that is another area for future research to explore.
Future studies can also aim to investigate how long-term behavior of Steller sea lions relate to trade-offs between resources and predation, providing insights into the vulnerability within endangered populations.
Finally: We want to make a big shout out to Ally Dubel, the summer intern in 2017 who analyzed much of this data and played a big role in getting this work published!!
Written by: Mary Keenan, ASLC Science Communication Intern 2020
Feature Image Credit: NOAA Fisheries
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Thank you for sharing the story of the research. It is a mystery worth pursuing.