Chiswell Chronicles: June 26,2017

Up to 74 births now on Chiswell!

As the pups grow older, females are starting to come and go from the island as they start foraging. Steller sea lions are ‘income breeders’, meaning they give birth, stay with and nurse their pup for a short perinatal period (~10 days on Chiswell), and then alternate going to sea to find food and coming back to nurse their pups. As we mentioned last week, this typically lasts for 1 year until the pup is weaned, but in some cases, nursing can last for up to 3 years!

A female returning from the sea to nurse her pup.


From some of the early research done on Chiswell (2001-2004) we found that the average duration of foraging trips were fairly short at the start of the summer (16.5 hours) and increased in duration through August (Figure 1). By fall, females spent on average 2.3 days at sea in between returning to their pups!


Figure 1: Average duration of foraging trips for all female Steller sea lions and all years combined in 5-day increments. From: Interseasonal and Interannual Measures of Maternal Care Among Individual Steller Sea Lions (Eumetopias jubatus). J Mammal. 2006; 87(2):304-311. doi: 10.1644/05-MAMM-A-163R2.1 J Mammal | (c) 2006 American Society of Mammalogists


Monitoring the duration of these trips can be important for understanding any changes to the availability of prey resources. In other systems, longer foraging trips have been linked to low food availability, but the durations observed at Chiswell were similar to those seen in other areas of the SSL range where populations are healthy and stable: suggesting females in that study were not food limited in summer months. You can read more about these findings in Maniscalco et al. 2006.

How Steller sea lions balance maternal care and foraging is very different to what we see with seals. Most seals are considered ‘capital breeders’: meaning they give birth, nurse their pup for 4-30 days (depending on the species) and then go back to sea– leaving the pup fully weaned and on its own. For capital breeders, the whole time a female is with her pup she will not forage, and instead relies on the blubber resources she built up before giving birth for energy.

A grey seal mom and new born pup. After 16-18 days, the female will wean the pup and return to sea to forage. (c) A. Bishop

Courtney Shuert, a former ASLC intern and research student, is currently in the UK studying grey seals for her PhD research. She is investigating how capital breeding mums partition their finite amount of energy into different activities. To learn more about grey seal breeding and her work check out their blog: “Studying Seals” and some posts on nursingpup growth, and energetics!



To continue with the theme of coming and going and foraging: this week we are highlighting Robbie. We first saw her in 2006. Robbie’s best identifying mark is a slight split in her left fore-flipper.

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Ref: Maniscalco, J. M., Parker, P., & Atkinson, S. (2006). Interseasonal and interannual measures of maternal care among individual Steller sea lions (Eumetopias jubatus). Journal of Mammalogy87(2), 304-311.

All photographs and activities conducted under NMFS Permit No. 18438-00

Written by: Dr. Amy Bishop and Pam Parker

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