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Ecology

Epic: sperm whales hunt squid

Aoki, K.; Amano, M.; Kubodera, T.; Mori, K.; Okamoto, R.; Sato, K. Visual and behavioral evidence indicates active hunting by sperm whales. Marine Ecology Progress Series 523: 233-241, 2015. doi: 10.3354/meps11141

A picture is worth a thousand words

Schutte 4.27Why we care

Since I began contributing to Oceanbites, I’ve been preaching. I’ve covered articles about conservation issues and some have had hopeful conclusions, but all start with a problem. Effective environmental conservation is extremely difficult given the necessity of human resource consumption. When I feel more aware of environmental challenges rather than the rewards of meeting those challenges, I look to nature for a pick-me-up.

So I’d like to share with you the latest article I’ve read that showcases the wonders of the natural world. Today we remind ourselves why science rocks so hard and why it’s worth it for us to work towards environmental protection.

4.27.1

One of the tags (inside the white circle in this image) deployed on a sperm whale.

This article elucidates how sperm whales capture giant squid, which we actually know little about. Giant squid are deep ocean animals and sperm whales are fantastic divers, so sperm whale hunting behavior can’t be observed without some serious planning. Sperm whales primarily eat squid and the earliest theories hypothesized that they used a sit-and-wait strategy rather than expending more energy on prey pursuit. However, more recent studies using accelerometers, sound recording devices, and speed sensors suggest that sperm whales actively pursue prey on deep dives. Prior to this study, there were no images available to be paired with any data suggestive of active hunting.

The authors of this paper attached cameras and accelerometers to sperm whales, then analyzed the images and diving data to determine how the world’s largest predator successfully captures and eats the world’s largest invertebrate.

Methods

The authors used long poles to attach suction cup tags to sperm whales around the Ogasawara Islands in Japan from 2010 to 2012. Each tag included a camera with LED lighting and an accelerometer. Cameras took photos every 4 seconds and recorded depth every second. Accelerometers also recorded depth (and swim speed as well in some models) every second and recorded acceleration every 1/16 second. Tags used earlier in the study yielded less complete data than tags deployed later as technological changes allowed more images to be recorded per dive in 2012.

Results

Tags gathered nearly 43 hours of diving data, including almost 18,000 images, from 47 dives made by 17 sperm whales. The average dive lasted 42 ± 10 minutes and to depths of 793 ± 248 meters, while the deepest dive lasted 57 minutes and went to 1354 meters. Statistical models applied to accelerometer data indicated that the LED lights on the tags’ cameras did not significantly affect whale behavior.

Most images were very dark because there was nothing in the frame to reflect the light of the LEDs. Images that contained visible material were assigned to one of five categories:4.27.3

 

 

 

 

 

Images of other sperm whales were only recorded to depths of 339 meters, indicating that whales were alone while foraging at deeper depths. In contrast, images including suspended material, unidentified particles, and possible animal body parts were only obtained at depths deeper than 400 meters. Images containing suspended material were also significantly associated with bursts of speed, with the whales sometimes achieving swimming speeds greater than twice the average dive speed.

Images of suspended material were only obtained at deeper depths (but not at the seafloor) and while the whales were swimming at higher speeds. This suggests that the whales were engaged in foraging and that the suspended material may be squid ink.  A video taken at depth (in the supplementary material for this article) shows that the suspended material does indeed look very much like squid ink. Taken together, the authors’ data does not support the sit-and-wait foraging hypothesis. Instead, their data imply that sperm whales actively hunt squid alone. This study supplies the very first images associated with such foraging.

4.27.2 (1280x642)

Examples illustrative of the categories researchers used for analysis: a) nothing recorded, b) suspended materials, c) unidentified particles, d) possible animal body parts, e) other sperm whales, and f) near-seafloor image (which is uniformly brighter than the “nothing recorded” image shown in a).

What about you, Oceanbites readers: if you could tag any marine animal, which would you tag? What would you hope to see with your data? Let me know in the comments!

Discussion

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