whales and internal waves

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     The methods by which animals take advantage of their environments to increase foraging and locamotive efficiency are sometimes astounding. Albatross (family Diomedeidae) can lock their wings and surf the pressure gradients in front of surface waves for days (see dynamic soaring). Striped bass (Morone saxatilis) are known to sit in one location, swimming in place, facing the current (this, by the way, is called positive rheotaxis, for those of you looking for your $2 word of the day) with their mouths agape, waiting for the current to bring them the food.
    A recent paper by Moore and Lien (2008) documents a pod of pilot whales (Globicephala macrorhynchus) following internal waves through the South China Sea. Moore and Lien suggest that the Globichephala may do this to take advantage of the increased concentrations of prey entrained in the physical mixing generated by the waves. (In fluid dynamics, to be entrained, literally, is to be picked up by and carried with a flow).
     While I think the ideas within the paper are very interesting, I'm disappointed that the authors didn't go further. How about some prey sampling in the waves? No nets? If the data was collected with a depth-sounder, there is post-processing software that can be used to estimate plankton and fish abundances. The other papers cited, Ramp et al. (2004) and Lien et al. (2005), describe methods used to investigate the internal waves in the South China Sea. Data collection methods included: ADCP, moored current, temperature, conductivity, and pressure sensors as well as other acoustically gathered data. If the pilot whale were observed by chance during the course of the internal wave research-cruises, Moore and Lien would have the capacity to describe the (probable) presence or absence of prey for the whales that was aggregated by the waves. Such inferences could be made from the available acoustic data.
     Although the authors mention that the shoaling waves bring nutrients and plankton to the surface, there is no mention of plankton in the cited paper by Lien et al. (2005) describing the internal waves in the area.
     Basically, my issue with the paper is that Moore and Lien don't offer any evidence that prey were present in the waves. They merely say the waves are capable of entraining likely prey for the pilot whales. That said, I think the phenomenon is interesting, and I agree with Moore and Lien's conclusion that the issue merits further study.

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This page contains a single entry by Peter Stetson published on February 4, 2009 12:20 AM.

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