04 August 2011

WHERE WHALES WANDER

Sperm whales. Credit: NOAA.
    
A new paper in MEPS (Marine Ecology Progress Series) describes for the first time the communities—villages, in a sense—of whales and dolphins living offshore in the northwest Atlantic and the Gulf of Mexico.

This is a mind-bogglingly difficult thing to assess, as you might imagine, in a world beyond our sight and wildly three-dimensional... where far-from-still waters run hundreds if not thousands of meters/feet deep. As the authors dryly observe:

Data collection on the distribution and abundance of marine mammals is costly, time consuming and complicated by logistical difficulties.

Humpback whales. Credit: NOAA.
   
Nevertheless, this team of 15 researchers—themselves clustered along the East Coast, with one outlier in the Bahamas—availed themselves of an archive of datasets on cetacean distribution and abundance. These covered the waters from Nova Scotia to the Gulf of Mexico.

Specifically, the team used OBIS-SEAMAP sightings, collected over many years from vessels both at sea and in the air during the summer months of June, July, and August.

Marine mammal sightings 1990-2010. Credit: OBIS-SEAMAP.

 
Duke University's OBIS-SEAMAP (Ocean Biogeographic Information System—Spatial Ecological Analysis of Megavertebrate Populations) is a seriously cool tool. Above is a map I generated for marine mammal sights between 1990-2010. You can play around with the datasets—which include seabirds and sea turtles too—and generate your own maps.

The authors were looking for signs of community structure at really large spatial scales, on the order of thousands of kilometers/miles. And they found it.

The regions North of Hatteras (NOH) and South of Hatteras (SOH). (a) Overview. (b) Sampling hexagons (NOH: light grey; SOH: dark grey). Sightings are depicted at the taxonomic guild level with different colors and symbols. Depth contours (200, 500, 1000, 2000 m) in light grey. GOM: Gulf of Mexico. Credit: R. S. Schick, et al. MEPS. DOI:10.3354/meps09183.












  
This glorious map shows the distribution of 16 species or species clusters (e.g., beaked whales, baleen whales) in waters North of Cape Hatteras and south of Cape Hatteras. Depth contours are included—though you can't really see them on the small map I posted here. For a higher resolution image, download the PDF of the paper.

As you can see, things get seriously busy up there in the colder, productive waters north of Cape Hatteras, where harbor porpoises and baleen whales rule, where bottlenose dolphins own the inshore realm, and where toothed whales work the drop-offs to the deep. These waters also had the biggest datasets.

And take a look at the map above and the fascinating zigzag distribution of Atlantic spotted dolphins (blue dots)—running back and forth across the Gulf Stream to the south of Cape Hatteras. Having worked many years filming those guys in the Bahamas, I find it really exciting now to see a bigger picture.

The Gulf of Mexico (GOM). (a) Overview. (b) Sampling hexagons in light grey. Sightings are depicted at the taxonomic guild level with different colors and symbols. Depth contours (200, 500, 1000, 2000 m) in light grey. Credit: R. S. Schick, et al. MEPS. DOI:10.3354/meps09183.



  
Turns out the spotted dolphins living in the Gulf of Mexico are distributed much like those along the Gulf Stream—zigzagging inshore and offshore from Mobile Bay to the Florida Keys (above).

You can also see how many species—Risso's dolphins, sperm whales, common dolphins, bottlenose dolphins, and killer whales—congregate in the waters enriched by the Mississippi River. Also the disaster site of last year's Deepwater Horizon blowout.


Sea surface temperatures on 4 Aug 2011 at 0756 GMT. Credit: NOAA, Rutgers University Coastal Ocean Observation Lab

The researchers connected the position of each sighting with each of seven environmental layers:

  1. Sea surface temperature (SST)
  2. Chlorophyll a concentration [a mark of ocean productivity]
  3. Bottom depth
  4. Distance to continental shelf (defined by the 200 m isobath)
  5. Distance to shore
  6. Probability of an SST front [a place where water masses of different water temperatures meet]
  7. Depth of the mixed layer (MLD)

Killer whales. Credit: NOAA.
 
The overall findings for the three biogeographic regions are interesting:

North of Hatteras, we found 2 main groups split along a temperature and chlorophyll [indicating phytoplankton abundance] gradient, with most piscivores [fish-eaters] being found in cooler, more productive waters of the continental shelf, and most teuthivores [squid-eaters] being found farther offshore in warmer, less productive waters at the shelf break (200 m isobath). South of Hatteras, we found 3 groups, with the largest group being in warmer, lower chlorophyll waters that are closest to shore. In the Gulf of Mexico, we found 7 groups arrayed along a bottom depth gradient.

The Domesday Book on display at Britain's National Archives.

  
To my mind, this paper is the ocean equivalent of Britain's Domesday Book, the great survey concluded in the year 1086 of much of England and parts of Wales. That effort provided a snapshot of the geography of human habitation and ownership at the time. This paper delivers something of the same for the shifting villages of ocean nomads—travellers in a fluidly moving realm.

The paper:

  • ♥ Schick RS, Halpin PN, Read AJ, Urban DL and others (2011) Community structure in pelagic marine mammals at large spatial scales. Mar Ecol Prog Ser 434:165-181. DOI: 10.3354/meps0918.

♥ Open-access paper.

Common dolphins. Credit: NOAA.

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