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Navassa cruise Day 3 - 4.22.06

Day 3: 4.22.06

Conch Survey

Conchs
queen conch (Strombus gigas)
by S. Moneysmith

Today we completed a conch survey dive on the south side of Navassa.  We targeted sandy and rubble substrates, as these appear to be preferred for both adult and juvenile conch.  Dave Hilmer and Christine Addison surveyed three 30 meter transects, each four meters wide.  Within this transect belt, 5 milk conch (Strombus costatus) and no queen conch (Strombus gigas) were observed.  In addition, fish counters found five adult conch during our morning habitat dive.  Adult conch are distinguishable from juveniles by the presence of a flared lip, heavily encrusted with algae and well hidden.  

Fish and Habitat Data Surveys

Sponges on spur & groove reef
photo: sponges on a spur & groove reef,
by A. Poray

We also surveyed two deepwater habitat sites today.  The second dive of the day was in a spur and groove habitat.  The term “spur and groove” describes a series of reef and/or rock ridges alternating with valleys of sand and/or rubble, as if a comb pulled through the reef. 

Ruth Kelty rolling up transect tape
Ruth Kelty reels in a transect tape at the end of her fish survey. (photo by J. Marr)
Paula Whitfield deploying surface float
Paula Whitfield deploys a surface float at the beginning of the ascent from her dive. (photo by A. Uhrin)
Schooling ocean triggerfish

Divers returned from this dive with sightings of:

  • schooling ocean triggerfish (+20) (photo by A. Poray)
  • a large hogfish 
  • sponges nearly the size of 50 gallon drums   
  • large stands of lettuce coral (Agaricia sp.)

Birds

Adult brown booby
photo: adult brown booby,
by C. Addison)

While waiting to dive, scientists did some bird watching.  Navassa Island has a large population of magnificent frigate birds, brown footed boobies, with an occasional tropic bird and red footed boobies.  One brown footed adult landed on the cliff face near the scientist’s boats. Higher up the cliff was another adult with a young chick. 

Booby nest
photo:  booby nest, by C. Addison

Bathymetric Mapping of Navassa Island

In accordance with a congressional mandate to map all coral reef habitat in United States waters, the Nancy Foster is using a technique called multibeam sonar to map a 100  km2 area of the sea-floor that surrounds Navassa Island National Wildlife Refuge.  The hull of the ship is outfitted with a sweep sonar, or transducer, which can send out 111 pings (sound waves) in a sweep formation 10 times per second.  The transducer then measures the time travel of sound waves from the boat to the sea-floor and back again at 111 angles, and mechanically compensates for the natural pitch and roll of the ship.  These highly accurate maps will help ground-truth maps put together from drop-camera work previously conducted by Dr. Margaret Miller of the NOAA-Fisheries, Southeast Fisheries Science Center.  We will use these maps in concert with our fine-scale SCUBA habitat survey data to get a more complete picture of the extent of coral reefs around the island.

Navassa with multibeam Western Navassa - small scale
(photos: Navassa with multibeam images, Western Navassa with multibeam-zoomed in, by M. Stecher and D. Boles)

Dan Boles, the resident survey tech aboard the Nancy Foster, and Mike Stecher, owner and founder of the survey contractor Solmar Hydro, plan and execute these surveys remotely from the command room.  Dan and Mike along with the ship’s crew have been working around the clock and have made significant progress to date.