Student Connection

August 19, 1999 The Hawaiian Monk Seal's, worldwide population is estimated at 1200 animals. The largest population of these seals is located at French Frigate Shoals Atoll (FFS) where survivorship has been declining since the late 1980's, due to apparent starvation. Since 1992, the fish populations in the shallow reef areas have been monitored for changes that might influence the seal's food base. However, recent evidence from seal-mounted, dive depth recorders, satellite tags and video cameras, indicates the Hawaiian Monk Seal forages mostly off the deeper slopes of the atoll, around the 200 foot depth. The seals often feed on animals they find by turning over large, loose rocks and debris that litter areas of the bottom at these depths. Much of the data collected on the actual foraging originates from video cameras temporarily attached to the seal's back. Unfortunately, the back of the seal's head blocks the view of the prey items and provides no clear indication as to the population density of the prey. On this cruise, technical divers are being utilized to descend to depths of 170-220 feet to video, identify and record the animal populations located under these loose rocks and debris. They'll also position various experimental models designed to measure the degree of seal foraging that may be taking place in a given area. The technical divers breath a mixture of Helium, Oxygen and Nitrogen (termed TRIMIX) enabling them to safely dive to depths of 200 feet for up to 20 minutes. They then have to stage their ascent through a series of decompression stops that take another 45 minutes before they reach the surface. During their ascent, they will switch to breathing and enriched air mixture (NITROX) and near the surface they switch to breathing pure Oxygen. This is designed to keep the dissolved nitrogen in their blood system to a minimum. To aid in a diver emergency, the U.S Navy's Mobile Diving Salvage Unit One, is collaborating with the project by providing the support of a diver decompression chamber and support staff. In addition to the Monk Seal foraging habitat study, the ship has also been tasked to travel to SE Brooks Bank (24 miles NW of FFS) and retrieve an Acoustic Doppler Current Profile (ADCP) mooring, and a Thermistor mooring. Both moorings have been anchored on the top of the bank (in 200 feet (67m) of water) for the past year. The ADCP mooring has been measuring ocean currents as they passed above it in addition to wave heights and tides. The Thermistor mooring consisted of a 50m (150 feet) length of cable which measured sea temperature every 5m (15 feet) along it length and sea pressure, depth and light levels every 10m (30 feet). The moorings and the data they contain, are very valuable and on August 16 we departed FFS for Brooks Bank and located each of the moorings using an acoustical (sound) generated transceiver to "talk" to each of the mooring's release mechanism. We also towed a video sled to actually see the moorings. The following day (Aug 17) we released each mooring from its anchor by transmitting its release code signal from the acoustical transceiver. One of our small boats then towed the mooring package to the ship after it floated to the surface. After the recoveries, we returned to FFS and anchored for the night. On the Aug. 18, we ran small boats from the ship to perform shallow reef fish surveys and to collect fish specimens for a Monk Seal nutrition study. Today, we performed 2 deep dive surveys, NE of Tern Island. Afterwards, we hauled in and reset the lobster trap string used to collect deeper specimens for the seal nutrition study. After we re-anchored the ship, the small boats departed for additional shallow reef surveys and fish collections.

Other Reports For This Cruise

• August 15
• August 19
• August 24
• August 27
• September 02
• September 07

Last modified September 03, 1999