The ADCP was an essential instrument for this cruise because we wished
to reference geostrophic currents to absolute ADCP velocities rather than
to the velocity at an arbitrarily chosen pressure level (e.g. 1500 dbar)
as had been done in the past for the Bering Slope Current. However when
the scientific party arrived at the ship on 27 March the ADCP did not work
owing to cable damage sustained when the ship's centerboard was raised
at the end of cruise MF97-02. Pacific Marine Center (PMC) had sent a spare
electronics box, cable and transducer for troubleshooting and replacement.
Tests by LET Rick Philips indicated a transducer or cable failure, both
of which required removing the transducer from the bottom of the centerboard
using divers. No attempt at transducer removal could be made on 27 March,
as first planned, due high waves at the Homer Spit wooden pier where the
ship was berthed.
On 28 March, two of the ship's divers, LTJG Mark Sramek and S/F Bruce
Thomassen, began the arduous task of removing the ADCP transducer. Field
Operations Officer (FOO) LT Mike Hoshlyk, remained topside providing coordination,
logistical support and safety monitoring as the divemaster. The work went
slowly in the sediment-laden, near-freezing water. The transducers were
not mounted for easy removal and installation. The divers first fastened
a basket under the ship as a makeshift working platform. Standing in it
and reaching above their heads, they had to weave their hands up into the
ADCP well, behind the transducers, and unscrew 8 small half-inch-sized
fasteners (6 bolts and 2 nuts) while wearing neopreme gloves as protection
against the cold. The space was so small that they could not simultaneously
see the bolts and unscrew them. Over several dives that day requiring 6
hours in the water, the dive team managed to remove 4 bolts. LT Hoshlyk
looked after his divers well, filling air bottles, logging time at depth,
and removing them from the water when cold began to hinder their efficiency.
Though disappointed by their slow progress, Sramek and Thomassen maintained
their optimism. They vowed to do better on the next day, and they did.
Soon after breakfast they had removed 3 more bolts with 1 remaining. Meanwhile
the Commanding Officer, CDR John Clary, arranged for a commercial diver,
Cecil Cheatwood of C and C Aquatics, to join the diving team. The divers
attacked the problem with renewed vigor, removing the last bolt and guiding
the ~80-lb transducer head from under the ship and onto the deck. There
we hooked up the new cable and tested the old transducer which worked,
thus confirming that the old cable was at fault. With the help of the deck
crew topside, the divers threaded the ADCP cable back through the centerboard
and reinstalled the ADCP head, again spending several hours in the water.
The ADCP worked perfectly for the rest of the cruise.
LTJG Mark Sramek and S/F Bruce Thomassen deserve special recognition
for their unselfish devotion to the task, taking risks and forsaking their
own personal comfort to make our scientific research possible. LT Mike
Hoshlyk is to be congratulated for looking after the dive team's needs
and maintaining a safe diving environment. CDR John Clary had overall command
and took a close personal interest in the details of the operation. He
was never far from the action, handling the lines on several occasions.
His hiring of a commercial diver bolstered the dive team with a fresh,
experienced member who helped expedite the repair.
Recommendation: The ADCP repair delayed our departure
by 28 hours. Bolt removal and refitting consumed most of the time. The
ADCP mounting must be redesigned for easier repair by divers. The transducer
head should be placed in a recessed housing with a flange that bolts directly
to the bottom of the centerboard via a ring of easily accessible bolts.
The ship departed Homer on 29 March at 1900 hours bound for Shelikof
Strait. There we launched a satellite-tracked drifting buoy and occupied
7 CTD stations along Line 8 (Figs. 1 and 2).
We proceeded to the mouth of Pavlof Bay to recover and deploy a temperature
measuring crab mooring (Fig. 1). The wind
was blowing at 40 kt from the NW with gusts to ~50 kt. The in-place mooring's
acoustic release would not respond to enabling commands despite repeated
transmissions from both the centerboard-mounted and hand-lowered 12 kHz
transducers. By listening inside the centerboard trunk, we determined that
the transducer there was transmitting. Disabling commands were sent to
the acoustic release. The new mooring was deployed with some difficulty
in the strong wind. It was initially placed in water too shallow, but was
subsequently lifted, dragged and released in deeper water.
The ship carried on through Unimak Pass into the Bering Sea where we
searched for spawning walleye pollock using the EK 500 research echosounder
(Fig. 3). When promising fish sign was
observed, the Poly Nor'easter bottom trawl with roller gear was fished
at mid-depth to catch a small amount of pollock. The first haul was small
but contained some spawning females. Eggs were set aside for the Alaska
Department of Fish and Game, and fin clips were taken for genetic study.
A second haul produced no spawners; therefore the Aleutian wing trawl was
used on the third haul providing adequate spawners. Eggs were stripped,
fertilized and incubated in 4 refrigerators by Debbie Blood to determine
the effects of temperature on egg development.
Following trawling, the ship sailed northwestward to the inward end
of section I to begin the Bering Slope Current study (Figs.
3 and 4). There commenced a series of 7 CTD-ADCP sections with stations
located nominally at 150, 200, 500, 1000 and 1500 m depth and then every
15 to 30 nm. Usually two CTD casts were taken at each station, one shallow
cast limited to 300 m because of the depth range of the ChLAM (chlorophyll
light absorbance meter) and one deep cast to the lesser of 1500 m or near
bottom. The shallow CTD, operating off the starboard winch, measured the
photosynthetically available radiation (PAR), fluorescence and chlorophyll
concentrations as well as temperature and salinity. Water samples for nutrients,
pigments, algal cells and primary productivity incubations were drawn from
Niskin bottles on this CTD for final analysis at the University of Alaska,
Fairbanks. The deep CTD off the port winch carried dual temperature and
salinity sensors for cross-checking precision. Water bottle samples there
were used for salinity calibration and to supplement the biological sampling.
CTD data were processed while at sea, and preliminary profiles and sections
worked up using EPIC programs.
The ADCP ran continuously while the ship was in the Bering Sea. Using
GPS navigation and attitude determination (with an Ashtech 3DF unit), it
computed the current velocity which will be used later to reference geostrophic
speeds and draw fine-scale current vector maps. Dedicated ADCP transects
connected the ends of the CTD sections (Figs.
3 and 4). A portion of the ADCP data was processed preliminarily at
sea, but reinstallation of the ADCP heads meant that several sections had
to be completed before adequate data were available to compute the transducer
misalignment angle and scaling factor. Also because the processing person
stood CTD watches and processed CTD data, he lacked time to process the
ADCP data as well.
While underway the ship continuously measured PAR on the mast and temperature
and salinity from the sea chest whose intake was located 5-5.5 m below
the surface. Fluorescence was measured with a Turner Designs 10-AU bench
top fluorometer situated in the chem lab and plumbed to the sea chest.
The flow-through cell was cleaned every 1-2 day. A DI water blank was run
periodically and automatically subtracted from the fluorometer reading.
Also the scaled raw fluorescence values were recorded, and reference samples
were drawn from the fluorometer stream and run through the calibrated University
of Alaska fluorometer.