The three quarter moon glows dimly behind a thin unbroken
layer of high stratus cloud. Tonight is nearly as black as any night can
be. I walked out onto the bridge
deck, eye level to the net reels, to look out at the horizon. I thought I saw
it, but it is just as likely that I made it up. We are cruising up the bank to a nighttime station Chris has
picked about 50 nautical miles east north east of the mouth of Chesapeake Bay. We have already finished fishing model
pixels representing “good” and “bad” habitat that we selected as randomly as
possible given what we still need to get done before Sunday. The northerly wind
has died down so the “Karen Elizabeth” has lost the heave she had earlier this
evening. The only waves to speak
of are those she throws up which cap as ghosts in her wake.
A few hours ago we fished Chris’s daytime station with
the net doors armed with real time and recording temperature and depth
sensors. He picked a bump along
the wall of the shelf break he said is “sometimes pretty fishy.” The record of
his trawl tows stored in his Navigation Software shows he’s worked this feature
a lot. His first tow followed the
114 fathom depth contour, and in real time the temperature at the doors held to
a pretty steady 53 degrees. The
low frequency acoustic machines lit up with bright orange and red targets and
the 200 kilohertz machine showed a lot of bottom haze. “It might be hake‚” he said. When we hauled back the net was indeed
full of small hake, a few dogfish, and about 4 pounds of butterfish.
Chris then steamed up onto the edge of the bank
into 55 fathoms of water, turned the boat east, and set the net for his second
tow. This tow was down slope and in the shallow water the bottom temperature
was 61 degrees. The hydroacoustic
screens were mostly black to the bottom. As we moved down the bank the
temperature held pretty steady; at 80 fathoms it was still 59 degrees. Then all of a sudden at 90 fathoms
orange and red targets began to slide into the acoustic screens, with the
38 kHz and 50 kHz machines showing hard red
targets. The animals kept showing
on the screen as we towed a few more minutes longer. Then the temperature began to drop 58, 56, 55 then 53
degrees as the bottom fell away under the boat to 140 fathoms. A huge mass of fish appeared on the
acoustic screens that our net, with its sensors a quarter of a mile or more
behind us, never caught. Our 20
minute tow was up - we had to haul back. The numbers of hake, dogfish and other
species caught in the net didn’t tell the story. We
plotted up the data from the temperature depth recorder and could see the
abrupt drop in temperature as the netdoor reached 90 fathoms. It showed we had pulled our net just
barely past the wall made by the warm water on the bank and the cold water down
on the other side where Chris’s first tow showed fish were abundant. I put the position of Chris’s station on the map of our habitat model “nowcast” we made made with measurements of the sea surface
from satellites in space, radars onshore, and the broad scale seasonal survey
of animals in the Northwest Atlantic NOAA performs twice every year.
Chris had sampled a hot pixel in our habitat map. But he had also drilled down to scratch
the surface of much finer scale processes and features, like that deep water
thermal front, that are embedded in broader scale features and affect the lives
of fish and the fisherman who depend on them.
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