The Seascapes

The Seascapes

Thursday, December 15, 2011

The Butterfish Smackdown: Fishing for Temperature

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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