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DISPATCH 2: - 9.12.2006 | Tammy Frank
We've been having some equipment problems, as is par for the course for oceanographic
research cruises. The net is fishing beautifully, but the timer isn't working properly,
so we have to send the net down open, and close it at depth. We're not doing quantitative
trawls, meaning we're not looking at the abundance of animals at various depths, which is
usually the reason for doing opening and closing trawls. The reason we like to send down
a closed net and open it at the depth we want to fish at is because there tend to be a lot
of gelatinous organisms in surface waters. Not only does mashed jelly clog the net, but
it also clogs the gills of the shrimp and fish we want, so it's difficult to get live
animals out of a trawl that has a lot of jelly in it. However, we think we've figured
out how to fix the timer, and this vessel is equipped with a mill, so after today's
trawling, one of the crew will hopefully do some precise milling for us.
In addition to the net problems, the cold room in which we wanted to keep live animals
isn't working. Many research vessels don't have walk-in cold rooms, but you can request
one from the research vessel equipment pool, so these cold vans get trucked from location
to location, and, as with all equipment that undergoes a lot of transit, problems do occur.
Our cold van compressors seem to want to constantly defrost, which keeps the cold room at
a comfortable 80 degrees. This is a wonderful temperature for a fragile Florida flower
such as me, but not so great for animals that live happily at 4° C (about 39° F).
Equipment breaking down is a common problem for ocean going vessels, and so one learns
to make do. In this case, the ship's cook very generously offered us space in his food
storage cold room to store 4 five gallon containers of sea water (cold water is needed
for every trawl and to do water changes on the animals we're trying to keep alive), the
lab refrigerator is filled with our black jugs containing our animals, and it looks like
the marine tech can turn one of the freezers into a refrigerator. Not as a convenient
as a walk-in cold room, but definitely a good solution to our cold storage problems
We had a great trawl yesterday, although unbelievably, it was actually a bit too full of
animals. When the collecting vessel is packed with animals, the oxygen is used up pretty
quickly after it's closed, and since it takes almost an hour to bring the net to the
surface after a deep trawl, many of the animals came up dead. We trawled for 6 hours
at between 600-800 m, and I just didn't expect to get that many animals towing at 1.5 knots.
There were some very interesting specimens that were still alive, however - a beautiful
swimming angler fish, and an equally beautiful, still swimming Anoplogaster (see picture
at right - note the inky black body to make it virtually invisible in the deep dark depths,
and the huge mouth to help it grab anything that should come its way). Tomorrow, we'll
tow at the same depth, but for a shorter period of time, and hopefully more of these
denizens of the deep will be alive.
We collect these animals in the cod-end, a long collecting vessel attached to the end of
the net. This cod-end is both light tight and temperature insulated when it is closed.
There's a mesh net inside, so as the net fishes, water filters out of the cod-end, but
the animals stay inside. The net has a 3.3 m (11 ft.) by 4.3 m (14 ft.) mouth opening,
and gradually tapers down its 20 m (65 ft.) length to an 11 inch opening, which fits the
end of the cod-end. The net is so long to give the animals inside a relatively gentle
"ride" to their eventual entrance into the cod-end. When the net closes, the ball valves
in the cod-end snap shut, so the animals trapped inside are maintained in cold water, and
often arrive at the surface alive and in excellent shape. There's a common misconception
that pressure changes are what kill most deep-sea animals when they're brought to the surface
in trawl nets. This is certainly true for fish that have swim bladders, as the air in the
bladder expands and bursts the bladder on the trip to the surface. However, most crustaceans,
squid and many species of fish don't have any air filled spaces (remember, they use gills
instead of lungs), and those caught down to about 1000 m can survive surface pressures
quite nicely. What does kill them is the temperature change on the trip to the surface,
as they're found at about 4° C in their natural environments, and surface waters can be as
much as 25° C warmer.
Once the net is on deck, at the surface, we detach the cod-end from the net and carry it
into a light-tight room where we can sort through it under dim red light. Last night, the
door to this room, which is also the storage van, was accidentally opened by one of the crew,
who had been sent over to get some tie-down straps. I screeched at him to close the door,
which he did, so no damage was done, other, apparently, than to the ears of everyone within
hearing distance of my screech. I tend to have a high voice under the best of circumstances,
and when stressed, it reaches those painful ranges. To prevent ever having to hear me screech
again, several of the members of the science party took matters into their own hands, and
formed a guard detail to guard the door of the storage van whenever we were in it. Luckily,
they were there for this evening's trawl, as we got lots of the species that we that we needed
for our experiments.
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