Sunday, 31 August 2008

Fish trawl sampling

A few posts ago, I mentioned that I would let you know about what had been keeping us all so busy. The start of trawling meant lots and lots of fish (and other creatures) winding up on our decks, which needed to be identified to the lowest possible taxon (preferably to species).

The first step in processing a trawl is to pick them all up off the deck and do some preliminary sorting into "recognisable taxa" - which may often be more than one species. Buckets which have a lot of different species are carefully sorted into different species if at all possible; some require considerable care to ensure that two (or more!) similar looking species aren't getting "lumped" together. For many species, this takes a long time, particularly if you are not familiar with that particular group. In the past, I've mainly worked on identifying tropical and subtropical reef species, so tropical trawl species, particulary pelagic ones, are quite new to me. Fortunately, Sean was quite familiar with some of them, which helped, and I have a reasonable grasp of general families of fish, so can get to the right sort of area fairly quickly. Still, tricky specimens (individual fish!) can take over an hour to get to a point where you're happy that you've done a reasonable job of identifying it accurately.

After the identification is done, all the different species need to be weighed and counted, and important fisheries species should be measured too; fortunately, we have electronic measuring boards which make entering the length frequency data relatively quick.

The fish lab looking unusually tidy (just after we cleaned it on Saturday).

Preliminary sorting of the catch into buckets. (Photo courtesy Jens-Otto Krakstad)

Sometimes you get a lot of the same kind of fish. Only those buckets probably contain more than one species, so they'll need to be sorted again. Carefully.

The Captain of the Dr Fridtjof Nansen, Leif Olsen, stopped by to check on our progress and this prawn caught his eye.

Some fish are really hard. This is a whole bucket full of a genus called Bregmaceros. There are at least 3 species in the area, and telling them apart requires microscopic examination of the number of rays in the fins and also the number of transverse scale rows. (Yes, keys aren't even easy to use until you get au fait with some fairly technical language! And don't get me started on subjective judgements in keys like "moderately long"... Unless you wrote that key, chances are you have no idea what "short" or "long" is for that particular character!).

This is the ugly side of bottom trawling; essentially, you're bulldozing a track along the seabed, and catching anything in your way. Most of that pile is large basket sponges, and on top is a typically rather slow-growing gorgonian. Sorting out the catch in what was essentially a tropical reef station took us well over 12 hours. In commercial trawls, all the stuff you don't want is termed "bycatch"; in some fisheries, such as prawns, bycatch to catch ratios can be well in excess of 6:1 (6 kilos of other stuff to every kilo of prawns). If you'd like to learn more about more sustainable seafood choices you can make, people in South Africa could look at the Sustainable Seafood Initiative. Similar initiatives exist in many other countries. (Photo courtesy Arrie Klopper).

Are we done now?


After that, we have to take DNA and stable isotope samples; each individual specimen is photographed before the samples are taken and then it is preserved so that a physical specimen exists to which the genetic sequences and stable isotope signatures can be tied. This specimen is lodged in a collection for further study.

Note the little sample tag in each photo; using this, we can link the colour photo back with the actual specimen; before it is preserved, we tie this label to the fish so it doesn't go missing. The colour chart can help us correct the colours in the image to be more accurate.

As you can probably imagine, a few trawls like this would keep us busy for some time!

We're currently on station here doing a CTD cast. The current is so strong the wire is getting pulled out at a crazy angle. Raymond Roman, our oceanographer, has never seen anything quite like it!

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