Thursday, 16 October 2008

Demersal Trawling

The demersal trawling programme is producing some very colourful fishes of fascinating variety. We caught 38 species in our first half-hour trawl and the total is now up to nearly 130 species. Five of our bottom trawls so far have been in about 20-30 m depth and the species found at this depth are now becoming very familiar to us, so sorting is becoming quicker and easier. I was able to name all of the fish in this morning’s sample in a very short time. Smith’s Sea Fishes is proving an excellent guide to identify these fishes even though we are well away from the sea area covered by this famous book. This illustrates the role of the Agulhas current in spreading these species down the south east African coast.

Before I left Grahamstown, Eric Anderson, SAIAB’s expert on deeper water fishes, told me that the deeper waters around the Mascarene Plateau were very poorly explored and that many species caught in waters around 300 m deep are likely to be new. I’m not sure how true that comment will prove to be, but I’m certainly having difficulty identifying some apparently common large species from deeper water trawls in 200 m and 300 m. I’m still searching the literature and thus won’t put pictures of these on the web yet to ask for help, but may do so before the end of the cruise.


Among the more interesting species landed was a black snoek, Thyrsitoides marleyi, not far short of 2 m long, held up in the photo by Jackie Hill, and also a sawshark, Pliotrema warreni, whose toothy snout became so entangled in the netting that it had to be cut free. Other common fishes are a variety of trigger fishes, fusiliers, butterfly fishes and other tropical reef fishes. The opening of the codend at the end of each trawl continues to be eagerly anticipated, to see what new species spill out on to the deck





Image captions (top to bottom): A collection of fishes from the trawl awaiting sampling of muscle tissue for genetic analysis; Balistoides conspicillum; Lactoria fornasini; Apolemichthys trimaculatus; Sargocentron spiniferum; A very colourful mantis shrimp trawled from 50 m depth; (bottom left): Jackie holding a large black snoek trawled at 300 m depth; and (bottom middle): A sawshark, Pliotrema warreni, caught at 200 m depth.
Text and images by Denis Tweddle & Oddgeir Alvheim


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