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SO173-2, 3. Weekly report, 25th August - 1st September 2003

After having concluded our investigations in the area off Guatemala, we used the 3rd and last week of our cruise to extend mapping work to the adjacent continental margin off El Salvador. In the course of the week the surveyed areas could be linked with those recorded during previous cruises off Nicaragua and Costa Rica. Thus, there is a now a complete map of almost the entire continental margin of Central America - from the southern border of Mexico into Panama. Bathymetric results obtained from the region off El Salvador reveal many deep canyons incising the upper and middle slope. Here, the trench reaches depth in the range of 5,500 m to 6,000 m; its sedimentary deposit is either surprisingly low or absent. The dipping oceanic plate is marked by pronounced horst- and graben structures with fault throws between 500 m and 1,000 m running at an angle of approx. 30° to the trench axis.

Mapping was interrupted on Friday for approx. 8 hours to recover 4 ocean bottom seismometers which had been deployed about 4 weeks before, during cruise SO-173/1, to record microseismics. All instruments could be recovered in good condition, first processing of data showed that they had all been operating and recorded correlational events.

With regard to the biological work group, the last three catches were by far the best among the six trawls of the third week. They were deployed off the Nicaraguan coast and produced the long awaited number of fish, both in terms of diversity and abundance, thus confirming the observations of the very first trawl in the same area. The fish obtained allowed us to set up the organ culture experiments with isolated pineal glands for the study of endogenous rhythms in mesopelagic fish. Two other groups also reported promising and interesting preliminary observations: The biochemical experiments destined to clarify the role of coelenterazine (a key enzyme in bioluminescence) successfully led to the detection of this substance in the blood plasma of mesopelagic fish. This would confirm the working hypothesis according to which the primary role of coelenterazine is that of an antioxidant scavanger of free radicals, and that its function in the context of bioluminescence is derived, and only possible in suitable conditions such as the deep-sea environment. The work on the visual pigments in the photo receptors of mesopelagic fish were mostly focussed on various myctophids. A total of 21 species were analysed, 17 of which had not been studied previously. In addition, several epipelagic species which were found by chance in our nets were also investigated and these yielded an unexpected result: The spectral sensitivity in fish from the mesopelagic and the epipelagic habitat showed surprising similarities. This would indicate that the absorption peak in the blue-green range of the spectrum which, traditionally, was interpreted as a special adaptation to the downwelling light at mesopelagic depth, may instead constitute a more general property of photoreceptors and vision of all fish (and possibly crustaceans) living in the blue waters of the open ocean.

Only rarely - and mostly far away - were other ships to be seen during the cruise. On Tuesday night, however, a naval ship of El Salvador sheered up alongside in order to check our research approval.

After the conclusion of the scientific programme, SONNE is now heading for Caldera. The work groups are preparing large quantities of samples, objects of study, and data for the homeward trip. The success of this cruise would not have been possible without the tireless crew's extraordinary efforts in supporting our work. The scientific work groups thank Captain Andresen and his entire crew very much for their efforts.
All people on board are fine, looking forward to the return voyage, and are greeting those at home.

At sea, 9°35' N, 86°08' W
W. Weinrebe, J. Wagner

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