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SO173-2, 1. Weekly report, 8th - 17th August 2003

The cruise SO-173/2 on R/V SONNE started late in the evening of 8th August 2003 when we left the roads of Caldera, Costa Rica after having taken on board some last, delayed items of baggage. For the first time, SONNE has been operating off the Pacific coast of Guatemala and El Salvador with 19 scientists from 11 nations on board to investigate this area. Two independent groups of people have shared time at sea and laboratory space, each applying their own methods to tackle completely different problems: the group of marine geo-scientists (scientific direction: GEOMAR, SFB-574) focusing on processes in the subduction zone to improve the knowledge on both the evolution of the continental margin off Guatemala and of Central America, and the group of biologists (scientific direction: Anatomisches Institut, Universität Tübingen) aiming at a better understanding of sensory systems of mesopelagic fish.

After two days of transit we reached the sea territory off Guatemala on Monday morning. As we are going to perform our work in Guatemala's economic region, the authorities there requested that an observer from the Navy be present on board the R/V SONNE. We picked him up from a coast-guard boat near Puerto Quetzal on Tuesday morning.

Geo-scientific work comprises mainly systematic bathymetric and magnetic surveys of the subduction zone from the upper slope across the deep-sea trench down to the oceanic plate. So far we have recorded 15 profiles of excellent quality running at right angles to the strike of the trench; the profiles are between 50 and 80 nm in length. In this region the trench is more than 6,500 m deep, i.e. it is considerably deeper than in the south off Costa Rica and Nicaragua. Here too, as off Nicaragua, the oceanic plate approaching the trench shows a strongly pronounced relief of horst- and graben structures with fault throws of more than 500 m. We noticed that the tectonic pattern of the plate, owing to its origin being in the spreading zone, is reactivated during bending and before dipping into the trench. There are "mound"-shaped structures at the continental slope - similar to those in the region off Costa Rica and Nicaragua -. However, we will only be able to describe the character of these structures in detail after smoothening of bathymetry data and interpretation of the level of back-scatter signals.

The group of biologists studying "Sensory systems of mesopelagic fish" consists of 10 scientists from 5 different countries. Among them are morphologists, biochemists, electrophysiologists, and specialists for physiological optics. During the first week 8 trawls were deployed in Nicaraguan and Guatemalan waters. We used two types of net: an RMT 8 (rectangular midwater trawl) with an opening surface of 8 m2 with an open cod piece, and a smaller Tucker trawl system with a timer controlled closing cod end which isolates the haul against temperature changes and light. This closing cod end brings the animals back in such a good condition that many of them will come on board alive. Trawls were performed at depths between 650 and 1,000 m by day and at about 300 m by night. The results were surprisingly variable: Off the coast of Nicaragua, catches were markedly richer both in terms of abundance and diversity, than off Guatemala. This is probably due to differences in currents and water quality. West of von Nicaragua, the waters are clearly richer in nutrients than off Guatemala. Another unexpected finding was that the night time trawls, aimed at catching migratory species living below a depth of 500 m by day and coming up to 200 m at night, yielded very few specimens. This may be attributed to the currently very high temperatures of the surface layers (32°C), reaching far deeper here than in the Atlantic. We also noted the absence of circumglobal species such as the viperfish (Chauliodus sp), Gonostoma sp., whilst other species of otherwise high abundance were rare (hatchetfish: Sternoptyx, Argyropelecus).
Certain species (especially Melamphaeids) were fixed on board in order to prepare them for a systematic study of their brains, lateral line, and auditory system. Previous investigations of deep-sea fish brains had shown that these sensory modalities are well developed in some mesopelagic fish, and are probably more important than vision and smell. Furthermore, we started a systematic comparative analysis of the visual pigments of lanternfish (Myctophids) in order to identify those species with a high sensitivity in the long-wave range of the spectrum.

With the sea mostly calm, brilliant sunshine, tropical temperatures, and now and then heavy rain and thunderstorms, people on board are in an excellent mood. All aboard are fine and send their regards to those at home.

At sea, 12° 1' N, 91°16' W, 17th August 2003
W. Weinrebe, J. Wagner

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