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The Southern Indian Ocean seamounts expedition achieved many of its sampling objectives. The data gathered are likely to form a significant contribution to knowledge in the following areas: • Hydrographic structure of the Sub-Tropical Convergence zone • Patterns of chlorophyll concentration, nutrient chemistry and phytoplankton diversity from the oligotrophic Sub-Tropical Anticyclonic Gyre system through the Sub-Tropical Front to Sub-Antarctic waters • Small scale current topography interactions around seamounts with differing summit heights, including evidence of tidally driven concentration and / or mixing of water and phytoplankton and influence on the distribution of zooplankton • Trapping of multiple deep-scattering layers of zooplankton and predation by resident seamount predators • Evidence supporting proposed biogeographic zones within the southern Indian Ocean • Evidence of the significance of both water masses and the presence of elevated topography on seabird distributions • Connectivity of populations of pelagic organisms across the South West Indian Ocean Ridge The extremely large number of specimens gathered during this expedition means that a large post-cruise effort will be required in order to extract the maximum information from the cruise. These data will be most significant when combined across the disciplines of oceanography, biogeochemistry, botany and zoology represented on the cruise by the scientists.
The hydroacoustic survey covered the shelf and slope to about 2500 m bottom depth. Continuous acoustic recording and analysis was carried throughout the survey. Pelagic trawling was carried out for pelagic species identification, mainly during dark hours, either as random blind trawl hauls close to the surface with pelagic trawl equipped with large floats, or on registrations. In addition blind surface hauls were made at intervals and at the start and end of each environmental transect to determine the distribution and species composition of juvenile fish. The highest acoustic densities where found over the shelf and shelf break while further out the recordings were lower and the catches more dominated by plankton and mesopelagic fish towards deeper waters. The dispersed fish distribution and high abundance of plankton made acoustic detection and separation very difficult, and thresholding according to the Cotel methods were used. The acoustic estimates for the pelagic species groups PEL 1 (Clupeidae) and PEL 2 (Carangidae, Sphyraenidae, Trichiuridae and Scombridae) shown a biomass of 6.1× and 3.5× higher than the estimate presented in the 2007 Ecosystem Survey report for the whole Mozambique coast. The differences between the 2007 and present survey acoustic estimates should not be taken as indications of large increases in species abundance. During the present survey new methods for analysing and interpreting the acoustic data were used and these methodological differences are probably the major cause in the differences in abundance between 2007 and 2009. Pelagic species were observed both acoustically and in trawl catches along the whole survey track. The PEL 1 group was observed from 10° -13° 30’S, then from 14° S to the end of the survey. In 2007 no PEL 1 was observed in this region. The main distribution was over the shelf, but PEL1 were also observed over deeper (>1000m) waters. The PEL 2 group (Carangidae, Sphyraenidae, Trichiuridae and Scombridae) were found from 11° 30’S to the southern end of the survey. The main distribution area was over the shelf and shelf break, but like the PEL 1 group the distribution extended into deeper (>1000m) waters. Compared to the 2007 survey the distribution was wider and extended further to the north. Zooplankton was sampled at 42 stations using the multinet. The multinet was deployed after measuring the fluorescence profile using the fluorometer mounted on the CTD rosette. Nets were deployed at different depths above and below the depth of highest fluorescence: two above f-max, one through f-max, two below f-max. Samples for the multinet have been preserved in formalin solution but not analyzed in any way. Therefore, no results from the zooplankton investigations can be reported in this preliminary survey report. During the survey a total of 213 species were identified. The fish species caught are grouped according to functional groups. Pelagic trawl catches of fish were grouped according to fish depth of the trawl. In the surface layer the ‘Other’ and ‘Scombrids’ groups dominated due to one or two very large catches. Down to 20 meters the ‘Other’ group still dominated but here hairtails as the second most important. At 20-100 meters Clupeids were the dominant group with one catch 10× as large as the second largest. Deeper than 100 meters the mesopelagic fish dominated, although the catch rates were less than closer to the surface. Dedicated observations of marine mammals were carried out on 19 August along the coast while steaming towards Pemba. The course was set close to shore to cover bays where whales and birds had been observed during surveying southwards. A total of 31 birds and 29 humpback whales were observed from 06:10 to 17:38. A large school of fish was seen feeding actively at the surface next to a mangrove forest. Feeding continued for over 15 minutes and the fish were repeatedly jumping out of the water.