Bettina Martin

Diets and standing stocks of benthopelagic fishes at two bathymetrically different midoceanic localities in the northeast Atlantic

Abstract Deep-sea benthopelagic fishes were sampled by trawl and free-fall longline at two latitudinally and bathymetrically separated stations in the northeastern Atlantic. The catch structure and the contents of the fish stomachs were analysed. The northern, 3000 m deep station in the Iceland Basin exceeded the deeper (4500 m) site in terms of demersal fish biomass and abundance by factors of 5 and 10, respectively. Synaphobranchidae of the species Histiobranchus bathybius and several species of Macrouridae contributed to the samples, the latter dominating with Coryphaenoides (Nematonurus) armatus most abundant in the north and with Coryphaenoides (Chalinura) leptolepis in the south. Diet composition was studied for numerical abundance and wet weight of prey organisms. In the Iceland Basin, where two size classes of C. (N.) armatus could be analysed separately, an ontogenetic change in diet was noted. Individuals shorter than 30 cm in length fed mainly on benthic and epibenthic Crustacea whereas the larger specimens showed a high proportion of fish and Cephalopoda in their stomachs. At the southern site (BIOTRANS station in the West European Basin) Amphipoda, fish and Cephalopoda were the main food source. Coryphaenoides (Chalinura) leptolepis fed on Malacostraca and fish at the northern site, but on Polychaeta and Cephalopoda at the southern station. Stomachs of H. bathybius contained food only in the Iceland Basin. Their diet consisted mainly of Malacostraca and Cephalopoda. Despite their state of digestion, most Cephalopoda could be identified by means of their beaks, which also allowed a recalculation of their original size. The taxonomic composition and the size structure led to the assumption that most Cephalopoda were epi- or mesopelagic and had been preyed upon near the bottom as carcasses. The possible trophic position of the predominant benthopelagic fishes in the benthic boundary layer is discussed.

Observations on deep-sea benthopelagic nekton at two stations in the northern Arabian Sea: links to organic matter supply?

Benthopelagic nekton were sampled by trawl and baited trap arrays at two abyssal stations (WAST and CAST) in the northern Arabian Sea. The natantian decapod Plesiopenaeus armatus and fishes comprised most of the nekton caught by the trawls, whereas the amphipod Eurythenes gryllus dominated in the trap samples. At station WAST, Plesiopenaeus armatus made up 80% of the benthopelagic nekton in terms of biomass, fishes contributing 20%. This relationship was reversed at station CAST. The fishes caught belonged to 13 species in five families. Ophidiidae dominated in terms of abundance, followed by Synodontidae and Ipnopidae. Zoarcidae and Alepocephalidae were scarce, and no Macrouridae were caught. Synodontidae dominated in terms of biomass. Most fish were rather small, rarely exceeding a length of 40 cm. The amphipod Eurythenes gryllus was largely restricted to the traps lying directly at the bottom. A total of only five specimens were captured in pelagic traps from 8 to 500 m above bottom. The predominance of ophidiids and ipnopids, the small mean fish size, and the high standing stocks of natantian decapods suggest that the structure of the Arabian Sea benthopelagic nekton resembles that at abyssal depths in the tropical Atlantic and may be typical for the deep-sea at low latitudes.

Onshore–offshore distribution of Thecosomata (Gastropoda) in the Benguela Current upwelling region off Namibia: species diversity and trophic position

Many Thecosomata (Gastropoda) produce an aragonite shell and are potentially threatened by the increasing ocean acidification. Information about these species is very important for future monitoring of the fate of this group. This paper investigates the distribution, species composition and trophic role of Thecosomata along a transect from the coast into the open ocean off Walvis Bay, Namibia, in September 2010 and January/February 2011. Twenty species were detected, but three taxa ( Limacina bulimoides , Limacina inflata and Desmopterus papilio ) dominated the community with more than 80% of the total standing stock. Diel vertical migration was observed for both Limacina taxa with higher concentrations in surface waters during night. Desmopterus papilio revealed almost no day/night differences. The highest diversities and abundances were detected at the slope and offshore stations, indicating the oceanic preference of this group; some taxa aggregated at the shelf–open ocean interface. δ 15 N measurements confirmed the first trophic level of this group; however, significant differences were detected between seasons with higher values in February 2011. This can be related to differences in seston values as the primary food source. Possible biogeochemical causes for these differences like an exhaustion of the nitrate pool or denitrification processes under suboxic conditions are discussed.

Ecological relevance of salps and doliolids in the northern Benguela Upwelling System

Thaliacea (Tunicata) are fragile pelagic organisms, switching between sexual and asexual reproduction during their life cycle, occasionally occurring in massive blooms. During these blooms they graze large amounts of phytoplankton and microzooplankton, subsequently providing an enhanced carbon flux into deeper layers by sinking of faeces and dead and moribund animals. Seasonal variations and spatial distributions off the northern Namibian coast were analysed for December 2009, September 2010 and February 2011. Three Doliolida and four Salpida taxa were detected with a decreasing diversity towards the coast. Generally, Thaliacea preferred the upper 200 m of the water column but were also found in the mesopelagic zone. The abundances accounted for <1% of the mesozooplankton in most samples, but blooms with up to 63% were detected at some stations during moderate upwelling conditions. Intense upwelling seems to hamper the development of Thaliacea. During bloom periods, the relative abundance of asexually bred blastoor gonozooids was highest. First estimates for a salp bloom off Walvis Bay (~11.000 ind. m, 0–200 m) revealed that 25–100% of the daily primary production was consumed during the bloom. Stable isotope analyses suggest a higher trophic level for Thaliacea than for herbivorous copepods.

Deep-sea fishes from Senghor Seamount and the adjacent abyssal plain (Eastern Central Atlantic)

Senghor Seamount is an important fishing ground around the Cape Verde archipelago in the Eastern Central Atlantic. On an experimental field survey in October 2009 and December 2011, a total of 115 deep-sea fishes of 26 species belonging to 18 families were caught on the seamount summit, along the slopes and on the adjacent abyssal plane, using longlines, fish traps, beam trawl and otter trawl. Here we report seven new records for the Cape Verde deep-sea fish fauna. Most species are known from other areas of the Atlantic Ocean, but our findings are an important contribution to our understanding of the distribution of deep-water fish species. The co-occurrence of northern and southern Atlantic ichthyofauna components at Senghor Seamount, and the Cape Verde area in general, can be attributed to the large-scale hydrographic regime with two water masses merging at the Cape Verde Frontal Zone, the North Atlantic Central Water and the South Atlantic Central Water.