|South Atlantic cruise on "RV Akademik Ioffe" 2009 |
31 October – 7 November
Author: Stas Kobyliansky
The first two SA MAR-ECO superstations (SST) were conducted north of the Romanche fracture zone. Four samples were collected by Isaac-Kidd Midwater Trawl (IKMT) at these stations; two of them focused on the composition and distribution of pelagic fishes inhabiting the sound-scattering layers (SSL). According our echosounder data, the SSL in this study area demonstrated interesting daily dynamics: during the day, there is only one common sound-scattering structure, located at 200 to 450 metres depth, but at night, the SSL is composed of two layers. The first rises up to the surface and has not been observed deeper than 50 metres. The other one remains at the previous (daytime) depth, and after sunrise both SSLs return to the deeper, common one. Because of these patterns, our trawl methods differ according to time of day. In the daytime, we only target the deep SSL, while at night, we sample both the deep and surface SSLs.
The ichthyocoene composition of the SSL at the first two superstations was dominated by fishes of the families Phosichthyidae (Vinciguerria nimbaria) and Myctophidae (Diogenichthys atlanticus, Myctophum cf. asperum, M. nitidulum, M.affine, etc.). Vinciguerria nimbaria was most predominant, comprising more than 90% of all individuals caught. The catches also included the larvae, juveniles, and adults of several other pelagic fish families: Trachipteridae, Tetraodontidae, Sternoptychidae, Gempylidae, Zenionidae.
The important objective of two other hauls was to determine the qualitative composition of ichthyofauna of the lower, middle and upper mesopelagic strata over the Mid-Atlantic Ridge (MAR), looking in particular for possible latitudinal changes. One of these hauls was carried out down to 1000 metres, above the seafloor in the vicinity of the MAR. The other one was conducted above the peak of a seamount, at 800m depth. Both trawls were carried out at 100 metres increments, with a sampling duration of 6 to 10 minutes at each stratum.
The first deep-water haul brought in over 20 deep-water species belonging to 11 families. The most abundant were representatives of Gonostomatidae (Cyclotone spp.), Sternoptychidae (Sternoptyx diaphana and Argyropelecus hemigymnus), and Myctophidae (Lampanyctus alatus, L. nobilis, Lepidophanes gaussi, Hygophum macrochir). The rare and poorly studied species Stylephorus chordatus (Stylephoridae) was also found. The second deep-water catch included at least 36 species from 13 families. The most abundant groups in this catch were also Cyclotone spp. (with about 10 times more abundant than in the previous deep-water trawl), Vinciguerria nimbaria and some Myctophidae (Hygophum macrochir, Diogenichthys atlanticus, Diaphus problematicus, Lepidophanes guentheri, etc.). Interestingly, in the catch above the seamount, some bentho-pelagic species also appeared such as Maurolicus walvisensis (Sternoptychidae) and Diretmus argenteus (Diretmidae). This may indicate a partial overlap of two types of ichthyofaunas, i.e. megopelagic (including SSL fishes) and demersal (the latter only characteristic of underwater elevations), over the seamount. Such “mixing” is likely associated with the wide, flat seamount top, which might concentrate pelagic species by not allowing them to descend to deeper layers during the daytime.
The captures of so many mesopelagic fishes provided numerous, excellent material for collections, as well as tissue samples for further genetic analysis of population structure, species identification, and phylogenetic relationships of different fish species.