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Bottom Fish Communities of the Mid-ocean Ridge

Gui Menezes and Manuel Biscoito.

Hard work at Madeira. From left: Odd Aksel Bergstad, Gui Menezes and Åge Høines.

Synaphobranchus kaupi

Synaphobranchus kaupi. Photo: Andrey Dolgov.

Coryphaenoides rupestris

Coryphaenoides rupestris. Photo: Alexei Orlov.

Halosauropsis macrochir
Halosauropsis macrochir. Photo: Alexei Orlov.

Madeira coastline
Madeira coastline.

Mid-ocean ridges offer diverse living conditions to deepwater fishes. A wide depth range from the coasts of islands and shallow seamounts to the bottom of fractures at 4000m or more. And the ridges are rugged, with only minor patches of level soft-bottom in-between bare rock and mountainous terrain.

The fish taxonomists associated with MAR-ECO recently completed the re-examination of the big collection of fishes from the near-bottom zone sampled during the 2004 expedition to the mid-Atlantic Ridge. This task is challenging and time-consuming because hundreds of fish had to be examined very carefully to  check characteristics that allow species to be identified and separated. A group that was given particular attention recently was deepwater eels of the family Synaphobranchidae. Ichthyologists Manuel Biscoito from Madeira and and Gui Menezes from the Azores found that two very similar species Synaphobranchus affinis and S. kaupii occurred in the Mar-ECO samples. To tell them apart, details such as teeth numbers and placement must be inspected on every fish.

Near-bottom, or demersal, fishes are those that spend most of their life at or immediately above the seabed. Scientists refer to benthic and benthopelagic species, living at and right above the bottom, respectively.

In a workshop in the beautiful town of Funchal on Madeira, Portugal, Gui Menezes (DOP, Univ. Azores), and Åge Høines and Odd Aksel Bergstad  (Institute of Marine Research, Norway), spent a week together to analyse the patterns of abundance and distribution of the demersal fish along the mid-Atlantic Ridge. The data investigated were catches from the bottom trawls deployed by the RV G.O.Sars in 2004.

Using maps and statistical analyses they analysed what species occurred at what depths and whether there were differences in species composition along and across the ridge. About 70 demersal fish species were identified with certainty, but the collection probably has more. Some difficult groups are still being dealt with by specialists, and some specimens are probably new species to science.

The shallowest areas of the ridge crest are inhabited by a sub-set of species different from that on the slopes and at the foot (rise) of the ridge. Changes in species composition with depth reflect differences in the depth preferences of individual species. And at the very deepest fishing stations, the number of species is low compared with the ridge summit. The overall biomass of fish is also declining with depth, probably because of reduced food supply.

Number of species increases going from the north towards the south along the ridge, and there are some typical “northerners” and “southerners” among the species. Examples are roundnose grenadier (Coryphaenoides rupestris) and Rouleinia attrita, respectively. Others are found almost everywhere, e.g. Halosauropsis macrochir.

Madeira is a magnificent Atlantic island, and spending time analysing fish data from the mid-Atlantic in the peace and quiet of the Marine Biological Station of the City Museum of Funchal was perfect. The station is seaside, and the roaring swells hitting the rocky shore and the many nice fish dishes being served in the cosy restaurants up the hill were particularly inspiring to the “number-crunching” workshop participants. The project and participants are grateful to Manuel Biscoito for his never-ending hospitality in Madeira.

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