Results from the longlinger MS Loran shows that chondrichthyans, it means sharks, skates, rays, and chimeras, dominated the catch by a large margin. It is inferred therefore that this group of fishes are the most abundant top predators of the benthic realm of the Mid-Atlantic Ridge.
By Chip Cotton, Jan Erik Dyb, and Inge Fossen
Skate - dorsal and ventral view
During the summer of 2004 an expedition was undertaken by MAR-ECO scientists to explore the deep-sea realm along the Mid-Atlantic Ridge. To assist in the sampling, a commercial Norwegian fishing vessel, MS Loran, was contracted to sample demersal (bottom-oriented) fishes with baited longlines. We found that chondrichthyans (sharks, skates, rays, and chimeras) dominated the catch by a large margin. It is inferred therefore that this group of fishes are the most abundant top predators of the benthic realm of the Mid-Atlantic Ridge.
This finding was quite different than the data recorded by the MAR-ECO companion ship, RV G. O. Sars, which sampled these areas with a trawl net. Among the tens of thousands of fishes captured by the RV G. O. Sars, only a few were chondrichthyans. This is not surprising as the trawl net was fished in mid-water areas as well as on the bottom. More importantly, an active gear such as a trawl net will generally catch quite different fishes than a passive attractant, such as a baited longline. This is because the larger fishes can easily avoid the trawl net as it is dragged through the water and therefore a different fishing method is required to capture them.
Sharks in continental slopes
Previous studies have shown that continental slopes exhibit the highest diversity of sharks. It is therefore interesting to see if this high diversity also exists on the Mid-Atlantic Ridge. The data from this MAR-ECO survey will help us construct a picture of the demersal fish community that exists along the Ridge. The reason behind the high diversity of “slopewater” sharks has to do with a daily vertical migration of fishes and plankton in the upper 1000 meters of the ocean. In continental slope areas, this daily migration impinges with the bottom and in effect offers daily home delivery of meals to the demersal predators. This “coupling” of bottom predators with the daily vertical migration provides the necessary energy to support such a high level of shark diversity along the continental slopes. Some areas along the Mid-Atlantic Ridge offer a similar “coupling” of the bottom with the daily migration of fishes and plankton. Therefore, it stands to reason that we may find similar diversity in the shark community of the Ridge.
Vulnerable to fishing pressure
Currently, deep sea sharks experience little fishing pressure compared with many other fish species. However, these sharks are likely to be particularly vulnerable to fishing pressure due to their life history. Most data on deep-sea sharks suggest that these sharks live to be quite old and have a low fecundity (number of eggs produced) when compared to their relatives, the bony fishes. To further investigate this theory of high longevity (life-expectancy), many samples were collected on the MAR-ECO cruise to determine the ages of several species of deep-sea sharks. These samples will be processed in the coming months to determine the age structure of deep-sea shark populations.
Found rare deep-sea skates
Among the noteworthy findings of the cruise were the rare deep-sea skates, Bathyraja richardsoni and B. pallida (see picture). Only six specimens of B. pallida have been captured since the original description of the species in 1967. Our catch represents the first record of this species from the Mid-Atlantic Ridge, with the other catch records being reported from the Bay of Biscay and Rockall Trough. Similarly, the 141 specimens of B. richardsoni seem to be the largest recorded number from a single expedition. Also, catsharks of the genus Apristurus are notoriously problematic to identify because most are so infrequently encountered. During the cruise, the initial examination was only able to identify these sharks to the Genus level. Further analysis back at the Bergen Museum indicates that there were at least two different species caught during the MAR-ECO cruise, one of which has not yet been positively identified.
Chondrichthyans captured by longline along the mid-Atlantic ridge