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6th Cruise Diary:Bottom trawl catches

R/V Fridtjof Nansen left Nouadhibou harbor in Mauretania the 16’Th of November loaded with excited scientists from 6 nations (Senegal, Mauretania, Morocco, Spain, UK and Norway). Since it has been change of crew and scientific personnel at the same time, it does take some time before everybody get into all the routines, get familiar with each other and the work onboard, but now we are on track.

Part of the cruise strategy is to assess the bottom (demersal) fauna and biodiversity of deep and shallow areas. This is typically done by demersal trawls in areas that are neither too steep nor rough nor covered in vulnerable species such as corals.

The management of marine living resources in the Canary current region is dependent on reliable identification of all marine species in the area, both to understand their population structure, distribution area, but also to help understand the different species importance as a food source or as predators. One of our first demersal trawls caused much excitement. Highlights of this trawl were the capture of a small and gelatinous Octopus (Pteroctopus tetracirrhus, fourhorn octopus). Luckily we have several octopus experts onboard (Prof. Fransisco Ramil and Dr. Addellatif Bomaaz) and the species was identified immediately.

What is most typical for this species is that it looks like it has 4 eyes (see picture). In addition they have four organs above each eye. The functions they have are not clear. Individuals normally live in two years and can be 28 cm in total length and 16 cm mantel length. The individual we caught had a mantle length of 9 cm. They live normally between 25-720 meters depth. This specimen was caught in the southern part of its distribution area. The diet of the octopus is based on crustaceans and teleosts. The octopus shows a marked preference for the benthic fish Symphurus nigrescens and the endobenthic crustacean Alpheus glaber. The bathymetric distribution of P. tetracirrhus coincides with those of these two main prey, which suggests that the distribution of the octopus might be strongly linked to its trophic resources.

Another octopus which is much more common in this area is the common octpus (Octopus vulgaris). O. vulgaris grows to 25 cm in mantle length with arms up to 1 m long. O. vulgaris is caught by bottom trawls and passive gears (special hooks and pots). In this area the catches is decreasing from 100 000 tonnes in 2000 to 35 000 tonnes in 2010. It is a bentic species which grow very rapidly. It lives for 15 months. The females do not eat while they guarded the eggs and they become an easy prey for predators, so most of them die after spawning.

The main spawning period is during spring. The Common Octopus hunts at dusk. Crabs, crayfish, and bivalve mollusks (two-shelled molluscs such as cockles) are preferred, although the octopus will eat almost anything it can catch. It is able to change colour to blend in with its surroundings, and is able to jump upon any unwary prey that strays across its path. The prey is paralyzed by a nerve poison, which the octopus secretes, and the octopus is able to grasp its prey using its powerful tentacles with their two rows of suckers. If the victim is a shelled mollusc, the octopus uses its beak to punch a hole in the shell before sucking out the fleshy contents. Training experiments have shown that the Common Octopus can distinguish the brightness, size, shape, and horizontal or vertical orientation of objects.

Many types of fish arrives in the bottom trawl catches and among them are several species that we normally like to define as pelagic. These are sardines (Sardina pilchardus), sardinella, horse mackerel and mackerel. One of the mackerel species does have swimbladder, which is not so common for this genus. Some species are easy to identify, like lobster (Palinurus mauritanicus), sea bream (Pagrus auriga), monkfish (Lophius budegassa) and John dory (Zeus faber). These are all species that tastes very good!!! Those we do conserve and bring with us to Norway for more detailed analyses and classification. Some species are very colorful and very nice to look at (see picture). Others are not so “nice” like the stingray (Dasyaties centroura ), Conger eel (Conger conger), weever fish (Trachinus draco). All being venomous or “dangerous”. Others species do not agree with all characteristics provided by species keys.

Contact

Kathrine Michalsen
454 29 971