Date:June 15, 2004
Author: Dr. Leif Nøttestad and Dr. Olav Rune Godø, Institute of Marine Research
A sperm whale
We accumulate an array of data day by day that gradually improves our knowledge about the ecosystem along the Mid-Atlantic Ridge (MAR). The hydro-acoustic system onboard G. O. Sars provides us with a broad view of marine life through 5 acoustic frequencies (18-200 kHz). Today there have been vigorous discussions among scientists working on trawl catches, acoustics and marine mammals / seabirds. A key ecological question is the connection between the acoustic layers seen on the echosounders, and the species and groups of species identified in the trawl catches as well as surface sightings of whales and dolphins. Some highlights are illustrated in the figures.
1. We can see fish on more than 2000 m depth. The Simrad EK 18 kHz echosounder system covers the depths well below 2000 m and we can actually see traces of fish along the bottom here (Figure 1). These exceptional recordings are also partly due to the low noise level of G.O. Sars. Precise trawling down to these great depths has been possible due to the newly developed long-range trawl instrumentation from Scanmar. This has definitely improved our ability to reach the deep with remote sensing technology. The catches from these recordings were a mixture of species where Maulisia sp. and Bothilagus auriops made up an important component. Most deepwater pelagic species have no swimbladder and therefore only reflect a weak echo. We are amazed by what can be achieved at deep water with appropriate instrumentation and a silent vessel.
Figure 1. Echogram from 18 kHz echo sounder reaching bottom at 2300 m. The echogram is partially contaminated by propeller noise and disturbance during trawling.
2. Marine life on MAR is structured. A surface layer (0-100 m) and mid-water layer (200-800 m) have been persistent over large areas along MAR (Figure 2). When passing over Seamounts (underwater mountains) we observed additional activity on the echograms associated with more baleen whale and sperm whale activity at the surface. A hypothesis is emerging: Are these large whales linked to Seamounts along MAR? What species do they eat and how do they manage to hunt for prey species down there in the deep? Time will show.
Figure 2. A picture of the activity of the day drawn onto an echogram (time is UTC). An echogram is the picture given by an echo sounder, and the structure and intensity tell us about the density and type of animals recorded. Vertical migration is indicated bychanges in the depths of the acosutic layers. Above the echogram is indicted the relative frequency of occurrence of sea mammels during that day.
3. We have a challenge to link the complex acoustic layer structure (Figure 2) to the true species composition. This will be an important issue for the rest of the cruise. If we can solve this overriding issue we will be able, in the end of the cruise, to present distribution patterns of various species and groups of species in space and time.
And finally, this evening, after several days of continuous searching a sperm whale was tagged with a satellite tag, hopefully giving us update of its behaviour in coming days.
Breeze of variable strength, cloudy, and good working conditions.
Tomorrows expected highlights
Tomorrow we will approach the fascinating Charlie-Gibbs Fracture Zone. Everybody is excited to see what meets us in this spectacular landscape with underwater mountain peaks stretching from 4000 m depth up to 1000 m depth over a distance of only a few kilometers.