We have now been back from the Sargasso Sea for some weeks and have reflected a bit on the outcome. One of the main projects on the expedition was to get an impression on the amount of eel larvae out there and, in this way, get an indirect measure of the amount of adults (that must have been there to spawn the eggs).
I was onboard Dana on the first part of the expedition, where we covered the 3 most westerly transects. As you might remember I reported quite low catches of eel larvae and hoped that the spawning area had shifted a bit to the east. This turned out not to be the case. Also on the more eastern transect the catches were quite low.
Thus we have to conclude – preliminary – that the amount of adult eel coming to the spawning area has unfortunately not increased substantially. Therefore, the improvement in the amount of glass eel coming to Europe in the most recent 3 years is more likely to be due to better ocean currents and the like than to sufficient management in Europe of the eel stock in Europe.
Seen from a scientific point of view, one very good discovery on the expedition was that the eel larvae are very evenly distributed in the Sargasso Sea. It was not the case that in a few hauls we caught thousands of larvae and in others only a handful. We caught a few in most hauls and never more than 65 eel larvae. This means that it is possible to monitor on a routine basis, say every 2nd year, the amount of spawners in the eel stock and in that way keep track of the eel management in Europe - does it work or not?
My judgment is that a three-week survey in the area will give a precise picture of the situation. It could be done as a European cooperation with USA and Canada because the American eel species is also spawning in the Sargasso Sea.
Scientifically, the expedition went extremely well I think. The entire ecosystem was investigated from physical-chemical features, bacteria, nana-plankton, etc all the way up to the top predators. The energy requirements and swimming speed of eel larvae, bacteria on fish egg surfaces and eel genetics were also covered. There was hectic activity on the deck and in the laboratories 24/7. A lot of data and samples have been collected and almost all went very well. I am sure that when all this is worked up and analyzed it will represent a large step forward for the science of the Sargasso Sea ecosystem and for the ocean phase of eel especially.
The eel larvae fishing positions in the Sargasso Sea and beyond.
the Azores suddenly appeared on the horizon after 16 days at sea with no land
Finishing the second leg of the cruise I must say that the cruise have been both fruitful and an unforgettable experience. We are very happy to have added additional 6 short transects to the 3 longer ones done during leg 1, leaving very comprehensive data material for the coming years of analysis and publications.
left the Azores this afternoon going towards Denmark on leg 3 with a new set of
scientists and students, and tomorrow I will follow by airplane together with my
We are now as far away from land as possible in the Atlantic Ocean – sampling directly above the Mid Atlantic Ridge.
water composition has changed quite dramatically - going from warm saline water to
less saline and colder water. The phytoplankton subsurface peak has moved
upwards from 150 to 110 meters.
not been catching any European eel larvae lately but instead the much talked-about box
jellyfish whose relative in Australia is said to be able to kill a man.
now approximately halfway between Bermuda and the Azores. Everything has more
or less worked out as we expected. Going eastwards, we are still catching eel
larvae and our hope is that this trend will continue.
Otherwise, we have all gotten into our daily routines. Breakfast at 7:30, lunch at 11:30, dinner at 17:30, bacon for breakfast on Wednesdays and wine at lunchtime on Sundays. Working around the clock, sleeping and exercising whenever time permits in between.
The weather has been rather good to us even though sleeping in the bow of the ship is challenging now and again.
We are now heading out into the Sargasso Sea once again - to continue the work done during the previous leg by Henrik et al. The first leg covered three long transect south of Bermuda. On the second leg we will be heading southeast, covering 5-6 short transects towards the Azores. Cannot wait to reach the first station around midnight.
The journey to Bermuda went fine and yesterday Dana came into St. George's Parish to exchange crew, scientists, and experience.
Henrik told me that they had a very nice cruise - with quite rough weather - and a very good catch of eel larvae, nicely reflecting the hydrographic front both vertical and horizontal.
My task on board Dana will be exactly this e.g. to map the hydrographic fronts by operating the standard CTD and the underway U-CTD along the coming transects … more on that later.
We are working our way thorugh transect 3 which should be in the core area of the European eel spawning area. We had a very succesfull day and night a day ago with our vertical eel larvae distribution sampling. We were lucky to choose a station with many eel larvae - that makes it easier to get a clear picture of the vertical movement of the eel larvae between day and night. In total we had almost 200 eel larvae. However, further up the transect the numbers are only modest. Not several times the Galathea III catch, as we had hoped. So a very preliminary and tentative conclusion is that there are not more mature eel this year than in 2007. Thus, the increase in glass eel we have seen coming to Europe the last 3 years seems not to be due to increased numbers of adult eel here at the spawning area but must be ascribed to improved survival of eel larvae at sea and better transport across the Atlantic . ....but this is only very preliminary - and there are several more areas to be covered and I am especially keen to see the results from the most easterly stations - the eastern limit of the eel spawning area has never been properly investigated before - that will be when Hjalte take over from me next week.
The weather has been beautiful today – sun and light wind. We have been at the same position all day and will stay here all night. We are conducting a special study on the vertical distribution of the eel larvae by making separate horizontal hauls at different depths. The result from the day hauls is that the eel larvae stay 10-30m below a distinct maximum algae layer at around 120m. We got a record catch (for Galathea III in 2007 and this cruise) of 34 eel larvae here in one haul and only a few in other layers. We expect that the larvae probably will move up to or a little higher than the algae layer at night time–we will see. Now the new moon is over – yesterday night – we can hope to find eel eggs (never done before) the next two days. After two days they will have hatched and the small larvae might be difficult to find here at sea – that will be for later analysis back home in the lab.
The weather has improved and we are now in full swing again. We are now in the new moon phase, so we will fish with a large pelagic trawl for spawning eel. The experience from the Japanese eel research is that this is when they spawn. We hope to see on our echo sounders that some fish are coming up from the deep water and aggregating at a depth of between 150-350 m. If we see this, we will target it with the trawl, because this is the likely depth of eel spawning – but nobody really knows. Yesterday night we did not see such a thing happening, but now we are closer to the new moon and close to the center of the eel spawning area. Last night we got quite a lot of deep scattering layer fish of 5- 15 cm - some looking quite scary, like this one.
Big waves have now prevented us from working for a day or so, but we plan to start again tomorrow at 8:00 - hopefully. I am not sure how high the waves were - that is difficult to judge - but some on the ship say 8-10m. Several of us were a bit weak and tired.
We are now steaming south and out of the bad weather. Not that the wind is very strong - it was 20m/s and is now about 12m/s - but it is the swell which is giving us problems. However, the echo sounders are running and we are now 3 days from a new moon, so it is now and over the next 3 days that the adult eel should move up from the deep waters to about 100-200m under the surface for spawning. If they do, we should be able to see them on the echo sounders....we will see.
It took us some days to get the acoustic equipment running appropriately. Now it has been running for some days and it reveals some interesting features. It seems that also here in the marine desert of the Sargasso Sea there is a so-called deep scattering layer - a layer of small fish and crustaceans at a depth of about 500m which moves up at night to about 100m..probably to feed... and down to 500m at day time to avoid being eaten by larger fish. This layer must be very important for the functioning of the ecosystem out here. Dana is very well equipped with 3 acoustic echo sounders 18 kHz, 38 kHz and 120 kHz and it might be the first time such good echo sounders have been used in the eel area. We have also seen some exiting schools of fish on deep water which - dare I say it - might be adult eels.
ICES crew onboard RV Dana, sailing the Sargasso Sea with the Danish Eel Expedition 2014 by DTU Aqua.
Henrik Sparholt, Deputy Head of Advisory Programme, ICES
Onboard Dana 13 March to 7 April 2014.
Hjalte Parner, Oceanography Data Scientist, ICES
Onboard Dana 5 - 25 April 2014.