28 January 2002 - A fishy tale or two....
RRS James Clark Ross Diary
Position at 1200: 53° 34.1'S, 37° 41.3'W - 30 Nautical Miles, NNE of Bird Island
Distance Travelled since Grimsby: 28143 Nautical Miles
Air temperature: 5.3°C; Sea temperature: 4.1°C
Current, frequent weather observations reported back to BAS Headquarters in Cambridge is used to plot the ship's current position and recent track. Meteorological data are also available from this page. The callsign of RRS James Clark Ross is ZDLP.
A fishy tale or two....and some krill for good measure!
This week is going to eclipse even last weeks in terms of content, I think. This means that either people are having great results from their research, or have too much time on their hands! No seriously we have more very good contributors this week, so first a quick resume of what we have been up to, and then over to them.
We finished off the Eastern Core Box (ECB) with the last of the drifter buoys ( see last week ) being deployed, and some more CTD's and target fishing. Since then we have moved back to the West end of South Georgia and are doing the Western Core Box (WCB) all over again.
While we were over there on the Eastern end it was good to have a chat on the VHF radio to both the Ernest Shackleton and the Dorada as well as an ex Third Officer off the JCR - Paul Heslop, who is now Chief Officer on the cruise ship Explorer. There are also a number of fishing vessels and yachts around the area, and you would be surprised how busy this supposedly remote area of the world is - as if we don't have enough to worry about with just the fog and ice!
So without further ado or rambling, the science bit ..... The keener among you may have noticed that some of the same people keep cropping up in each weeks science bit as well as us mentioning the same stuff quite often; this just goes to prove that all of this research is linked together somehow!
So how many krill are there at South Georgia? - By Rachael Shreeve
Blue whales eat up to 4 tonnes of krill every day, enough to fill two VW beetles!! Fur seals crowd the beaches around South Georgia during their breeding season, and hundreds of thousands of penguins breed around South Georgia. All of them depend on krill for food. Can you imagine how much krill is taken daily by these higher predators? (Coming up next week!) There is also a fishery for krill in this area, and this has prompted questions about how many krill are present, how fast they grow, and ultimately how many tonnes could be taken annually by the fishery and leave enough for the wildlife. This is one of the key questions that we are trying to answer on this cruise. To find out how many krill are around we use acoustics. (See acoustics below).
How fast do they grow?
To understand how we measure krill growth, it will help if we first explain how they grow. Krill are like prawns, having their skeletons on the outside. It is a bit like a suit of armour, a hard layer protecting their body. However, if you imagine being put into a suit of armour when you are born, you would have to change it many times before you were fully grown, and this is exactly what krill do. Every couple of weeks they moult the old skeleton and form a new, bigger one. We can work out their growth rates by taking freshly caught animals and incubating them in individual pots. Those that have moulted have their old and new shells measured, and from this we can work out the growth rate. So far we have found some high growth rates in parts of the South Georgia region. Although cold (about 3°C) the waters here have plenty of their favourite food: phytoplankton.
Krill’s smaller relatives
We are also looking at the really small animals in the water, the copepods (pronounced Cope-e–pods). Although only the size of a grain of rice, they occur almost everywhere, and their relatives can be found in your fresh water pond at home. Copepods are eaten by larval fish, birds and even whales, so are just as important as krill in the marine food web. We catch copepods in the ‘Bongo net’, which has changed little from the nets used on the Discovery Expeditions that Pete talked about last week. We want to know how abundant and how fast copepods grow in relation to krill.
As an interesting comparison our resident photo-artists better known as Nathan and Alex have put together a little series of piccies showing how bongo nets have not really changed that much over the years, along with the PSO's hats (look carefully on the deck of Discovery) The safety equipment the crew are wearing has changed mind you! From left to right: Discovery, Endurance, James Clark Ross
A short day in the life of the great krill hunters.....aka Cathy Goss and Jon Watkins
Just like the submarine in Hunt for Red October, as she travels on our survey route the James Clark Ross is sending out underwater sound pings and listening for the echoes that return. The target for the submarine was another submarine, but in our case our scientific echo sounders are tuned to detect fish, krill and even zooplankton. If we ever sailed over a submarine, our display screens would fill with a giant red blob. The ship has several other sounding devices – also called SONAR (SOund NAvigation and Ranging) – they are designed to detect the seabed when it is thousands of metres below us (bathymetric sounder), to measure movement in the water column (Acoustic Doppler Current Profiler, ADCP) and to record the depth of the seabed in a wide swathe (swath bathymetry - Ed's note....for swath info see previous cruises or wait until April when we are actually on a swath cruise!). We generate so many pings, all at different sound frequencies, that we need a device to make sure they all ping together at the same time (a 'trigger'). By doing that, we know from the time of the returning echoes how far away the targets are that we're interested in. We can map out the strength of echoes on a continuous chart of depth below us (range) against time (or distance) – an echochart.
Right at the start of the cruise we sailed over huge swarms of targets that our acoustic sensors told us weren't krill (and krill usually seem to dominate round here). Dense layers and patches appeared on the echocharts from the lowest frequency we use, but were washed out at the higher frequencies, the exact opposite of krill patches when they appear on our charts. The bridge echosounder, set up to record the depth of the seabed, responded to the schools too – mysterious shapes spreading through the water column. Since this sounder uses the lowest frequency of all on the ship, this is a clue to the identity of the creatures causing the echoes. We trawled our plankton net through the thick of the marks we could see, for up to 10 minutes (nearly a kilometre) but were rewarded with a tiny catch. Tiny but significant. If the patches had been caused by krill a trawl like that would have been full to bursting. Instead we caught a handful of myctophiids.
Myctophiid fish, lantern fish, Mick-the-Fid to mention their nicest names, are often fishermen's least favourite catch. Small, oily and quick to deteriorate, they aren't even usable for fish-meal – their eyeballs clog the processing machinery. Perhaps that's why there are so many about. The few we had caught were stunning – the plankton net had solid cod-ends, designed to catch live specimens, and in this case the fish had kept many of their mirror scales, so often lost when they are caught in ordinary nets. Most were less than 10cm long, but huge mouths and huge eyes – often reflecting emerald green in the artificial light of the laboratory, dominate their faces. Counted, measured and volumed, the treasured specimens were frozen so that they could be carefully weighed and analysed in Cambridge. The aim of this analysis is to arrive at a value for the echo strength of one fish (jargon term ‘target strength’). The target strength calculation will then allow an estimate to be made of the fish contained in those huge marks that caught our attention.
"So what do they eat?" asked everyone. Peter was persuaded to open one up to see. After a few minutes came the half expected answer ‘Themisto’. ‘Themisto’ are everywhere. Tenacious, tough, wiry and spiky these crustaceans, the size of a pea, are found in most plankton hauls where they eat almost anything. One Themisto in one myctophiid is hardly a representative sample, but the rest of the specimens must be kept whole for other analyses, and we will have to answer the diet question later.........
Written by Cathy Goss
Stromness....... through the eyes of a South GeorgiaG virgin........By Matt Swarbrick
(This should have been included in with last week but Matt was even later than last weeks update!!....Ed)
In a foggy sea, such as the one we had been in for the week, one could be anywhere and were it not for the wealth of wildlife all around the ship, and the frequent icebergs, I would not have believed the crew that we were near South Georgia at all!
However, on the morning that we sailed into Stromness; the fog lifted, the sun shone, and the birds (though mainly the noisy fur seals) defiantly sang. All in all not a thing could be wrong in the world - apart from the fact that we had not as yet been told we could go ashore, and there was still the risk that we may not be allowed off the ship. After a couple of tense hours however notice was given, carrying with it strict warning not to approach the old whaling station as it is now very ramshackle and dangerous. The ship was anchored in the bay, and we were run ashore in the ships Humbers (small inflatable boats).
For a land lubber like me, it would have been great just to smell grass, but here you could (mainly due to the fur seals) both smell and see a lot more. We landed on the shore in a small, and very green valley surrounded on all sides by some of the great and mean looking peaks which characterize South Georgia. It seemed that to even get past the jetty we had to push our way past lots of elephant seals. As the world's biggest seal, they are just astonishing. The seals at Stromness were small in comparison to the big males, but even some of these measured over ten foot of pure blubbery seal.
Matt's birdwatching buddy, Daffyd , takes a closer look at an elephant seal, and the seals have a good wallow! Click on the images to enlarge them.
The whole valley was waddling with wildlife. It seemed so strange, that not only did all the wildlife mix happily amongst one another, but that they also let us humans mix with them, and hardly batted an eyelid at our presence. It seemed strange that whilst they saw so little threat in us, many were sat among the ruins of a building, which amongst others, was responsible for the death of a great number of the world's whales and seals. It seemed as if seals now inhabited the houses of those who used to hunt them!
We all stretched our legs, and many also their camera lenses to take photos of the fur seals, king and gentoo penguins, terns, and skuas. There was also a lot of reindeer which were introduced by the whalers for food. Many of us also stretched our legs a bit further and visited the waterfall that Shackleton climbed down at the end of his great journey, before seeking help at the whaling station.
Although I believe that everybody came on this cruise with the sole purposes of answering questions about the secret lives (and they do seem very secret) of Krill and the South Georgian ecosystem, one thing that everybody has in common is the love of this great ecosystem. And it is days like these which keep that love going for years to come.
Quote of the week....!
"Has the weather significantly improved?" Nathan, whose role as watch leader has given him ambitions for higher things...watch out Pete!
Cathy for the fishy tale, Matt for his views on Stromness, Rachael for krilly bongos and to Nathan and Alex for some good photos.
Next weeks delights.......
We are hoping to persuade our higher predator observers (bird watchers to me and you) down from their 'perch' on the monkey island for long enough to tell us what they have seen over the last few weeks.....and it is a lot, let me tell you!
Also we will be dropping Daffyd back into Bird Island after his summer hols on the JCR, and finishing off the last transect.