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Fishing for Data

July 21
Testing the waters

​“Catch a wave, and you’re sitting on top of the world.” ​– The Beach Boys​​.

Yesterday, Monday, was spent cruising southwards again, picking up both IBTS trawls and saithe acoustic sampling on the way. Progress seems to be good – a small trawl net deficiency at one station aside – and I'll update the blog with a cruise map tomorrow.

The big news was that a whale was spotted breaching by the captain. "It doesn't happen very often here," he explained to me later. "We see a lot in the Barents Sea."  

​​​Guding lights
Of course steering and navigating the Johan Hjort is more than a step in a process, it's a round-the-clock endeavour. From the control room at the top of the vessel, known as the bridge, captain John Gerhard Aasen pulls the strings at a desk which holds a suite of screens, panels and buttons. Around the room, which provides a panoramic of the sea, there is also communications equipment, and chairs on either wing from which he can further run the boat. With the Johan Hjort on the water for​​​​​​ 320 days of the year, only the finest, most reliable equipment will do.

captain1.jpg

The focal point of the operation is a high-tech map, which is dotted with geographical and physical information, including the delimited rectangles in which the trawls are conducted and all historical trawl sites mapped by the boat. Using this, John Gerhard can guide the boat to the paths that the vessel will tow down with a few mouse manoeuvres. He can also call up a projected depth range for the path. Other information shown on the map includes oil and gas pipes, oil rigs and oceanographic features such as deeper and shallower water. In the centre is the boat itself, the position visible though GPS.

captain3.jpg

John Gerhard has spent four years in charge of the Johan Hjort and has captained many other vessels mostly in the Barents Sea ("The biggest difference between the North and Barents seas is that there is more traffic here. More oil rigs, oil pipes, gas lines and things like that.").

He explains to me how the trawl is deployed at each site by setting the cable that links the net to the back of the boat into motion. A monitor relays information on the length of the cable and an echosounder shows the trawl nets depth. When bringing it in, he stops the mechanism with 50 metres to spare so that the crew on deck can finish at their own speed (the net sometimes gets tangled towards the end so it helps if they can control the movement of the cable). Once the trawl is in, he gives the green light for the parallel process, the CTD reading.​

captain2.jpg

Sound of the CTD

Alongside the fish-catching part of each trawl, the local environment is also tested for a range of variables, with the seawater being sampled at each station is for its conductivity (C​​) and temperature (T) relative to its depth (D). From the conductivity reading, the water's​​ salinity can then be deduced, and salinity and ​​temperature data can be combined to give the density.

The instrument that takes the measurements is called a CTD (after the things it's monitoring) – a barrel-shaped construction called a rosette which is armed with a temperature and pressure sensors, weights to help submerge it, a remotely-operated cylindrical tube which captures a water sample, and a pump which flushes water through to measure conductivity.

Once the trawl net comes in, the captain gives the go-ahead and the depth to which to lower the gadget. This particular one, manned by deckhand Anders Strønen, is going in at 150 metres and will stop five metres from the seafloor. Here the water is more stable.

The process then goes as follows:

ctd1.jpg

First, a panel large enough to drive a van through is opened on the side of the boat.

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It is then lowered to the required depths. Though here this time, because the CTD happens so quickly after a trawl, the water around is usually carpeted by seabirds waiting for what they think is a​ feeding. 

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It is then underwater for ​​about five minutes.​​

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Then returned to its resting position. The data is delivered automatically to a computer up in the instrument room and stored on a database.

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The tube containing the seawater sample is decanted into a glass bottle.

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Bottles from each station are labelled and boxed up for instrument calibration purposes.​

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The rosette can actually hold up to 12 of these water cylinders. During the hydrographic section of the IBTS - the final leg that will take place after I leave the vessel - experts will analyse the water within these for such things as nutrients and chlorophyll. When the rosette is fully stocked, each of these tubes samples water at a different depth during its ascent from the depths. 

All this hydrographic and environmental information mapped over time provides a valuable record of the conditions within which marine organisms live, offering trends and the ability to produce temperature maps across the surveyed area, amongst other things. 

​I will cover how this information is used by ICES in a later post.

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