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Fishing for Data

July 17
Day 2: first stage: the trawl

"The sea! the sea! the open sea!, The blue, the fresh, the ever free!" – Bryan W. Procter

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Back of the net: the first IBTS net makes its reappearance from the depths

Having left the undulating stretches of Norwegian coastline overnight and ventured out into the open sea, we steamed north overnight towards the upper limits of our survey area to a latitude of 62°. Far from feeling at one with the ebb and flow of the sea in some sort of foetal state in bed, I actually managed about half an hour's sleep all night. And the wise heads on board have been telling me that this is a calm sea. I'd like to see it lose its temper.

Heading south from our northern tip we performed our first IBTS swoops in two ICES rectangles as well as the separate saithe trawl. Two birds, or fish, with one stone if you like.

But before we get into the fish and monitoring intricacies, there's the small matter of first catching the creatures. This is where I've decided to type the blog entries based on stages of the IBTS operation rather than a daily log, presenting a kind of storyboard of the process rather than the calendric crossing off of days. First up, the trawl. 

Access trawl areas​

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The net at the end of the cables is left in the sea to work its magic

How do the scientists on the expedition know where to fish? Sure, there are the rectangles as guidelines, but what about the swathes of sea area within each? Well, these guys have a database of all IBTS trawls ever made (back to the earliest ones) coupled with trawl positions from other countries (this is stored in ICES DATRAS database) and fishermen – or should I say fisher to be gender-neautral? – records. Sometime the positions this year will coincide with these historical ones and sometimes not; in reality, any and every spot in a rectangle can be chosen as long as the sites are at least ten nautical miles apart. It's semi-random, with some randomness helping to decrease bias. There are limiting factors, however, such as delicate seafloor features that shouldn't be disturbed.

The cruise leader, in this case IMR's Jennifer Devine, makes the ultimate call on where the boat should stop.

As the ship eases down to within 3.5 – 4.5 knots, the crew set to work manoeuvring the gear seawards using a huge winch. In goes the net, the rubber discs that will make contact with the seafloor, then the buoyancies and net itself and finally the otterboards, which are deployed closest to the boat and help with the spreading of the net. ​​At the very top of the formation is the kite - a metal device ​​lined with floats that provides lift and holds the net open to its fullest vertical extent. 

This whole arrangement is draped into the sea with a thick wound wire holding the whole thing to the back of the boat. The gear used for the IBTS is known as a GOV – or Grande Overture Verticale (large vertical opening), unsurprisingly designed by the French.​​

The tows typically last for 30 minutes, although we also carried out a 15 minute one today. Much talk is now of whether the survey needs to be modified in any way based on the amount of new knowledge scientists have accrued. Maybe there's a need for more trawls within each rectangle? This year's survey is experimenting with 15 minute trawls to see whether the abundance indicies​ formed are any less reliable. Pros of the shorter hauls could include more of them in one rectangle (and thus greater survey precision) and the sampling of more habitats and potentially endangered species. 

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Double down: the two nets rest side-by-side

The Johan Hjort has a plumage of gear mounted on her back. Here's some of the equipment on standby. On the right is the IBTS bottom trawl gear, with rubber discs (called rockhoppers) on the left designed to bounce along the sea bottom – the benthos – and to collect samples of the animals that live there: urchins, starfish and so on. On the left is the netting used to capture saithe. This is lined not with rubber discs but with ball bearings, designed to slip over rocks on the seafloor without upsetting them. ​​​

Into the deep

The first trawl went in down to 170 metres – with a maximum of 200m at the stations – but of course the idea is to get a cross-section of depths across the area. This hinges on the physical make-up of the region and whether or not there's any oil activity taking place.

Saithe and sound

Today we also traversed one transect in search of any saithe that might show up on the screen courtesy of the acoustic equipment mounted on the underside of the boat. ​

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There are several different frequencies in simultaneous operation here. As explained to me by cruise leader Jennifer Devine (of IMR), the larger and the redder any patches on the screen, the bigger the fish underfoot​. Could it have been saithe? After all, the species is a 'stealth' one; hard to see on an echosounder. So we towed to find out. And indeed, when the nets were deployed, there were some sizeable ones in the ensuing catch. 

This method is clearly different from the IBTS one, where we call in at stations to trawl. 

And here's some of that saithe!​

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