Jul 19
Stage three - taking the details

"Measure what is measurable, and make measurable what is not so." Galileo Galilei

The survey continues at a steady pace, and operations on the Johan Hjort now seem to operate like clockwork. Having steamed southwards from the 62nd parallel north, making trawl-stops at IBTS stations and sniffing out saithe in the process, we've spent the last day heading back up north slightly to the west. From a satellite viewpoint, our vessel's leaving a pattern like the teeth of a comb vertically over the water. Eventually we'll skirt the outer edge of UK waters, although being a Norwegian operation and not part of the EU, we can't go within 12km of the coastline to sample.

Made to measure

Time then to focus on the second step in the IBTS timeline – the measuring, weighing and noting of other key biological features.

By now our sea creatures have been organized into species as far as possible. The scientists working that particular shift weigh them collectively as species and then set about lining them up on electronic measuring boards.

A basket of saithe on the scales

All fish and shellfish pulled up by the trawls – both the open-water species and the deep-water/sea bottom ones – have their lengths taken and stashed in on-board databases, whether a target species (so one of the commercial ones the survey has set out to specifically examine) or not. How long a target species is, coupled with its age, becomes the foundation for valuable output further down the line.


The brilliantly-named 'greater forkbeard' on the measuring board. The scientists agreed this particular specimen was larger than usual.

I watch scientist Silje Elisabeth Seim, a researcher specialising in demersal fish, age determination and diet analysis, as she weighs and measures some of the morning's specimens.

First, she calibrates the board, before entering a species-specific code so the board knows what fish it is dealing with.

Next she lays the fish out and measures it from the tip of its snout to the tip of its caudal find. It's important to note that all fish are measured in terms of a length class, or a bracket within which the actual length falls. For the demersal commercial species (cod, hake, et al) and the mackerel (which is pelagic), this is 1cm and for the rest of the pelagics it's 0.5 as they tend to be smaller.

So whilst the board shows an exact length in centimetres, what's important is the category. A fish measuring 23.4 centimetres, therefore, would fall into the 23 centimetre length class. 


Silje calibrating the board


A pelagic Norway pout being measured on the smaller scale. 

Lending an ear
If the fish is a non-target one, then after she takes the length and weight that's job done and she can move on to the next. However for the commercial ones of interest to the survey, the board prompts her for a larger profile of the fish – so sex and age. For the former that means removing the relevant reproductive parts; for the latter that means removing the otoliths – the tiny calcium carbonate ear stones buried behind the fish's brains whose rings tell us the age of an individual, like with a tree.

A certain amount of otoliths are removed per 1 or 0.5 centimetre length class. This differs depending on the species.​ The board will indicate whether an otolith removal is required or whether the quota has been filled. 



Top: the tiny ear stones known as otoliths are picked out; bottom: reprodutive parts

The information varies according to the taxonomic rank. Skates, rays and sharks (known as elasmobranchs due to their cartilaginous rather than bony skeletons) have their length, weight and sex recorded. Lots of these are kept alive in water after being caught and are returned to the sea after being quickly examined. Bottom organisms like urchins and starfish, meanwhile, are identified, counted and weighed. For crustaceans it's like with fish: parameters taken depend on whether the species is a targeted commercial one or not.


A blackmouth catshark. See if you can identify its gender. 

All data is wired directly to a computer from the boards for storing and processing. Each creature looked at is entered and listed with the details taken on it alongside auxiliary statistics such as the station at which it was caught, the date, the vessel, the type of trawling gear used and so on. This is used to generate graphs of species length distribtion ​over what are known as 'roundfish areas' (delineated zones over the survey area larger than the statistical rectangles)

The data is used to generate what is known as an 'age-length key' – and this gets stored in ICES DATRAS database for the experts to pull out and use in stock assessments and modelling. This will be covered in more detail in a later post.

But biological data such as length also feeds into equations that help generate estimates of biomass (the total weight of individuals in a stock), crucial in stock assessments.

Each and every piece of data recorded is submitted to the DATRAS database at ICES, once they've been processed by the respective marine institute – in this case Norway's IMR.​inger.jpg


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