Blog post

Sampling the fish

Published on
April 8, 2011

Another two shelf crossings have been completed, resulting in 2 more hauls, targeting blue whiting aggregations.

The first one took place at 01:01 GMT this morning at 56º52.3’N 9º8.7’W close to a steep part of the continental slope. The aggregation was not very dense at all, seen by its colder blue and green colours:


The next aggregations seen further north at 10:31 GMT at position 57º32.2’N 9º30.8’W was more dense, containing warmer, yellow, green and red colours, which indicate higher sound backscatter values:


While hauling the net, the cod-end (last bit of the net) popped to the water surface and could already be seen at a distance away from the vessel. This phenomenon is caused by the rapidly expanding gas-filled swimbladders of the fish when brought up from higher pressure at deeper depths. As a consequence, the fish become more buoyant and “float” at the water surface.

The catch turned out to be relatively “clean” containing blue whiting to a large extent:

However, there are usually also other species present, which are sorted out and also measured:

These were:

and a squid
and a squid
grey gurnards
grey gurnards
horse mackerel
horse mackerel

The blue whiting part of the catch is then sub-sampled: only part of the whole catch is actually measured, but this can then be raised due to statistical laws to be representative of the whole catch.

As demonstrated by Jérôme and Sharon, the fish are then measured for lengths:

The blue whiting are then further divided into lengths classes to take further data of maturity, weight and age:

Dirk Tijssen, a fish biologist from the Danish institute DTU-Aqua divides the blue whiting samples by lengths…

We were glad to see some smaller, 1 and 2 year old blue whiting contained in the sample!

Fish biologist and Imares colleague Thomas Pasterkamp shows how to determine the maturity stage…

…and take out the otoliths (2 per fish) used to determine the age of the fish…


Otoliths are essentially bone structures and part of the inner ear system of the fish. They are primarily responsible for sensing gravity and movement and consist of layers of calcium carbonate, which differ depending on growth conditions: denser layers in summer, less dense in winter. The result are discrete growth rings that resemble tree rings. By counting the rings, it is possible to determine the age of the fish. As otoliths and especially the rings on them are quite small, this has to be done using a microscope!

Here an example microscopic image of a blue whiting otolith showing the distinct growth bands:

In the end, all the data is entered in the data base for subsequent analyses: