From raw data to meaningful analysis – Fisheries Data Collection

Authors: Celina Eriksson Bjånes, Lea Marie Hellenbrecht, Frida Reinsfelt Klubb & Ine Moksness.


On this page, you will be guided through a step-by-step manual of how one collects biological data from a trawl-catch and transferred to digital data ready for analysis.

1.0 Trawl catch

For this catch, a MultiSampler with 3 different cod-ends that sampled at fixed depth layers was used. Once the trawl comes on deck, the catch from different cod-ends is put into buckets and handled independently. Here we expect to get different catches from each depth, depending on the time of day and if it is fixed depth range or follow echo layer.

2.0 Handling the catch in the fishlab

2.1 “Big fish and mixed catch”

The catch is transported to the fish lab, where all the sorting and registration of data is done. To make things easy and thorough, usually one person is in charge of noting all the data (this person keeps track of all the measurement that are taken for that net (catch)). The catch in each bucket is examined separately. First the total weight is registered, then all the big fish, periphylla and red jellyfish is sorted out. These are separated into different species. The rest of the catch is “mixed catch”.

2.2 “Mixed catch: Subsample, sorting and measurement”

The total weight of the mixed catch is registered, and if the mixed catch weighs more than 2kg, then a 1kg heavy subsample is separated and analyzed. It is important that the mixed catch is well homogenised, so that the subsample will give a well estimate of the total catch. When extracting the subsample, make sure to take the whole vertical layer, so the smaller fishes that tend to collect at the bottom are part of the subsample as well.

The subsample of the mixed catch is sorted into species and waste. Then the species are counted and weighed. For the fish species, a hundred individuals are counted and the individual length measurement is registered, as well as the weight for these hundred. If there are less than a hundred, all are length measured and weighed. This data gives a mean weight and length per species. All mesopelagic fish are length measured from snout to caudal peduncle.

2.3 “Measurement of big fish”

The big fish is separated from the mixed catch and sorted into species. Individual measurements are then registered:

      - Total weight and length (round down to nearest cm)
      - Lever weight
      - Stomach: Examine the gallbladder, and estimate stage (sometimes at predator houls, diet analysis is conducted by identify the stomach content, and as much as possible, determine its prey species)
      - Determine the sex, the maturity stage and measure the gonadal weight.
      - Otoliths are extracted and collected.

3.0 Registration of fish data: transferring analogue into digital data

For the registration of the fish data, we used two main sheets for manually filling in the data (a and b), and further two excel-sheets (c and d).

  1. a. The total catch sheet

For each cod-end (T1, T2, T3) one of these sheets was filled out with information about the gear used, depth, total catch weight, mix catch and number of each species (over 100 specimens -> subsample)

  1. b. Length measurement sheet

All of the fish was length measured. However if we got more than 100 specimens of one species, subsampling was done. Each species have one sheet, and one mark on the sheet is for one fish. In this example the length of 100 Benthosema Glaciale were measured.

The information from these two sheets was transferred into two excel-sheets (c and d) for later analysis and statistics.

  1. c. The total catch Excel-sheet (Excel sheet for pelagic Data Collection)

To this sheet you transfer the information from you first manual sheet (a). If you don't know what to fill in on some of the columns, use the “Coding”-sheet to find out.

Every catch (also meaning T1, T2 and T3) has each one line of information, and an unique serial number.


  1. d. The length measurement excel-sheet

In this sheet you fill in the length measured for each species from (b)-sheet. Additionally you have to fill in some of the same information together with some station information as in the last Excel-sheet (c).

How to fill in step-by-step:

Excel Sheet for pelagic Data Collection (c)

General information about the station:

  • What kind of gear was used?
  • When, where, how long/deep?

Use the “coding”-sheet shown with an arrow to find out what to fill in if you do not understand the heading.

Coding sheet:

This sheet gives you guidelines in what to fill in. For example what kind of sampling was conducted? Use the codes.

Remember to put the right cod-end number. T1 is the deepest one, then T2 and T3 for most shallow.

From sheet-(a) you will have the number of total catch weight. Fill this in, together with the total catch for each species.


  • (fill inn numbers of specimens)
  • B.glaciale.g (how many grams?)


If we get a lot of specimens of one species, we take a subsample of 100 specimens to save time.

To find out how many specimens we had in total, we use this equation:


Not all the species are listed in this sheet, if you get other species, they are filled out under “other” together with discard. Remember to add number of specimens and the weight of these species.

Length measurements Excel-sheet (d)

Fill out the station data in the first columns, it is the same information as the previous Excel-sheet (c).

Use the sheet (b) where you marked the length of each specimen of each species. This information will be put in this Excel-sheet.

The general information is plotted in (same as Excel sheet (c))

In our example we use the length of Benthosema glaciale.

We have 100 subsamples of this, therefore we can list B.glaciale hundred times for hundred different length measurements. Furthermore we fill in the length of each specimen.

The length (in millimeters) is filled in for all the B.glaciales.