The echogram contains distinctive patterns which are characteristic of certain fish species. These patterns are called echotraces or marks. For example, the tall dark pillar like objects on the echogram are schools of herring. These schools can be very big. The larger one is 40 metres in height and 140 metres in length. Note that the echogram is a little distorted - the horizontal axis is on a different scale to the vertical axis: this results in objects appearing much taller and thinner than they really are.
What Does the Echogram Tell Us?
To confirm the identification of the echotrace, a trawl sample is taken. The echogram indicates exactly where in the water the fish school is, so the trawl is directed straight at it to obtain the sample.
This identification of echotraces is known as ground truthing after a similar type of identification process in the science of satellite imagery (a person is required on the ground to tell the truth about what the satellite is detecting). In the case of fisheries, the nearest thing to the truth is what can be caught with a trawl. An echosounder mounted in the mouth of the trawl (a netsonde) allows for reasonable verification that the trawl is not telling any lies!
Once the species and size of the fish which have contributed to the echotrace are known, the echo intensity can be converted to fish density (numbers of fish per unit distance). Fish densities are therefore measured all along the track of the ship. The ship's track is designed to survey the entire area thought to be occupied by the whole fish stock.
(A cruise track (black solid line) for the west of Scotland herring acoustic survey in 2002.)
The density is then averaged (or interpolated) and multiplied by the area surveyed to produce an estimate of total numbers (total numbers = density x area). The numbers can be be divided into age groups. The fish in the trawl samples are aged using standard techniques to determine the proportion of fish at age.
Indices of Abundance
The actual results of an acoustic survey serve as an index of abundance at age of the particular fish stock. An index is a relative estimate; this means that the absolute numbers are not known, but the change in numbers from year to year is known - i.e. it is known in any one year how many fish there are relative to previous years. One of the main reasons for this is because there is still uncertainty about the acoustic properties of the fish - the so-called target strength.