Monday, 19 October 2009

The sound of (sound) waves: What is fisheries acoustics?

Fisheries acoustics rely on the physics of sound travelling through water to quantify the distribution of biota in the water column. By sending a signal of a given frequency through the water column and recording the time of travel and the strength of the reflected signal, it is possible to determine the size and location of fish and estimate biomass from the acoustic backscatter. The intensity of the returning echo is a measure of the target strength of a given individual at a particular sound frequency. As a fisheries assessment tool, fisheries acoustics technology is an efficient, non-intrusive method of mapping the water column at a very fine spatial and temporal resolution. It provides a practical alternative to bottom and mid-water trawls. However, validation of fish targets is an essential part of interpreting the acoustic signal for any given location. Output from acoustic surveys can be mapped in relation to bottom habitat type, bathymetry, temperature, time of day etc. The ability to census aquatic organisms are more difficult in deep water as many fish species here lack a swimbladder – the major acoustic reflector in most shelf species.
In its most direct form, we could say that fisheries acoustic produces relative information on size distribution of targets, numbers of organisms in the water column, and estimates of biomass, based on the physical properties of sound traveling through water. However, there are many factors that limit the confidence with which this information can be interpreted into accurate measures of absolute numbers of fish, fish lengths and total biomass.

Acoustic surveys onboard Dr Fridtjof Nansen
Dr Fridtjof Nansen uses ER-60 echo sounders (with ER-60 software) and LSSS (“Large scale survey system”, also called “El-triple-S”) for scrutinizing of echoes. The acoustic transducer is attached to an adjustable keel that can be lowered in rough weather to avoid the damping effect of bubbles. Echo intensities per nautical mile are integrated continuously, and mean values per 1 nautical mile are recorded for mapping and further calculations. The echograms, with their corresponding sA-values, are scrutinized every day. Contributions from the seabed, false echoes, and noise are deleted.
The acoustic survey has been carried out by zigzagging between 50 and 500 meters bottom depth around the Islands, as well as along all the transects (north-south, west-east and oblique transects). Four frequencies are being used (18, 38, 120 and 200 kHz). The survey will target firstly plankton, mesopelagic fish and pelagic fish aggregates. Secondly, the dynamics of the migrating scattering layer and the pelagic layer communities will also be studied in more detail using fisheries acoustic and multinet trawling.
The corrected values for integrated echo intensity are allocated to species according to the trace pattern of the echograms and the composition of the trawl catches. Data from pelagic trawl hauls and bottom trawl hauls considered representative for the pelagic component of the stocks, which is measured acoustically, will be included in the stock abundance calculations.
The echo sounders are watched continuously, and trawl hauls in addition to the predetermined hauls are carried out whenever the recordings change their characteristics and/or the need for biological data makes it necessary. Trawling is thus carried out both for identification purposes and to obtain biological observations, i.e., length, weight, maturity stage, stomach data, and age.

So far the R/V Dr Fridtjof Nansen has undertaken acoustics transects around the 3 Island of the Comoros and Mayotte to determine the distribution and abundance of small pelagic fish shoals. Mid-water trawls have been used on fish aggregations to determine species and size composition. However, very few pelagic shoals have been recorded. Schools of fish have been observed in the surface a couple of times, and we have tried to catch them by rod fishing, but so far with little success. These fish aggregations have not been recorded acoustically either. The reason for this could either be that they are too close to the surface (the echo-sounder can only record fish deeper than 6 m depth, or that the fish swim fast and/or actively avoid the research vessel. Strong scatters of mesopelagic fish have been recorded, but no schools which resemble commercially important species.
The conclusion is that there are very few fish in this area. We keep an eye on the echo-sounder all the time and we will conduct pelagic trawling whenever we see any acoustic signals that are strong enough to suggest higher fish abundance.

author: Katherine Michalsen (Norwegian cruise leader); image: Pascal Cotel.
We also made a post about Acoustic Surveys last year with some other details - take a look at this post on acoustic surveys

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