SPLIT BEAM Echo Sounders:
Myths and Truths
The Rise of the Split Beam
During our work in countries as diverse in their fisheries as the United States, Argentina, Peru or Spain, we have been collecting experiences about what is possible to achieve with SPLIT BEAM acoustic technology and also about the limits that today exists with this kind of equipment. We have learned on site about different fish behaviors and environmental situations that make sizing measurement very difficult or even impossible.
Although SPLIT BEAM equipment is not a new technology – they have several decades in the market – they are indeed for many fishermen, especially for small or medium artisanal boats, which gain access for the first time to this equipment. Previously they were out of reach of these fishermen, because of their high cost and complexity of operation and, therefore, they were almost reserved exclusively for scientific use.
The existence of equipment such as the Seaman DBF-4000-SB has contributed to democratize this technology, making it more accessible to all types of fishermen, but at the same time creating the need to explain its possibilities and limitations to those who are not familiar with its use. Fishermen’s training is a key factor for the successful use of these equipment. Here is a compendium of the most frequent doubts presented when evaluating the acquisition of a SPLIT BEAM acoustic tool for fish measurement.
What does it mean that an echo sounder is SPLIT BEAM?
All echo sounders project a beam of sound similar to the light of a flashlight. If the object is taken with the center of the flashlight beam it can be seen clearly, because there is more concentrated power. Towards the edges, the object looks more diffuse. The same thing happens with sound. If you take a fish with the center of the acoustic beam, it is captured with all the power and a strong echo returns, while the same fish, with the edges of the beam will be acoustically weaker. When converting the sound to size, we might conclude that they are different fishes, a big and a small one. To avoid this problem is that the SPLIT BEAM echo sounders were created for. The SPLIT BEAM echo sounders have a special transducer or projector with three or more beams instead of just one. The most common design is with four beams. This allows it to position the fish within the sound beam, thus being able to compensate the difference that exists when it measures a fish with the edge of the beam. For this purpose, SPLIT BEAM echo sounders demand special calibration, that allows it to know how much the echo should be corrected, depending on where it is located. If an echo sounder is not SPLIT BEAM, then it cannot apply this correction and will not measure accurately.
Is it possible to measure any kind of fish with a SPLIT BEAM echo sounder?
Fish, in general, can all be measured without any problem using a SPLIT BEAM echo sounder. However, there are limitations for measuring with underwater sound. The sound has horizontal and vertical resolution, that is to say, a sound pulse has a volume itself. Everything within that volume, will return an echo in response. If there are several fish within that volume the echo will be that of several fish added and therefore may make us think that it is a single big fish. That is why, when fish are present in very dense shoals (many fish in small space) the measurement of individual echoes with sound can be very difficult, even impossible. Fish without a swimming bladder, like some species of mackerel, have a weaker echo than those that do have it. The air inside their bodies offers an echo easier to capture by acoustic waves. So that the species can be of help or complicate the task of the measurement.
Is it possible to identify the fish species using SPLIT BEAM echo sounders?
The identification of the fish species has been the holy grail of fishing acoustics for several decades. Being able to know not only the size, but also the species of an echo before catching it, is something that would definitely help the fishermen. Given the level of technology available, it is not easy to achieve the identification of the species by using only acoustics. In order to identify a fish and know what species it belongs to, it is necessary to consider many variables and not only the “Target Strength” (TS). Depth of the target, geographical location, form and size of the shoal, time of the year, water temperature and salinity are among other variables to analyze. Each one of these variables should be given an importance and not always the same for all species. Many of these things are done by the human brain, as the experienced fisherman knows where to find the different species and how its echo is seen in the sounder and sonar. With the single use of an acoustic instrument, it is impossible to evaluate all this data in a few seconds and make a decision to show a result on screen. So, be cautious and don’t trust in definitive solutions in this matter. There is equipment at a very high price and great complexity that will bring you closer to the solution, but always have a degree of error or uncertainty, never be foolproof.
What does the bar graph shown on the screen of all SPLIT BEAM echo sounders mean?
The bar graph is known as size histogram and is a way to represent information for easy and quick reading. Each bar represents a size and its height tells us the importance of this size over the total measured. Then the average size is shown in a large number. However, it is very important to know how to read this chart to decide if the average size is valid. It is always better that the graphic has a single higher peak instead of several. As an aid in the identification of species, DBF-4000 allows to establish areas in the TS bar graph. Each area can be related to one or two species in particular. Some fishermen, depending on the fishing zone, can link a certain bar color with a particular species. However, this procedure is only valid for the particular case. The tool is very versatile and allows multiple interpretations which cannot always be generalized.
Should I select a particular area or full screen to measure fish?
In some places, given the way in which the shoal is presented, as is the case of Peru, fishermen set an area to the right of the screen in the form of a vertical rectangle and so they leave the equipment fixed without altering this selection. Then the fishermen look at the average and check if high bars of small sizes appear, which could indicate the presence of juveniles or “múnida”, a species of little shrimp accompanying the Anchovy. On the other hand, in other places like in the north of Spain, they prefer to analyze each shoal measuring different parts of it to see the results. Measuring very small areas can be misleading, as long as measuring very large areas can cause species that are not our goal to be measured in depths that are out of our range of work. Fishermen always have to pay attention to the target species and their behavior.
Is it possible to always trust the size data is given by the equipment?
The acoustic measuring equipment work with statistical methods. The echo sounder program filters non-measurable echoes and only present those that have gone through a very strict series of discard criteria. So, like all statistics, the greater the sample of well measured fish, the better the final result will be. It is important to know that the very dense shoals will be acceptable measured the edges, rather than in the center. Finally, a single peak in the shape of the histogram indicates that the average is more realistic. All of these are quick visual indicators of a more reliable measurement, so fishermen will only trust the average size when the elements mentioned above are present. The equipment can be adjusted, if deviations are detected, in order to achieve a result closer to the actual size. For this purpose, it is necessary to keep track of the catch and compare with the measurements, in order to detect a trend, previous to make any adjustment.
Are SPLIT BEAM echo sounders expensive and complex to maintain?
In the past, SPLIT BEAM echo sounders were much more expensive and difficult to operate and maintain, than they are today. Given the actual advance of computer science and electronics, as well as new piezoelectric materials for transducers manufacturing, this equipment are within the reach of any commercial fisherman. They require a minimum training for its use, since they are not “fit and forget” equipment. However, it is not difficult to learn for anyone who operates a personal computer. The power of observation and personal intuition of the fishermen will finally make a difference. But it is very important to know how the equipment really works and its limitations, so as not to ask for impossible results or not to be exploiting only a part of its real possibilities.
Does SPLIT BEAM equipment make any difference in fishing?
Properly installed and configured, used by a fisherman who understands its possibilities and limitations, without any doubt, it makes a big difference in the greater efficiency of fishing effort and in reducing incidental catches. Its price product relationship, compared with the benefit it brings, certainly make it convenient to purchase. From the perspective of sustainability of the fisheries at the global level, the incorporation of these or other acoustics technologies for size measurement will be a growing trend.
Is there any improvement in the size measurement using acoustic equipment?
New pulse compression or “CHIRP” technologies have already contributed to improve the performance of measuring equipment, but they are far from solving the problem definitively. Multibeam acoustic equipment which sweeps in 3 dimensions and uses CHIRP pulse are a great advance in the task of trying to measure dense shoals, but they are always within the boundaries of sound. The more complex the equipment are the more expensive and difficult to operate they become. Actually, they are out of reach of the fisherman and reserved for scientific uses. However, hydro-acoustics is a field of permanent study in which there are constant advances. Our challenge, as echo sounder manufacturers is to transform them into useful tools for commercial fishing.