WO1995019103A2

WO1995019103A2 - A method for counting fish, a device for implementing the method and a method for counting fish in an enclosure

Info

Publication number: WO1995019103A2
Application number: PCT/NO1995/000008
Authority: WO
Inventors: Magnar Uthaug; Arent Wiken
Original assignee: Magnar Uthaug; Arent Wiken
Priority date: 1994-01-10
Filing date: 1995-01-09
Publication date: 1995-07-20
Also published as: WO1995019103A3; NO940085D0; AU1468595A; NO300477B1; NO940085L

Abstract

A method for counting fish comprises providing at least one detector with a transmitter and a receiver in each grid element in a counting grid and connecting the detectors to a control unit which is coupled to a display unit, together with detecting the passage of a fish through a grid element on the basis of a recorded digital level change and the duration thereof in a signal transferred between the transmitter and the receiver, recording the passage of a fish as one count and transferring the result of the count to the display unit for display. A counting apparatus for fish for the implementation of the method comprises a surface part with a display unit and an underwater part with a counting grid formed from separate grid elements, each of which constitutes an opening in the counting grid. In each grid element there is provided a detector for detecting the passage of a fish and the detector comprises a transmitter and a receiver. There is further provided a demultiplexer connected to the transmitters and a multiplexer connected to the receivers, and a control unit connected to the demultiplexer and the multiplexer and also coupled to the display unit in the surface part. In a method for counting fish swimming in an enclosure and by means of the counting apparatus for fish, the enclosure in which the fish are to be counted is connected with an additional enclosure, the underwater part is lowered and placed in the enclosure net at the connection point, the counting grid thus constituting a connecting opening for fish between the enclosures. When counting takes place the bottom of the enclosure in which the fish are to be counted is raised towards the surface, thus forcing the fish to pass through the counting grid and into the additional enclosure.

Description

A method for counting fish, a device for implementing the method and a method for counting fish in an enclosure

The invention concerns a method for counting fish, a counting apparatus for fish for implementing the method and a device for counting fish which are swimming in an enclosure or cage, where the counting apparatus is employed.

In connection with the management of fish resources, fisheries and fish farming there is a great need to be able to obtain an estimate of a stock or population of fish. In practice this can be done by counting the number of fish in the population. A large number of devices are known for counting fish. Most of these devices, however, are designed for counting under very specific conditions and thus have a very limited application.

As examples of such known counting devices for fish, reference can be made to Finnish patent application no. 85 4235 wherein a pipe device is used with a counter, the pipe device accelerating the velocity of a water current and thinning out the number of fish, thus enabling fish which are swimming closely behind one another to also be registered. In addition it is possible to determine the size of the fish. In Norwegian patent application no. 85 0566 there is disclosed a device for counting fish wherein there is used a container vessel and a sluice device with openings which lead to a number of channels, each of which is equipped with a counting apparatus for recording the fish which pass the bottom of the sluice device. The channels are designed in such a manner that the water current is accelerated, ensuring that the distance between following fish is increased, thereby making them easier to count. The fish has to be guided into the container vessel and then made to almost slide through the counting apparatus.

NO-PS no. 156 811 discloses a counting device for fish, especially smolt. In this case too a container vessel is employed, but connected to a pipe socket where there is produced a water current which is accelerated towards the outlet of the pipe socket, a known apparatus being provided at the outlet for recording each individual fish. NO-PS no. 168 151 discloses a method and an apparatus for counting preferably live fish, wherein the fish is guided in a channel past a recording unit based on a CCD camera which is oriented perpendicularly to the channel. By means of a light source on the opposite side a shaded area is created when the fish passes and this shaded area is recorded by the camera. Other kinds of counting apparatus are also known for use in connection with sorting devices for fish. A common feature of all of these is that the fish has to be guided into a counting channel or a counting chamber.

The common disadvantage of the known devices for counting fish is that they are unsuitable for counting fish in situ, e.g. free-swimming fish or fish which are swimming in an enclosure. If, e.g., they have to be used for counting fish which are swimming in an enclosure, the fish have to be transferred from the enclosure into the counting device and this is both a time consuming as well as a risky operation. In order for the fish to be able to withstand the strain of being removed from the enclosure, it has to be starved for a week beforehand. In addition to this, after having been taken out, counted and replaced, the fish will normally not take any food for the first week. Thus by taking the fish out for counting at least two weeks' growth is lost. In addition to the fish starving for two weeks, the actual counting operation will also be a strain on the fish and this in turn can result in an increased risk of contracting diseases such as, e.g., furunculosis.

The object of the invention is therefore to avoid the above-mentioned and other drawbacks of known devices for counting fish. In particular the object is to provide a method and a device for counting fish which enables fish to be counted in situ, for example free-swimming fish or fish which are located in an enclosure, without exposing the fish to injury, stress or other strains and without having to remove the fish from the environment in which it is situated. A further object of the present invention is simultaneously to enable an accurate count to be made of each individual fish in a fish population, and possibly to provide the possibility of performing an efficient sampling of the stock, thus enabling it to be estimated with a relatively high degree of accuracy.

The knowledge of the number of fish in a population is crucial in a great many connections, and by means of counting or sampling of the stock of free-swimming fish this knowledge can be used to obtain a more efficient management of fish resources. By counting fish swimming in an enclosure in a fish farm, the result of the count will be important for feeding, thus making it possible, for example, to optimize feeding and avoiding both overfeeding and underfeeding which can easily occur in one and the same fish farm. This will give economic gains in the form of both savings in feed costs and better growth in the fish. Counting fish can also be important in connection with the fish trade, e.g. in connection with transport of live fish. The above-mentioned objects and advantages are achieved according to the present invention with a method which is characterized in providing at least one detector which comprises a transmitter and a receiver respectively in each grid element in a counting grid, connecting the detectors via an address bus and in and outgoing signal lines with a control unit which is coupled to a display unit, the method also comprising successive steps for: a) setting a specific signal level in the detectors; b) testing the connection between the detectors and the control unit; c) addressing each of the detectors from the control unit in order to initialize a counting sequence; d) detecting the passage of an object such as a fish through a grid element in the counting grid on the basis of a recorded digital level change and the duration thereof, in a signal transferred between the transmitter and the receiver; e) recording the detection of the passage of a fish as a counted unit in the micro- controller; and f) transferring the result of the count to the display unit for display, and with a counting apparatus for fish which is characterized in that the counting apparatus comprises a surface part, the surface part containing a display unit and a power supply and an underwater part with a counting grid in the form of a matrix with I rows and J columns which form I x J = N elements in the matrix, an element in the matrix constituting a grid element in the counting grid, that there is provided in each grid element at least one detector arranged to detect the passage of an object such as a fish through the grid element, that each detector comprises a transmitter and a receiver provided on the chords which form the sides of the grid element, the trans- mitter being arranged to transfer a signal to the receiver, that there is provided a demultiplexer connected with the transmitters and a multiplexer connected with the receivers, and that there is provided a control unit which is connected via an address bus with the demultiplexer and the multiplexer, the control unit being further connected with the demultiplexer via an outgoing signal line and with the multiplexer via an ingoing signal line and also connected to the display unit and the power supply in the surface part.

The invention will now be described in more detail in connection with an embodiment and with reference to the attached drawing.

Fig. 1 is a block diagram of a counting apparatus for counting fish according to the present invention. Figs. 2a and 2b illustrate a counting grid used in the counting apparatus according to the present invention and a grid element in the counting grid respectively.

Figs. 3a and 3b illustrate schematically the counting apparatus according to the invention located in an enclosure and the connection between the underwater part and the surface part of the counting apparatus.

As shown in fig. 1 , the counting apparatus according to the invention comprises a surface part wherein there are provided a display unit and a power supply in the form of a battery. Wherever possible the battery can be replaced by an external power supply or connected to a battery charger operated by an external power supply, the battery thus being charged when the counting device is not in operation. The display unit can give a visual or other indication of the number of fish which are counted and may be connected to an external printer for printing out the results of the count. The display unit will also have a reset input and a pulse input, a signal pulse which is supplied to the pulse input representing a count unit and indicating, e.g., the passage of a fish through the counter.

The surface part is connected to an underwater part which substantially comprises a counting grid as illustrated in fig. 2a. The counting grid is in the form of a matrix with I rows and J columns, thus comprising I x J = N equal-sized grid elements of the type shown in fig. 2b. In each grid element there is provided a detector consisting of a transmitter or receiver. The detector can be an optical detector or an acoustic detector. The use of optical detectors in the form of infrared detectors is preferred. In this case the transmitter in the detector emits an optical signal in the form of infrared light which is received by the receiver. If a break now occurs in the radiation path of an object, e.g. a fish, which passes through the grid element, the level of the received signal will naturally drop, theoretically right down to zero, and such a level change is recorded as the passage of, e.g., a fish and constitutes a count unit.

The signal which is used in the optical detector may be continuous, but can also be pulsed, for example with a pulse frequency of, e.g., 30-50 kHz. Instead of a pulse a burst signal can be used, i.e. the pulse is then divided into a very large number of intense and extremely brief pulses with a corresponding burst frequency of 30-50 kHz. One of the advantages of using a pulse signal or burst signal is that power consumption is reduced. Two or more detectors may also be provided in each grid element and the detection of the passage of a fish can then be based on coincidence between the recording of a level change in the received signals and the duration of the level change in the signals. This will give a more reliable detection in cases where fish swim through the counting grid in close proximity to one another, which could result in two or more fish being counted as one with the use of only one single detector and without the use of coincidence in the signal detections.

As already mentioned, the counting grid is designed in the form of a matrix with equal- sized, similar grid elements. Typical dimensions for the grid elements may be approxi- mately 200 x 150 mm and the counting grid which in the figure comprises 32 grid elements thereby has a cross-sectional area of approximately 1300 x 1000 mm. If a counting grid is used, for example, for counting fish in fish farms where the fish are swimming in enclosures or cages, the fish in a specific enclosure will generally be distributed about an average size and thus the counting grid will at any rate act as a sorting device, which will not allow fish over a certain size to pass through. If at the same time the dimensions of the grid elements are in reasonable agreement with the average size of the fish, it will not be possible for two or more fish to pass simultaneously. If less fish have to be counted, there can be provided in front of the counting grid a combined sorting/guiding grid where the grid elements or openings are of smaller dimensions than the grid elements in the counting grid. At the same time as the sorting/guiding grids are guiding the fish into the counting grid, there now continues to be a safeguard that two or more fish cannot simultaneously swim through the grid elements in the counting grid. In this case the sorting/guiding grid is provided in front of the counting grid, i.e. in the downstream direction of the passage of the fish and in such a manner that the openings in the sorting/guiding grid centredly overlay corresponding grid elements in the counting grid. If fish with several different dimensions have to be counted in respective counting sequences, sorting/guiding grids with grid elements of different dimensions can be used, while the same counting grid is used the whole time. In fish farms it has been shown that as a rule it will be necessary to count fish in three different size categories and in this case the counting grid can be adapted to suit the largest fish, only two sorting/guiding grids thus being necessary for counting the other size categories. In fig. 2a the counting grid is shown with rectangular grid elements, but it should be understood that the grid elements can also have a different shape, e.g. they can be circular or rhomboid, given that the grid elements are uniform and permit a practical arrangement of the detectors and the passage of the fish. If the counting grid is designed as a chord grid, the chords can be made of tubes of a suitable plastic material, since the tubes have to form the sides of the grid elements.

It is by no means a condition that the detectors should be optical detectors. Acoustic detectors can also be employed and in this case preferably ultrasonic detectors which will be provided in the same way as the optical detectors. With ultrasonic detectors too a pulse signal can be used with a suitable pulse frequency, and account must be taken of the fact that the ultrasonic frequency may be from 30-40 kHz up to several hundred kHz.

It will be obvious to a person skilled in the art that the detectors can be provided in other ways than those mentioned above. If a detector is used, transmitter and receiver can be combined to form a transceiver element and located on one side of the grid element, while on the opposite side there is provided a reflector, either an optical reflector or an acoustic reflector, depending on the type of detector employed. When using two or more reflectors other arrangements of transmitters, receivers and possibly reflectors may be possible. In this case, e.g., the reflectors can be combined and the signal path does not require to be perpendicular to the sides or chords which form the grid element, or the signal can be distributed to several reflectors by means of fibre optic devices and then transferred to several receivers, while only one single transmitter is used. In every case the object of a preferred arrangement of the detector elements will be to achieve as reliable detection as possible of the passage of a fish.

It can be mentioned that experiments with the use of infrared light under water have demonstrated that there is no problem in obtaining reliable detection of the signal with the small distances involved here, even though some absorption of the signal occurs in the water.

The actual counting grid can be constructed as a chord grid with chords of plastic tubing, e.g. PVC tubing, detectors and detector elements being provided in the tubes together with the electronic equipment which is necessary for the detectors.

The underwater part of the counting apparatus according to the invention thus consists substantially of a counting grid and the other electronics in the counting apparatus are preferably built into the underwater part, e.g. in the tubes which compose the chords in the counting grid. The electronic equipment in the counting apparatus comprises a control unit, preferably a microcontroller with a clock or oscillator. The microcontroller is connected with the detectors via an address bus and via a demultiplexer for the transmitters and a multiplexer for the receivers respectively. An outgoing signal line leads from the control unit to the demultiplexer and an ingoing signal line from the multiplexer to the control unit. The control unit, which is preferably a microcontroller, comprises a processor, memories, including a programme memory, preferably in the form of a ROM, as well as an oscillator or clock.

When the counting device according to the invention has to perform a counting sequence the control unit first has to set the signal level in the detectors, and then test the connection between the detectors and the control unit before the counting starts. This is implemented by the control unit selecting each detector element in turn via the address bus in order to set a signal level which is adapted to suit the existing transfer conditions. This is performed via the demultiplexer which, for example, may consist of two MC14067 1:16 analog demultiplexers, if 32 detectors are employed. The demultiplexer circuits can be separated by a blocking input on the control unit together with an inverter in the control unit. The detector signal is generated by the control unit, preferably in the form of a pulse signal or a burst signal if the detector is an optical detector and is passed through the demultiplexer into a buffer which operates the transmitter in the detector. At the same time the control unit also addresses the multiplexer which, for example, can be two MC14067 16:1 analog multiplexers, if 32 detectors are employed, via the same address bus and initializes the receiver in the detector. The received detector signal is amplified and passed via the multiplexer on the ingoing signal line to the signal input on the control unit. In connection with the initialization of the detectors, as mentioned the connection between the detectors and the control unit is also tested and a test is also carried out to see whether or not the signal path between transmitter and receiver in the detectors is blocked.

When a fish passes through a grid element, this will be recorded as a digital level change, in practice a reduction in the signal level which is recorded by the detector's receiver. In order to avoid recording erroneous signals a count can be made conditional on the level change having a specific, predetermined duration. This can be advantageous if air bubbles or small particles occur which could cause an erroneous signal. Thus the control unit can distinguish between random signal disturbances and genuine detection signals.

As mentioned above, it can be a problem to count fish which are swimming close behind one another as separate count units. As indicated above, therefore, this problem can be advantageously tackled by the use of two or more detectors in each grid element and by basing the detection on coincidence between the detected signal in the receivers with regard to signal level and duration. It is, of course, also possible to use more than two detectors and coincidence detection. In the case where two or more detectors are used the detectors can be suitably arranged in such a manner that coincident detection is only obtained within an area where the width of the fish exceeds the distance between the transmitters and thereby the distance between the signal paths in one and the same grid element. The tail and nose sections of the fish will then not be detected and a natural division is thereby obtained between each fish, if the fishes are swimming close behind one another.

In general the combination of different detector arrangements and coincident detection will offer a number of possibilities for increasing the detection reliability and avoiding erroneous detections.

It is an advantage if both the multiplexers and the control unit in addition to the detectors are provided in the underwater part. By placing the control unit in the underwater part, the cable connection between the surface part and the underwater part is reduced to a minimum, thus giving a reduction in costs. All the electronics in the counting apparatus according to the invention are advantageously implemented with low power circuits, e.g. CMOS circuits, thus reducing the power consumption. By simultaneously using pulse signals or burst signals a further reduction in the power consumption is obtained. The pulse signals and burst signals are preferably transmitted at a pulse or frequency which for optical signals lies, as mentioned, in the range from 30-50 kHz. The receivers can then have a band pass filter which is adapted to this frequency range, and this contributes to a great extent to the elimination of the influence of any noise sources. Instead of using a demultiplexer and a multiplexer, coded signals can be employed, a codec being provided in the control unit and each detector being assigned a specific coded address. The detectors are advantageously addressed sequentially, with a distinction between the transmitter side and the receiver side, thus preventing any influence between the various detectors and detector elements.

Finally the operative function of the counting apparatus can advantageously be implemented by means of software which, for example, is stored in a memory in the control unit. A counting sequence can then be performed automatically according to the specified procedures which are included in the software and recording of the results of the count is performed on the basis of detection algorithms which constitute elements of the software.

A method for counting fish which are swimming in an enclosure or cage will now be described, wherein a counting apparatus for fish according to the present invention is employed. The counting is performed by connecting the enclosure in which the fish are to be counted with a second enclosure which is preferably empty of fish, which enclosure, e.g., is towed into the enclosure in which the fish are located. The under¬ water part of the counting apparatus is lowered and arranged in the enclosure net in such a manner that the counting grid forms a connecting opening for transfer of fish between the enclosure in which the fish are to be counted and the empty enclosure. This is illustrated schematically in fig. 3a, where the counting grid is shown located in the enclosure net, i.e. a side wall of the enclosure, and leads into the empty enclosure upstream of the counting grid. In order to make the figure clearer two of the sides and the bottom of the enclosure in which the fish are to be counted have been removed. Similarly, fig. 3b schematically illustrates the connection between the counting grid in the underwater part and the electronics, i.e. the display unit in the surface part, the grid elements in the counting grid being omitted. As mentioned, downstream of the counting grid in the enclosure in which the fish are to be counted there can be provided a combined sorting/guiding grid (not shown) whose openings or grid elements are of a size which constitutes a certain fraction of the grid elements in the counting grid, but otherwise a similar shape. In this context the grid elements in the not shown sorting/guiding grid are provided overlaying and centered in relation to the corresponding grid elements in the counting grid. When counting the fish in the enclosure, use will be made of a sorting/transfer grid which is preferably adapted to the size of the fish in the enclosure. The size of the fish in the enclosure will usually be normally distributed around a mean value, but with a reasonably low standard deviation which can easily be roughly determined by measuring a smaller number of fish in the enclosure. However, the use of the sorting/guiding grid is primarily intended to ensure that two or more fishes do not swim too close to one another or pass through the grid elements in the counting grid at the same time, thus making the counting as accurate as possible.

When the fish in the enclosure have to be counted, the bottom of the enclosure will be raised while at the same time the fish the fish will be apt to swim in circles in the enclosure. The fish will gradually find their way through the counting grid and into the adjacent transfer enclosure, and once a fish has found the way through the counting grid, experience shows that the other fish will follow suit relatively quickly. Experiment shows that the passage through the counting grid takes place calmly with no sign of panic reactions in the fish, nor are they exposed to any kind of stress.

For fish farmers, the determination of the number of fish in the enclosure will enable them to optimize feeding with reduced feed costs as a result, while at the same time preventing uneven feeding of the fish, i.e. some of the fish are overfed and some are underfed. Normally automatic feeding of the fish will be used in the different enclosures and this is based on having the same number of fish in each enclosure, which is by no means always the case. In practice it means that in some of the enclosures the fish are overfed, while in the other enclosures underfeeding takes place. At the same time as reducing the feed costs by avoiding overfeeding, an increase in the amount of food in the enclosures in which underfeeding occurred will naturally give increased feed costs, but this increase is compensated by better utilization of the fish's growth potential, with the result that all things considered a not insubstantial net gain is achieved.

Also when delivering fish from a fish farm to, e.g., well boats, it will be important for both the farmer and the purchaser to know the number of fish, since this will improve price fixing.

Finally it can be mentioned that counting fish in farming enclosures will be important in connection with fixing insurance premiums for the fish farm and possibly documentation of the fish farm's assets.

It should also be mentioned that better feeding based on knowledge of the exact number of fish in the enclosures not only will result in a reduction in waste, which has favourable environmental consequences, but will also provide a better quality in the fish, since it is obvious that feed which rots on the bottom under the enclosures produces poisonous gases which in turn lower the fish's resistance and cause an increase in the frequency of disease. If disease in the fish can be avoided, it will be possible to reduce the consumption of antibiotics in fish farms, which is known to be a major environmental problem as well as a serious financial burden for the farmer, while at the same time the connection is established between the consumption of antibiotics and the quality of the fish and possible consequences for the health of the consumers.

The method and the counting apparatus according to the invention are not only suited for counting fish which are swimming in an enclosure, but can also be used for count- ing free-swimming fish, e.g. anadromous fish species swimming up watercourses, and this can provide a better management of the fish population in the watercourse. In this case the counting apparatus according to the invention can be suitably provided in a salmon ladder or the like or in connection with a sluice or guiding device which is provided in the watercourse. In each case the fish can be guided through the counting grid of the counting apparatus without the use of physical or other constraints which could cause the fish stress or damage it in other ways.

However, the use of the counting apparatus for counting free-swimming fish is not limited to use in watercourses with sluice devices for the fish or salmon ladders, but it can also be used on free-swimming fish in rivers, lakes, fjords and even in the open sea. In this case the stock or population count must be based on sampling, e.g. where a specific number of counting devices, i.e. underwater parts of the counting apparatus with counting grids, are provided in a section of the area, which may lie in a horizontal plane. The dimensions of the grid elements should naturally be adapted to a maximum height/width dimension of the fish which has to be counted, thus preventing the fish from being injured in any way or from becoming stuck. Erroneous counts as a result of several fish passing through the opening in the counting grid simultaneously or almost simultaneously are in no way crucial in cases where an estimate of the fish stock in a specific area of the sea is desired, e.g. in connection with spawning migrations. Moreover the accuracy can generally be maintained by using several detectors in each grid element and coincident detection. By recording the number of fish which pass a specific number of counting grids at a given time and observing the variation in the recordings with the grids' geographical position and over time, it will be possible to obtain plausible estimates of a fish stock, e.g. in connection with spawning migrations and the infiltration of the species into the spawning areas, which will be of great importance for both the research and the management of fish resources in the sea.

The above-mentioned applications are intended as examples and those skilled in the art in this field will in all probability find applications for the method and the counting apparatus according to the present invention in other areas than those described here.

It should further be understood that other variants and designs of the method and the counting apparatus for fish respectively according to the present invention will be possible within the scope of the attached claims and that the above-mentioned embodiments are only quoted as examples of preferred designs of the invention and are not intended to be limiting in any way.

Claims

PATENT CLAIMS

1. A method for counting fish, characterized by providing at least one detector which comprises a transmitter and a receiver respectively in each grid element in a counting grid, connecting the detectors via an address bus and in and outgoing signal lines with a control unit which is coupled to a display unit, the method also comprising successive steps for: a) setting a specific signal level in the detectors; b) testing the connection between the detectors and the control unit; c) addressing each of the detectors from the control unit in order to initialize a counting sequence;, d) detecting the passage of an object such as a fish through a grid element in the counting grid on the basis of a recorded digital level change and the duration thereof in a signal transferred between the transmitter and the receiver; e) recording the detection of the passage of a fish as a counted unit in the microcontroller; and f) transferring the result of the count to the display unit for display.

2. A method according to claim 1 , characterized by providing two or more detectors in each grid element in the counting grid, the detection of the passage of a fish through one of the grid elements in step d) being based on coincidence of the recorded digital level changes and the duration thereof between the signals transferred from the transmitters to the receivers in the grid element.

3. A method according to claim 1 or 2, characterized in that the detectors are addressed sequentially.

4. A method according to one of the claims 1-3, characterized in that as a detector there is provided an optical detector, preferably an infrared detector.

5. A method according to claim 4, characterized in that a continuous optical signal is employed for detection in the optical detector.

6. A method according to claim 4, characterized in that for detection in the optical detector there is employed a sequence of brief optical pulses with a preferred pulse frequency of 30-50 kHz.

7. A method according to claim 4, characterized in that for detection in the optical detector there is used a sequence of brief optical burst with a preferred burst frequency of 30-50 kHz.

8. A method according to one of the claims 1-3, characterized in that as a detector there is provided an acoustic detector, preferably an ultrasonic detector.

9. A method according to one of the claims 1-3, characterized in that a programme stored in the control unit is employed for the control unit's functions and the detection of the passage of a fish.

10. A method according to claim 9, characterized in that a microcontroller is used as a control unit.

11. A counting apparatus for fish for the implementation of the method according to claims 1-10, characterized in that the counting apparatus comprises a surface part, the surface part containing a display unit and a power supply, and an underwater part with a counting grid in the form of a matrix with I rows and J columns which form I x J = N elements in the matrix, one element in the matrix constituting a grid element in the counting grid, that in each grid element there is provided at least one detector arranged for detecting the passage of an object such as a fish through the grid element, that each detector comprises a transmitter and a receiver provided on the chords which form the sides of the grid element, the transmitter being arranged to transfer a signal to the receiver, that there is provided a demultiplexer connected to the transmitters and a multiplexer connected to the receivers, and that there is provided a control unit which is connected via an address bus with the demultiplexer and the multiplexer, the control unit being further connected with the demultiplexer via an outgoing signal line and with the multiplexer via an ingoing signal line and also coupled to the display unit and the power supply in the surface part.

12. A counting apparatus according to claim 11 , characterized in that the counting grid is designed as a chord grid where the spaces between pairs of adjacent chords form grid elements, and that the transmitter and the receiver in each grid element respectively are provided on parallel chords which form the opposite sides of the grid element.

13. A counting apparatus for fish according to claim 12, characterized in that the chords in the chord grid are formed from tubes of a suitable plastic material, all the electronic components in the underwater part being placed in the tubes.

14. A counting apparatus for fish according to one of the claims 11-13, characterized in that the transmitter and the receiver in each grid element are combined into one unit and provided on a chord which forms one of the sides of the grid element, there being provided on the opposite side of the grid element a reflector for transferring the signal between the transmitter and the receiver.

15. A counting apparatus for fish according to one of the claims 11-14, characterized in that there is provided a pair of detectors in each grid element.

16. A counting apparatus for fish according to claim 15, characterized in that each of the transmitters and each of the receivers in the detectors which constitute the pair are provided respectively with a certain spacing on the chords which form the sides of the grid element.

17. A counting apparatus for fish according to one of the claims 11-16, characterized in that the detector is an optical detector, preferably an infrared detector.

18. A counting apparatus for fish according to one of the claims 11-16, characterized in that the detector is an acoustic detector, preferably an ultrasonic detector.

19. A counting apparatus for fish according to claim 11 , characterized in that the demultiplexer, multiplexer and the microcontroller are provided in the underwater part.

20. A counting apparatus for fish according to claim 11 or 19, characterized in that the control unit is a microcontroller.

21. A counting apparatus for fish according to claim 11 , characterized in that in front of or downstream of the counting grid in the underwater part there is provided a combined sorting/guiding grid, whose grid elements form a matrix of I x J = N elements corresponding to the counting grid, each of the grid elements in the sorting/guiding grid being of a shape corresponding to the grid elements of the counting grid together with an area which constitutes a certain fraction of the area of a grid element in the counting grid and moreover is provided overlaid and centered in relation to the corresponding grid element in the counting grid.

22. A method for counting fish which are swimming in an enclosure, where there is employed a counting apparatus for fish according to claims 11-21 , characterized in that the enclosure in which the fish are to be counted is connected to an additional enclosure which is preferably empty of fish, that the underwater part is lowered and placed in the enclosure net at the connection point, the counting grid thus constituting a connecting opening for transferring the fish between the enclosure where the fish are to be counted and the additional enclosure, the bottom of the enclosure where the fish are to be counted being raised towards the surface, thus forcing the fish which have to be counted to pass through the grid elements in the counting grid and into the additional enclosure.

23. A method according to claim 22, characterized in that in front of the counting grid on the side facing the enclosure in which the fish are to be counted, there is provided a combined sorting/guiding grid adapted to the size of the fish in this enclosure.